PUBLICATION 666

APRIL 1957

■ *■

AsncuHure Canadian Agriculture Library

Bibliotheque canadienne de I'agriculture
Ottawa K1 A 0C5

Canada

NADA DEPARTMENT OF AGRICULTURE, OTTAWA , ONTARIO

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3

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PUBLICATION 666
REVISION

APRIL 1957

CANADA

STEM RUST OF CEREALS

by

J. H. CRAIGIE

BOTANY AND PLANT PATHOLOGY DIVISION

CANADA DEPARTMENT OF AGRICULTURE
OTTAWA, ONTARIO

Price 25 cents

3M— 22328-4: 57

This bulletin was first issued in 1940 as Part XII of Studies
in Cereal Diseases. In the present edition, certain sections,
particularly those dealing with the control of rust by chemicals and
plant breeding, were revised by Thorvaldur Johnson, Officer in
Charge of the Plant Pathology Laboratory, Winnipeg, Manitoba.

TABLE OF CONTENTS

Page

Introduction 5

History of Stem Rust 6

Cause of Stem Rust 8

The Common Barberry 10

Life Cycle of the Fungus 11

Physiological Specialization 17

Varieties of Stem Rust 17

Races of Stem Rust 19

Hybridization in Stem Rust 23

Compound Pustules 25

Action of Pycniospores 25

Crossing of Races and Varieties 27

Sources of Infection 27

Infected Barberry 27

Survival of Red Stage 28

Sources in North America 28

Influence of Weather 29

Moisture 29

Temperature 29

Wind 29

Relation of Spore Abundance to Epidemics 30

Factors Influencing Spore Abundance 30

Characteristics of Spread 31

Within Region of Source 31

Beyond Region of Source 31

Stem Rust Spread in North America 32

Mississippi Valley and Prairie Provinces 32

Remainder of Continent 33

Effect of Stem Rust on Cereal Plants 35

Control of Stem Rust 36

Destruction of Barberry 37

Early Maturity 37

Chemicals 37

Plant Breeding 38

Summary 42

Useful References to Stem Rust Literature 43

82270— l1

3

2

STEM RUST OF CEREALS

J. H. CRAIGIE

INTRODUCTION

Stem rust is a disease of wheat, oats, barley and rye, as well as of a number
of different grasses. It occurs in practically every region where cereals are
grown. One or two of the other five cereal rusts may occur at the same time
on the same crop plant. For instance, stem rust and leaf rust very commonly
occur together on wheat, which in some regions is subject to stripe rust as
well. Similarly, stem rust, dwarf leaf rust, and stripe rust may be present
on barley. Crown rust may be present on oats, or brown leaf rust on rye, at the
same time as stem rust. Wherever prevalent, each of the other rusts causes a
greater or less amount of damage, and in some regions or localities the damage
from stripe rust or crown rust or from one of the leaf rusts may be greater than
that from stem rust. Of all the cereal rusts, however, stem rust is the one
most universally dreaded. The loss from it far exceeds that from any of the
others.

Some idea of the enormous losses caused by this disease may be gained
from the estimated damage done to the wheat crop in three of the north-central
States (Minnesota, North Dakota, and South Dakota) and in Western Canada.
In 1916 alone, the loss in these two areas was estimated at 180,000,000 and
100,000,000 bushels, respectively. In the three States mentioned above, the
average annual loss from 1916 to 1922 was estimated at 44,000,000 bushels.
During the years from 1925 to 1935, the average loss in Manitoba and
Saskatchewan has been calculated to be about 35,000,000 bushels, or nearly 11
per cent of the possible yield. These estimates are for yield alone, and do not
take into consideration the very appreciable loss in the value of the crop due to
reduction in grade and quality.

Because of the great economic importance of the crops affected, stem rust
has been the subject of investigation in many countries. Much information
has been brought to light concerning its cause, its method of spread, and the
environmental factors that favor or retard its development. In recent years,
attention has been given to controlling stem rust and notable progress has been
made, particularly in the direction of producing resistant varieties of cereals.

Canadian rust investigations, and the work on breeding for rust resistance,
have been carried on, for the most part, at the Rust Research Laboratory
established at Winnipeg by the Canada Department of Agriculture in 1925.
In 1938 the laboratory was divided into two sections designated as the Plant
Pathology Laboratory, and the Cereal Breeding Laboratory, and the original
name, Rust Research Laboratory, was discontinued.

The Canadian universities, in particular the Universities of Manitoba,
Saskatchewan, and Alberta, have engaged at various times in rust research and
have made valuable contributions in both plant breeding and rust pathology.
Some of these investigations have been supported by grants from the Canada
Department of Agriculture and the National Research Council. Throughout
the years there has been close collaboration between Canadian rust investigators
and those in the United States, Mexico, and certain Commonwealth countries.

6

The distribution of rust-resistant wheats in Canada began in 1936, and by
1938 somewhat more than half the wheat acreage in Manitoba was sown to
resistant varieties. By 1939 these wheats were being widely grown in eastern
Saskatchewan. It has been estimated that the growing of resistant wheats in
the rust area of Manitoba and eastern Saskatchewan over the period 1938-1943
increased the average annual wheat production of that area by about 41,000,000
bushels. Rust-resistant varieties of oats were also distributed in Western
Canada in 1936 and soon became widely grown in the area affected by oat stem
rust.

As a result of the extensive growing of resistant varieties of wheat and oats,
damage from stem rust in Western Canada was reduced almost to insignificance
during the decade 1940-1950. In more recent years, and especially since 1950,
changes in the rusts have taken place that have enabled them to overcome the
resistance of the new varieties to the extent that losses from stem and leaf rust of
wheat, particularly in 1954, were comparable to losses sustained in the years
before the production of rust-resistant varieties. The nature of these changes
and the countermeasures taken by plant scientists are outlined in the section on
plant breeding.

HISTORY OF STEM RUST

How far back in the history of agriculture stem rust first began to do damage
to cereal crops, no one can say. It is known, however, that "rust" was destructive
to such crops in Greece and Italy two or three hundred years before the
beginning of the Christian Era. Other cereal rusts may have been included
under this name, but the likelihood is that a good part of the damage was done
by stem rust. The disease has probably been present in southern Europe ever
since that time. It has been causing damage in northern Europe for two or
three centuries at least. It was probably present in Asia long before the time
of the Greeks and Romans. Some authorities are of the opinion that the
"blasting" and "mildew" mentioned in the Old Testament (for example I
Kings 8: 37; II Chron. 6:8; Haggai 2: 17) refers, at least in part, to stem rust.
Others are inclined to question that opinion. Undoubtedly the disease is a very
ancient one.

Little is known of how or when the disease became distributed throughout
the world. A severe outbreak of stem rust occurred as early as 1703 in South
Africa. It was causing serious damage in the New England States about the
same time. It appeared in Australia not long after the cultivation of wheat
was undertaken there, the first heavy attack occurring in 1803. It was found in
South America before the opening of the present century, but it was probably
there much earlier. For the last 65 years, and probably for centuries before, it
has been a serious menace to wheat production in India.

With regard to Canada, it is probable that stem rust began to attack the
crops in what is now the provinces of Quebec and Ontario fairly early in the
agricultural history of these provinces. About the middle of the last century
one writer stated that there was "nothing the Canadian farmer suffered so much
from as rust in their wheat crops." From his account, there is little doubt
that he refers mainly to stem rust. He does not state how long before that
time rust had been causing serious losses to Canadian farmers, but it is plainly
evident from his statement that the disease had been troublesome for many
years and that farmers and others were earnestly endeavoring to find some
means of controlling it. At that time, and even much later, the popular belief
was that certain climatic and soil conditions caused the crops to rust, although
about 50 years earlier the parasitic nature of the disease had been definitely
established.

The first record of its occurrence in Western Canada seems to be that of
Dr. John Dearness, who, while on a visit to Manitoba in the summer of 1891,
found it at several points in that province. There is little doubt that it was
present previous to that year. From what is known concerning the spread of
the disease and its occurrence in the Mississippi Valley, there is a possibility
at least that stem rust was present on the wheat crop from which the first
export shipment was made from Manitoba in 1876. In those earlier years it
was apparently of no great importance.

In the Report of the Dominion Experimental Farm, Brandon, Man., for
1896, Dr. Bedford, the Superintendent, records that the months of April and
May, were wet and temperatures for May were from 3° to 6° F. above average.
On June 27 the crops began to lodge, and a few days later rust attacked the
stems of wheat and oats. Temperatures and rainfall in July, August, and
September were about average for those months, "but in spite of this," he says,
"the ravages of rust continued, and were shown in delayed ripening, rusty
weak straw, shrunken heads, reduced yield and a light weight sample". There
is no doubt that the rust referred to in his report was stem rust. In 1897, on
the other hand, there was "almost a total absence of rust among grain crops".
"Rust" was therefore recognized at that time as being injurious to cereal crops
in Manitoba, although it is apparent from the reports that stem rust was not
distinguished from any of the other cereal rusts.

The year 1916 is generally regarded as the first bad "rust year," and
undoubtedly stem rust was more destructive in that year than in any previous
one. It was, however, by no means altogether absent from Western Canada
in the earlier years of this century. In 1902, Dr. Bedford states in his report that
"this disease (rust) is one of the chief factors in reducing the yield of wheat,
especially during seasons of abundant rainfall such as we have had during the
last two years." Rust was "very prevalent on the stronger soils" in the district
around Brandon in 1903 and, on the Farm, both the "yield and sample were
greatly injured, especially on valley land". The Superintendent of the
Dominion Experimental Farm, Indian Head, Sask., reported that in 1902 rust
"hitherto almost unheard of in the Territories" did a small amount of damage,
and that on the Farm in 1903 a number of varieties were "struck by rust."
In the latter year wheat on fertilizer plots was so badly rusted that the yield
data were of no use. Stem rust was severe enough in 1904 to cause widespread
alarm and rather heavy loss in Manitoba and in Saskatchewan (then a part
of the Northwest Territories). It was fairly prevalent in 1905. In 1911 it did
considerable damage in western Manitoba and eastern Saskatchewan, where
cold wet weather during August delayed ripening of the wheat crop.

The years from 1900 to 1939 may be divided into light-, medium-, and
heavy-rust years, according to the severity of stem rust on wheat in Western
Canada. The heavy-rust years are 1904, 1916, 1923; 1927, 1935, and 1938, the
worst ones being 1916 and 1935. In the medium-rust years, 1911, 1919, 1921,
1925, 1930, and 1937, infection was somewhat less severe and usually involved a
less extensive area. The years not classed as heavy- or medium-rust years may
be regarded as light-rust years. During the first half of the period under review,
the information concerning the prevalence of the disease is generally scanty,
and it may be possible that certain years, for example 1903 and 1905, should be
classed as medium- rather than as light-rust years.

From 1940 to 1949, because of the widespread cultivation of rust-resistant
varieties, little damage from rust occurred. Following a rather sudden
change in the parasitic behavior of wheat stem rust in 1950, slight damage was
done to bread wheats in 1951 and 1952, moderate damage in 1953, and severe

8

damage in 1954. In this period, the formerly rather rust-resistant durum
wheats were far more susceptible than the bread wheats and, in 1952 and 1953,
and especially in 1954, durum wheats suffered heavy rust losses.

CAUSE OF STEM RUST

Stem rust is caused by a fungus. The other cereal rusts are caused by
other closely related fungi. Fungi are plants, but, unlike ordinary plants, they do
not possess green coloring matter in their cells and hence they are unable to
manufacture their own food. On this account, they must obtain their food
ready-made. Some of them can attack living plants or animals and procure

Figure 1. — Red spores (urediospores) of stem rust. Notice their

egg-shaped appearance, also the denser color of their centers due to the

presence of a reddish coloring substance in oil globules collected there.

Magnification, 230 app. (Photograph by Mr. A. M. Brown)

their food from them. Such fungi are known as parasites. Others are able to
feed on dead matter, and are known as saprophytes. The fungus causing stem
rust (Puccinia graminis Pers.) is a parasite and obtains its food from green
plants, some of which have already been mentioned. As a matter of fact, all
of the rust fungi are strictly parasitic.

In the course of its development, the stem-rust fungus appears on its hosts
in three different guises, usually referred to as "stages" of the fungus. Two of
these stages, the red (uredial) stage and the black (telial) stage, occur on cereals
and grasses. The red stage is the one present on such plants while they are
green, and on this account is often spoken of as the summer stage. It appears
as red spots, or pustules, chiefly on the stems and leaf sheaths, but sometimes
on the leaves. The pustules owe their color to the spores (Figure 1) produced
in them, which, in mass, are red. The black stage (Figure 2) appears when the
infected plants begin to ripen. This is the stage that is present at harvest time,
and from it the disease sometimes gets the name "black rust". In this stage,
the spores (Figure 3) are dark brown or black. The third stage (Figure 4)
occurs on species of barberry, particularly the common barberry. No other
species has been so widely and frequently associated with the outbreaks of stem
rust. The three stages just mentioned constitute the chief divisions in what is
known as the "life cycle" of the fungus. In it the common barberry plays an
extremely important part.

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Figure 2. — Portions of wheat stems showing
pustules of stem rust in the black (telial)
stage. (Photography by Mr. A. M. Brown)

Figure 3.— Black spores of stem rust. Notice that they are two-
celled and that the cell-wall of each spore is much thickened to-
ward the tip. Magnification, 240 app. (Enlarged somewhat, after

Bailey)

82270—2

10

THE COMMON BARBERRY

The common barberry is a shrub (Figure 5), much used in recent times
for hedges and other ornamental purposes. Formerly it was cultivated for its
berries, from which jellies and wine were made. Under favorable conditions,
it may grow to a height of 10 or 12 feet. The leaves, either green or purplish,
occur in small clusters of three or more. At the base of each cluster there is

Figure 4. — Under side of a barberry leaf heavily infected with
stem rust. Aecia have developed in most of the infections. Some-
what enlarged. (Photographed by Dr. D. L. Bailey)

usually a three-pronged spine, which in reality is a much-reduced leaf. The
margins of the leaves are saw-toothed, each tooth ending in a short sharp bristle
(Figure 6). The small yellow flowers are arranged in loose drooping clusters.
The berries are scarlet, and they remain attached to the plant 'long into the
winter.

This shrub is not a native of North America. Probably its native home is
Central Asia. It seems to have been introduced into southern Europe by the
Arabs in the seventh century, but apparently was not planted to any great
extent in northern Europe until about the seventeenth century. It was brought
to America by the ear^ colonists, and eventually became naturalized in the
New England States. As settlement progressed westward, the shrub became
rather widely distributed through the Middle- Western States. Here, to some
extent, it escaped from cultivation. It does not appear to have been so exten-
sively introduced into other parts of the United States.

Little is known about its early introduction into Canada. Probably in
Eastern Canada some plantings were made fairly early. A bulletin issued in
1888 by the Ontario Agricultural College advised farmers against the planting

11

of barberry. Evidently it had been planted to some extent in rural communities
before that time. Its introduction into the Prairie Provinces was limited, and
probably occurred mostly in the early years of this century. It was probably
introduced into British Columbia about the same time.

The common barberry (Berberis vulgaris L.) should not be confused with
the Japanese barberry (Berberis thunbergii DC), a shrub immune to stem
rust. The Japanese barberry is a low spreading shrub, having smooth-edged
leaves and usually a single spine at the base of the leaf cluster.

Long before the exact relationship between common barberry and stem rust
was discovered (1864-65), farmers had observed that grain in the neighborhood
of this shrub was usually a failure. Between 1726 and 1766, three of the
New England States (Connecticut, Rhode Island, and Massachusetts) enacted
laws for its eradication. From 1800 onward, different countries of Europe
passed similar legislation. Following the severe epidemic of stem rust in 1916,
the common barberry was outlawed in thirteen of the north-central States and
in the Prairie Provinces, and its eradication was undertaken.

■■■■ i

Figure 5. — A common barberry bush, the "alternate host"

of stem rust. On it develops the spring, or cluster-cup stage

of the fungus. (Courtesy of the United States Department

of Agriculture)

LIFE CYCLE OF THE FUNGUS

An account of the life cycle of the stem rust fungus may be conveniently
begun at the red stage of the cycle. Suppose that one of the red spores (uredio-
spores) lodges on a green wheat plant as it grows in the field. So long as the

82270—2*

12

plant remains dry, the spore cannot germinate and, therefore, cannot infect the
plant; but if a film of moisture — either of dew or rain — covers the plant, the
spore germinates (Figure 7) within an hour or two, and sends out a rootlike
process, called a germ tube, which enters the plant through one of its numerous
breathing pores (stomata). Once the tip of the germ tube is well inside the
breathing pore, outside moisture is no longer necessary, for the germ tube is
then able to make use of the water in the plant itself.

The tip of the germ tube soon begins to branch, and within a few days a
system of rootlike fungus threads, called a mycelium, penetrates between the
cells of the plant, just around the spot where the germ tube first entered. These
threads send out suckers (haustoria) that are able to pierce the walls of the plant
cells and feed on the plant food within the cells. After four or five days, a new
development begins. Just beneath the epidermis, or outer covering layer of the
plant, the mycelium begins to produce young spores at the ends of stalklike
cells that point outwards towards the epidermis. As growth of the spores and
mycelium proceeds, the pressure beneath the epidermis becomes too great and
the epidermis ruptures, exposing a mass of red spores. Around the margin of
the pustule thus formed can usually be seen the ragged edge of the ruptured
epidermis. Figure 8 shows a cross section through such a pustule.

^

Figure 6. — A young twig of the common barberry.
Notice the saw-toothed edge of the leaf and the three-
pronged spine at the base of each group of leaves. About
one-third natural size. (Photograph by Mr. A. M. Brown)

In ordinary summer weather, this whole development, from the entry of
the germ tube to the eruption of the pustule, requires about seven or eight
days. The spores readily become detached from the stalk cells on which they

13

are produced, and are easily carried by winds to other plants where, as soon
as weather conditions are favorable, they germinate and cause new infections.
A single spore, after it infects a plant, may thus give rise in the next generation
to several hundred other similar spores. As long as the crops remain green
and weather favorable, this process of infection and multiplication of spores
continues. In due course, however, the t?rops mature and ripen. Further
infection can no longer take place, and the spread of the disease is over for that
particular season.

Figure 7 — Germinating red spores (urediospores) of stem
rust, showing the germ tubes growing out from the spores.
The germ tubes enter the breathing pores of cereal plants
and thereby infect them. Magnification, 300 app. (Photo-
micrograph by Mr. A. M. Brown)

While the plants are ripening, however, a peculiar change takes place in
the pustules. As stated earlier, the pustules cease to produce red spores (uredio-
spores) and in their stead produce black ones (Figure 9). After their formation,
the black spores (teliospores) enter a dormancy period which lasts for upwards of
six months. They pass most of the winter in this state, hence they are often
called winter spores. When moisture and warmth return in the spring they
readily germinate. They are two-celled, have a dark-brown wall much thickened
at the spore tip, and remain firmly attached to the stalk cells on which they are
produced (Figure 10). They are not, therefore, adapted for distribution by the
wind, as are the red spores. In fact, their distribution would be of little or no
value to the fungus, even if green crops were present, for these black spores,
unlike the red ones, are quite incapable of infecting any cereal or grass.

The germination of the black spore is different from that of the red spore.
Each cell of the black spore sends out a rather short stoutish curved tube
which in turn produces four small conical projections (sterigmata), arranged

14

along the outer side of the curve, towards the end of the tube. At the tip of each
of these four projections is- produced a very small colorless spore (Figure 11).
These tiny delicate spores (sporidia) form very rapidly, in 20 to 25 minutes,
and almost immediately afterwards they are shot off into the air and are carried
away by currents of air. Eventually they either sink to the ground or lodge on
plants of one kind or another. They cannot infect cereal plants, but if any of

Figure 8. — Cross section through a wheat stem, showing
pustules of stem rust in the red stage, with the urediospores
borne on their stalk cells. Notice that the mycelium develops
in the thin-walled green tissue between the thick-walled woody
bundles. The ruptured epidermis can be seen thrown back at
the left of two of the pustules. The spores are somewhat
collapsed by the fixing treatment. Magnification, 90 app.
(Photomicrograph by Mr. A. M. Brown)

Figure 9. — Cross section through a wheat stem, showing
pustules of stem rust in the black stage. Notice that the
outer layer of plant cells, the epidermis, has been burst open
and thrown back, and is visible at the margin of the pustules.
The black stage follows the red stage and hence develops in the
same thin-walled tissue between the thick- walled woody
tissue. Magnification, 90 app. (Photomicrograph by Mr. A.
Mr. Brown)

15

them happen to fall on young leaves of barberry (and a film of moisture is
present on the leaves) they germinate. On germination, each spore produces a
sharp peglike tube that punctures the leaf surface. Thus the fungus gains
entrance to the inside tissue of the leaves, and there begins to grow, much in the
same way as it did after the red spore germinated and entered the wheat plant.

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Figure 10. — Section through a pustule of stem rust,
showing the black spores supported on their long stalk-
cells, from which the spores are not easily dislodged.
Magnification, 180 app. (Photomicrograph by
Mr. A. M. Brown)

Figure 11 —A drawing to represent the germination of a
black spore. The top cell has produced a promycelium on
which have been developed four sporidia, borne at the tips
of the sterigmata. One spore has been shot off into the air
and one is only partially developed. Usually the sporidium
nearest the spore matures last .

16

Within five or six days after infection has occurred, small circular spots
appear on the leaves, and in the next week or two these spots, or pustules,
enlarge and may reach a diameter of from an eighth to a quarter of an inch,
sometimes more. The mycelium in the leaf tissue of each pustule gives rise, on
the upper side of the leaf, to a number of vase-shaped structures called pycnia
(Figure 12). Usually some pycnia occur on the underside also. In the pycnia
are produced extremely small spores (pycniospores) and a honeylike fluid,
called nectar. The function of the pycniospores is described in detail on page 25.

Figure 12. — Part of a cross section through a stem-rust infection on a barberry leaf,
showing a pycnium opening to the upper side of the leaf. Notice the brush of hyphae,
or threads, projecting through the mouth of the pycnium. A mass of the minute
pycniospores is seen at the center of the cavity, and, around the inside wall, can be
seen the short parallel hyphae, at the tips of which the pycniospores are produced.
These spores flow out through the mouth of the pycnium, carried by the nectar
produced in the pycnium. Magnification, 350 app.

Projecting through the opening at the top of each pycnium is a brush of fungus
threads. These threads are of two types. In one type, they are awl-shaped
and stiff; in the other they resemble, and are probably the same as, the fungus
threads within the leaf tissue. Those of the second type are longer and less
rigid than those of the first type, and occasionally are branched. The nectar
and the pycniospores ooze out through the opening in the top of each of the
pycnia and collect on the upper surface of the pustule.

On the underside of the pustules, structures of a different kind are formed.
These structures are generally referred to as cluster cups (Figure 13). They
are tubular in shape and are packed full of spores arranged in parallel chains
(Figure 14). The cups are called aecia and the spores, aeciospores. The aecia
open at the exposed ends, and the aeciospores are shot into the air and are
carried away by wTind. However, the aeciospores cannot infect barberry, but
if they happen to reach green plants of some susceptible cereal or grass (and
moisture conditions are favorable), they infect them. Infections by these
spores give rise to the red, or summer, stage of the fungus, thus completing
the life cycle. The relation of the pycnia to the aecia is shown in Figure 15.

17

In regard to the production of aecia and aeciospores by pustules on barberry, it
may be mentioned that they arise as a result of a sexual process. This aspect
of the life cycle can, however, be more conveniently discussed in a later section
which deals with hybridization in stem rust.

Briefly stated then, the life cycle (Figure 16) of the stem-rust fungus has
three principal stages: (1) the red, or summer, stage in which the disease spreads
from one cereal plant to another; (2) the black, or winter, stage in which the
fungus passes the winter in a dormant condition; and (3) the cluster-cup, or
spring, stage which occurs on barberry and which serves as a link between the
black stage of one year and the red stage of the following year.

Figure 13. — Cluster cups (aecia) on the underside of a
pustule of stem rust on a barberry leaf. Magnification,
8 app. (After Bailey)

PHYSIOLOGICAL SPECIALIZATION

Varieties of Stem Rust

There are several different varieties of stem rust. One variety attacks
wheat and barley, but does not attack oats. Another attacks oats, but not
wheat or barley. A third occurs commonly on rye. It can infect barley, but
not wheat or oats. For convenience these three varieties are generally referred
to, respectively, as wheat stem rust, oat stem rust, and rye stem rust. Each
of them attacks certain grasses. For example, wheat stem rust is very common
on wild barley, and rye stem rust on couch, or quack, grass. Then again
there are other varieties of stem rust that do not attack cereals, but occur
on particular kinds of grasses. One of these attacks timothy, another attacks
species of Agrostis, and still another, species of Poo.

82270—3

18

Figure 14.— A section cut lengthwise through a cluster cup
(aecium) of a stein-rust pustule on a barberry leaf. Observe
that the aecium is only partly open at the exposed end and
that the aeciospores are produced in parallel rows, the
youngest ones being farthest from the exposed end of the
aecium. (The spores have been considerably disturbed
and the rows broken by the sectioning process.) Magnifi-
cation, 120 app. (Photomicrograph by Mr. A. M. Brown)

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Figure 15. — A cross section through a pustule of stem rust
on a barberry leaf, showing a pycnium on the upper side
and aecia on the lower side. Fungus threads, or hyphae.
penetrate between the cells of the leaf in all directions, and
connect the pycnium with the aecia. Many pycnia besides
the one shown were present in the pustule. Magnification,
90 app. (Photomicrograph by Mr. A. M. Brown)

19 .

All the varieties look much alike, though the urediospores of the varieties
that attack the above-mentioned grasses are somewhat smaller than those of the
varieties that attack cereals. But although alike in appearance they are actually
quite different because they do not usually attack the same cereal or grass hosts.
The difference between them lies in their physiological, or constitutional, make-
up. Hence they are called physiological varieties, and are said to be physio-
logically specialized to their respective hosts.

Figure 16.— A schematic representation of the life cycle of the stem-rust
fungus. During the summer, the red spores infect green crops. When the
crops begin to ripen, black spores are produced. They survive the winter
and germinate in the spring. On germination, they produce sporidia, which
infect barberry. The infections give rise to the cluster-cup spores, which, in
turn, infect cereals and grasses. (Drawn by Mr. W. E. Clark)

Races of Stem Rust

Specialization occurs also in the varieties themselves. The varieties are,
therefore, said to be composed of physiologic forms or, as they are presently
known, physiologic races. Over 230 races have been distinguished in wheat
stem rust, although only 14 have been differentiated in oat stem rust and 14 in
rye stem rust. In each of these three varieties of stem rust, the physiologic
races are distinguished by the types of infection which each race produces on
seedlings of a given set of cereal varieties. For identifying or distinguishing
races of wheat stem rust, twelve different standard varieties of wheat are used;
for races of oat stem rust, five varieties of oats; and for races of rye stem rust,
five varieties of rye. The differential varieties are sometimes called differential
hosts.

The types of infection are shown in Figure 17. They vary from whitish
flecks in which no spores are produced to large pustules in which there is an
abundance of spores. Each type is represented by a symbol. The 0-type
represents immunity — the complete absence of any evidence of infection, the
4-type represents very high susceptibility. Actually in the figure, the 0-type
is not shown, but in its place a type showing flecks (0;) is substituted. The

82270—31

20

flecks indicate that infections occurred and killed groups of cells at a number
of points. As the fungus cannot feed on dead tissue, it was unable to develop
further and hence failed to produce any spores. The intermediate types, 1, 2,
and 3, represent increasing susceptibility between the 0-type and the 4-type.
The X-type represents a mixture of small and large pustules, for example, of
the 1-type and the 4-type. It occurs rather commonly on durum wheat varieties
when inoculated with certain races of wheat stem rust.

0;

X

Figure 17. — Types of infection and the symbols by which they are repre-
sented. The type on the left (0;) indicates very high resistance. The
next four types (1 , 2, 3, and 4) indicate increasing susceptibility. The type
on the right (X) indicates an indeterminate reaction, but usually more
resistance than a 4-type. About natural size. (After Newton and Johnson)

To illustrate how one race differs from another race, there are given in
Table 1 the infection types produced on the differential wheat varieties by
several of the physiologic races of wheat stem rust that have occurred more or
less commonly in Canada during recent years. Variations from the infection
types shown in Figure 17 are present in this table. By way of explanation, it
may be said that the addition to a given infection type of a plus ( + ) or of a
minus ( — ) denotes, respectively, a slightly greater or slightly less susceptibility
than that represented by the infection type shown in Figure 17. A double plus
( + +) or a double minus ( ) denotes a slightly greater departure than the

TABLE 1. — The infection types produced on the twelve differential varieties by several races of wheat

stem rust commonly occurring in Canada

Physiologic; race

3

o
h3

OS

"5

3

3

T5

3
3

3

C

3
3

s
<

n

3

-a

3

%

a
o.

X

03
M
a
a

3

g

<

3
O

a

m

3
>

a.

3

M

Race 15

4

4
4

4+

4

4

4

4

4-
2-

4

4-

4

2 =
i 4-

3 +

4 =

0;

0

4-

4-

4-

0

3+

3+ +
3-
3+ +
4 =
3+ +
3-
4 =
3+

4 =
4 =
4-

4
1 =

x+
1 =
1 =

4 =
4 =
4-
4 =
1 =
x +
1-
1 =

4 =

4 =
4-
4 =
0;

x+
0;

1 =

3+ +
3+ +
4 =

4 +

X

x+
x =

3 +

3 + +
3+ +
3+ +
3+ +
3+ +
x+ +
3+
3+

3 + +
3

1 =
1 =
3+
4-
1-
1 =

4 +
0;
0;
0;
0;
1 =
0;
1 =

" 19

" 21

" 34

" 36

1 +

" 38

" 49

" 56

1+

21

single plus or the single minus. A combination of plus and minus ( ± ) denotes
that the susceptibility varies slightly, being sometimes greater, sometimes less,
than that denoted by the type itself.

From the few examples given in Table 1, it will be seen that at ^ J least
on one of the differential varieties and generally on two or more of them,
the infection types produced by any pair of races are distinctly different.
For example, Race 19 can readily be distinguished from Race 21 by the types
of infection that they produce on Marquis and Einkorn. Race 49 and Race 56
differ very markedly on Kanred. Similarly, a comparison of any other two
races on these twelve differential varieties will show differences in infection
type to a greater or less degree.

This fact is made evident by an examination of Figure 18. In this figure,
seedling leaves of five of the twelve differential wheat varieties are shown.
The upper set of leaves was inoculated with Race 21 and the lower set, with
Race 36. It will be seen that the same wheat variety may be very resistant
to one race, as Kanred to Race 21, and very susceptible to another race, as
Kanred to Race 36. Of course, the difference between one race and another
is not always so clearly marked as in this particular case; but, in general,
the distinction is pronounced enough to make the separation of one race from
another a matter of no doubt. The same is true for races of oat stem rust.
Owing to the fact that rye is a cross-fertilized cereal, the varieties in it are
usually more or less impure, and on this account the distinction between one
race and another of rye stem rust is not generally so clear-cut as in wheat stem
rust or oat stem rust.

The existence of physiologic races within the varieties of stem rust largely
explains why a variety of wheat may appear resistant to stem rust in some years
and be severely attacked by it in other years. If, in any season, only stem-rust
races to which the variety is resistant are present in the area where the variety is
growing, the variety will be more or less free from rust. If, in another season,
races of stem rust to which the variety is susceptible are very prevalent, the
variety will become badly rusted. It is clear, therefore, that before a variety,
say, of wheat can be declared highly rust-resistant, it should be tested against all
races of wheat stem rust, or at least against enough of them to ensure that there
is little possibility that any race will do it injury. Similarly, an oat variety
should be thoroughly tested against all races of oat stem rust.

For this reason, it is an important phase of rust research to discover what
physiologic races of stem rust are present in a country, particularly if an attempt
is being made in that country to develop rust-resistant varieties of cereals.
This phase of the work must be continued over a period of years, for it is possible,
and indeed it happens, that a race not present in one year, or even for several
years, may be very prevalent in other years. Occasionally, too, new races are
discovered. In order to have races with which to test cereal varieties, it is
necessary to keep on hand the races found in the field, or, at any rate, representa-
tive races, so that they may be available when needed for use in such tests.

A complicating factor in the identification of physiologic races is the exist-
ence of biotypes within certain races. A biotype is a strain of a race; it is
distinct from the race in disease-producing ability but cannot be distinguished
from the race by the infection types it produces on the regular differential
varieties. To distinguish it, another differential variety has to be used. Thus
Race 15B cannot be differentiated from Race 15 by the regular differential
varieties, but, as Race 15B heavily attacks the variety Lee and Race 15 does not,
the two can be distinguished by using Lee as the additional differential variety.

22

In consequence of the existence of strains within races, the rust investigator has
to be on guard against not only new races but also potentially dangerous strains
in races already known.

r

'

RACE 21

MA. KRD.

SPM. ENK

RACE 36

MA. KRD. MND. SPM. ENK.

Figure 18. — Infection types produced by two different physiologic races of stem rust on seedling
leaves of the same wheat varieties. Notice the contrasting types of infection produced by the
-two races. (Ma. = Marquis,' Krd. = Kanred, Mnd. = Mindum, Spm =Spelmar, and Enk.=

Einkorn.) Somewhat enlarged.

23

HYBRIDIZATION IN STEM RUST

In the account already given of the life cycle of the stem-rust fungus, it
was said that when a black spore germinates (Figure 11) each of the two cells
of the spore sends out — although not always simultaneously — a stout tubelike
process (promycelium) that gives rise to four tiny colorless spores (sporidia)
and that the sporidia are able to infect young leaves of barberry. Throughout
most of its life cycle the stem-rust fungus has two nuclei (conjugate nuclei) in
each cell. For instance, each aeciospore, each red spore (Figure 19), and each
cell of the fungus mycelium in the wheat, oat, or rye plant has two nuclei. Be-
fore germination, each of the two cells of every black spore also contains two
nuclei. When, however, one of these cells begins to germinate, the two nuclei
in it fuse together to form one nucleus. This fusion nucleus now divides into
two daughter nuclei, and then each daughter divides, so that finally there are
four granddaughter nuclei. These pass from the promycelium into the sporidia,
one entering each sporidium.

Figure 19. — A section through a young unruptured pustule

of stem rust on a wheat stem. Notice the two conjugate

nuclei in the immature spores. Magnification, 700.

(After Newton and Johnson)

s

In this process of nuclear fusion and division, there is opportunity for
segregation and recombination of character-bearing factors, such as sex factors,
to take place. (Other factors also may be reassorted during the process, but
the sex factors are the ones of immediate concern here). It has been found
that two of the four sporidia are of one sex and two of the opposite sex. As the
four sporidia are exactly alike in appearance, the terms "male" and "female"
are inappropriate. To indicate, however, that the sporidia differ in their sexual
behavior, two of them are said to be of plus ( + ) sex and the other two, of minus
( — ) sex.

Now, if one of these sporidia, say, of ( + ) sex, infects a barberry leaf, it
produces a pustule (Figure 20) of the same sex as itself. The vase-shaped
structures (pycnia) develop and produce pycniospores and nectar, but no aecia,
and consequently no aeciospores are formed. In other words, the pustule

24

does not fulfil its proper function, namely, to produce aecia and aeciospores.
Similarly, if a sporidium of ( — ) sex infects the leaf, no aecia or aeciospores
are produced.

Figure 20. — Underside of a barberry leaf, showing
two pustules of stem rust. Each pustule arose
from a single sporidium, and is of the same sex as
the sporidium from which it originated. Neither
has produced aecia. Slightly enlarged. (Photo-
graph by Mr. A. M. Brown)

Figure 21. — A compound pustule of stem rust on a
barberry leaf. An infection by a (+) sporidium
has fused with an infection by a ( — ) sporidium, and,
as a result, aecia have been produced in the compound
pustule thus formed. Magnification, 4 app.

25

Compound Pustules

If, however, a ( + ) sporidium and a ( — ) sporidium infect the barberry
leaf so that the two infections are near to each other (a quarter of an inch or
less apart), each infection develops into a pustule having pycnia, pycniospores,
and nectar. In the course of some days, the two pustules meet and fuse
together to form a compound pustule. The mycelium in one portion of the
compound pustule thus formed is of ( + ) sex, and the mycelium in the other
portion, of ( — ) sex. When the fusion occurs, the mycelium of one portion
comes into contact with that of the other portion, and, being of different sex,
they interact. The interactions consist essentially in an exchange of nuclei, so
that the ( + ) mycelium receives ( — ) nuclei, and the ( — ) mycelium ( + ) nuclei.
The binuclear (conjugate) condition is thus re-established, and the mycelium
then proceeds to develop aecia and aeciospores (Figure 21).

There is just as good a chance, of course, that two sporidia of the same
sex may cause two infections near together on a barberry leaf as there is of
two sporidia of opposite sex doing so. If the two sporidia are of the same sex,
both ( + ) or both ( — ), the two pustules arising from the infections may meet
and fuse, but no aecia develop. In other words, such a compound pustule
behaves like a pustule that originates from a single sporidium.

The chances of compound pustules being formed depend very largely on
how numerous the infections are. If they are many, there is a better chance of
compound pustules being formed than if they are few and scattered. In any
case, only about half the compound pustules develop aecia, for only about half
of them have one portion of (+) sex and the other portion of ( — ) sex. In fact,
if the fungus did not have another method by which it could induce aecia to
develop, it would probably not be so successful in continuing its existence.

Action of Pycniospores

In this second method, it is not necessary that pustules of opposite sex
develop close together: contact between such pustules is established in a totally
different way. Owing to the fact that the nectar produced by the pycnia is
sweetish and that the upper surface of the pustules is of a rather bright orange color,
flies and other insects are attracted to the pustules. As they go from pustule to
pustule, sipping the nectar, they carry pycniospores from one pustule to another.
As a result of this intermixing, pustules of (+) sex receive pycniospores of ( — )
sex, and pustules of ( — ) sex receive pycniospores of ( + ) sex. The fungus
threads (hyphae), referred to earlier, that protrude through the openings of the
pycnia (Figure 22) come into contact with the pycniospores of opposite sex,
brought by the insects, and an interaction takes place between the pycniospores
and the threads or hyphae. Nuclei of the pycniospores pass over into the
hyphae and thus re-establish the binuclear (conjugate) condition in the mycelia
of the pustules. Something akin happens as takes place when a flower is ferti-
lized by pollen.

When insects are plentiful and active, as they usually are, this is a more
effective method than the other one for re-establishing the binuclear condition,
for by it most of the pustules develop aecia and aeciospores. This method of
inducing single pustules to form aecia has been found very convenient in experi-
mental work. The nectar can be transferred from pustule to pustule by hand, with
the aid of a toothpick, knife blade, or other implement, just as effectively as insects
can transfer it. Figure 23 shows how effectively the transfer can be done by
hand. The nectar of all the infections on the right of the midrib was intermixed,
receiving composite nectar collected from other infections, and practically all
of the infections developed aecia; the nectar of the infections on the left of the
midrib was not intermixed, and none of the infections developed aecia.

26

Figure 22. — A thick cross-section through a stem-rust
pustule on a barberry leaf, showing a side view of a
pycnium. Notice the two types of hyphae emerging
through the opening at the top of the pycnium. Magni-
fication, 400 app.

Figure 23. — The under side of a barberry leaf showing
a number of infections without aecia on the left of the
midrib, and a number with aecia on the right of the
midrib. Composite nectar (collected from many other
infections) was applied to the upper surfaces of the infec-
tions on the right. The infections on the left received no
nectar from any other infection. Through the action of
the pycniospores in the composite nectar, the pustules
on the right were induced to develop aecia. Magnification,

2 app.

27

Crossing of Races and Varieties

Because the stem-rust fungus passes through this sexual phase of its life
cycle on barberry, the opportunity therefore exists in this phase for crossing to
take place between physiologic races of the rust. Suppose a barberry bush is
growing in the vicinity of a wheat field in which, say, Race 21 and Race 36 of
wheat stem rust are present on the crop. These races, in the black-spore stage,
will survive the winter on straw and stubble. In the spring the black spores
will germinate, and sporidia of both races will infect the barberry leaves. Some
pustules of Race 21 will likely arise near pustules of Race 36. Wherever a ( + )
pustule of Race 21 and a ( — ) pustule of Race 36, or a ( — ) pustule of Race 21
and a ( + ) pustule of Race 36, are near enough together to fuse, the compound
pustule so formed will give rise to hybrid aeciospores.

Many of the pustules, of course, will be too far separated for fusions to
occur, but flies and other insects visiting such pustules will intermix the pycnio-
spores, so that pycniospores from pustules of one race will be transferred to
pustules of the other race, and as a result hybridization between the two races
will occur, and hybrid aeciospores will be produced in these pustules also. There
is good evidence that crossing between different physiologic races of stem rust
actually takes place.

Under greenhouse conditions, numerous crosses have been made between
races of wheat stem rust by transferring by hand the pycniospores of pustules
of one race to pustules of another race. From such crosses, upwards of 50
different physiologic races of wheat stem rust have been produced. Most of
these races have never been found on crops in the open field. None of these
new races, however, attacks wheat more severely than do some of those races
that occur commonly outside in nature. This is not to say, however, that a
race worse than any now known may not arise through hybridization.

Crosses have also been made between different varieties of stem rust, such
as between wheat stem rust and oat stem rust, or between wheat stem rust and
rye stem rust, and true hybrid races have been procured. There is, however,
considerable intersterility between such varieties of stem rust, while within any
variety the races appear to be completely interfertile, so that hybrids between
any two races, say, of wheat stem rust can readily be obtained.

SOURCES OF INFECTION

Of the five spore types of the stem-rust fungus, only two, the cluster-cup
spores (aeciospores) and the red spores (urediospores), can infect cereals and
grasses. If, therefore, an outbreak of stem rust occurs in any particular region
or area, it must first have originated either from infected barberry or from the
red stage that has survived from the previous year. Within some particular
region, of course, it may happen that neither source is present, the outbreak
being started by wind-borne spores that originated elsewhere.

Infected Barberry

For barberry to be a source of infection, stubble or straw of grain or grasses
bearing black spores must be present in the vicinity of barberry so that, when
the black spores germinate, the sporidia formed can reach it and infect it before
they dry up and die. Weather conditions that are such as to permit free ger-
mination of the black spores are usually favorable for abundant infection of
barberry.

In order, however, that the disease may spread from barberry, it is not
necessary that grain crops be growing in the immediate neighborhood of an
infected bush, although the nearer they are, the greater is the chance for heavy

28

infection to occur. Aeciospores, although perhaps not so resistant as uredio-
spores to heat and dryness, may be carried considerable distances and still be
able to cause infections. Moreover, grasses growing in the vicinity of an infected
bush may become heavily infected, and urediospores produced on them may
then spread to cereal crops.

Survival of Red Stage

When the source of infection is the red stage that has survived from the
previous year, the early infections are caused by urediospores, instead of by
aeciospores. The period between two crop seasons corresponds, in cooler climates,
to the winter months, and, in warmer climates (where winter crops are grown),
to the summer months. Generally speaking, this period is considerably longer
than the period that urediospores can remain alive under ordinary outside
conditions. Usually they either germinate when there are no green plants present
to infect, or they perish without germinating. In both cases, the end result
is the same.

It is true that, under certain constant conditions of temperature and air
humidity (about 40° F. and 50 per cent relative humidity), urediospores may
remain living for upwards of nine months or even for a year, but such conditions
rarely occur in nature. The possibility, however, of their living long enough to
infect the next year's crop cannot be altogether excluded; but wherever the red
stage is known definitely to survive from one crop to the following one, it does so
apparently on susceptible grains or grasses that remain green during at least
most of the intervening period. The fungus continues to grow and feed, although
perhaps at a much reduced rate, during the whole or the greater part of the
period between crop seasons.

Whether infection in an area arises from the one or the other source, or
from both sources, depends very largely on the climate of the area concerned,
and, of course, on whether or not barberry is present. If barberry is not present,
it cannot be a source of infection. Generally speaking, the red stage persists
from one season to the next only in areas or countries having a mild climate.
For instance, in Europe, the chief source of infection is the barberry, for the red
stage of stem rust seems to be quite generally killed out during the winters,
although in Southern Europe it appears that this stage may overwinter to some
extent in very mild winters. The opposite situation exists in Australia. There
the source of infection is the red stage. This stage, owing to the mildness of the
climate, survives from one crop season to another on self-sown, or volunteer
grains. Barberry has been introduced only to a limited extent, and it plays
little or no part as a source of infection.

Sources in North America

In North America, climatic conditions vary from cool temperate in the
northern agricultural zone to subtropical or even tropical in the southern zone.
Therefore on this continent both sources of stem rust are present: infected
barberry in the cooler parts and the overwintered red stage in the warmer
parts. Barberry rusts quite generally in Canada and in the northern United
States. A native species rusts farther south in some of the mountain districts
in the eastern part of the United States. Native species occur in the southern
Great Plains region and in the Rocky Mountains but they rarely become infected.

There is one important cereal-growing region in which barberry is present
but seldom becomes infected with stem rust. This is the southern half of the
Mississippi Valley. In this region cereal crops ripen in early summer, and the
black spores, formed at that time, are therefore exposed to the intense mid-
summer heat, which kills ^them. Consequently, they do not germinate in the
following spring, and so barberry very generally escapes infection.

29

Of the stem rusts that attack cereals, the only red stage known definitely
to overwinter in Canada or the northern United States is the variety on rye.
It is able to survive the winter on heavily infected couch grass as far north as
Winnipeg. The variety of the rust that attacks timothy can survive the winter
in the northern United States and adjacent parts of Canada, and the varieties
that attack red top and blue grass may be capable of similar survival. The
overwintering of wheat stem rust and oat stem rust seems to be confined to
Mexico, the American states bordering on the Gulf of Mexico, and the Pacific
coastal region, particularly southern California.

INFLUENCE OF WEATHER

Stem rust cannot increase rapidly or spread widely unless climatic or
weather conditions are favorable for its development. Generally speaking,
weather conditions that are favorable for rapid growth of crops are also favorable
for the development of the disease. Certain other factors besides weather
conditions influence the rapidity with which the disease develops and spreads,
but these other factors would be largely inoperative in the absence of weather
conditions that favor the development and spread of the disease. A combination
of favoring conditions would allow the disease to make greatest progress. The
principal climatic factors influencing the development of stem rust are moisture,
temperature, and wind or air currents.

Moisture

A film of moisture must be present on the plants before the spores will
germinate, and it must remain long enough to allow the germ tubes to enter the
breathing pores of the plants. A few infections may occur within four or five
hours, but for abundant infection the plants must remain damp for upwards of
24 hours. If the plants become dry before the germ tubes enter them, the germ
tubes wither and die. This is one of the reasons why thin crops, which dry
quickly after a rain or dew, are usually less heavily rusted than thick rank crops,
which dry much less rapidly. However, once the fungus gains entrance into
the plants, it is no longer dependent on outside moisture. It is then able to
make use of the moisture in the plants themselves.

Temperature

Certain limitations on the development of the disease are imposed by
temperature. Little infection can occur below 50° F. or above 90° F., and
growth of the fungus inside the plants ceases at about these limits. Infection
proceeds most freely and growth most rapidly between 65° and 75° F. There
is a marked slowing down of growth when the temperature falls below 60° or
rises above 85° F.

Wind

Stem rust cannot spread unless spores of the fungus are distributed, and
wind is the greatest distributing agent. Usually there is sufficient wind during
the growing season to carry the spores from plant to plant, from field to field
and for long distances. Spores have been thus carried for upwards of a thousand
miles. Strong winds passing over a large rust-infected area pick up and carry
along enormous numbers of spores, thus causing at rather distant points the
so-called "spore showers". Such showers, in Western Canada, occur after a day
or two of strong south wind.

Severe outbreaks of stem rust usually occur in seasons when moisture
is plentiful and the average temperature is above 62° or 63° F. Damp misty
days alternating with bright warm ones, or dewy nights followed by warm

30

sunny days favor infection and growth of the fungus in the plant tissues. Con-
tinued cool weather retards the development of the disease; but if during a
period of cool weather rains or dews occur, many infections may take place, and
if the weather then turns warm, the heat hastens the development of the fungus
in the plants, and within a few days there may be a sudden increase in the amount
of rust. Dry cool weather gives a susceptible crop the best protection against
stem rust. Dry hot weather has a similar influence, but it tends to induce
premature ripening and hence prevents proper filling of the grain. During
the summer, dry hot weather occurs more commonly than dry cool weather.
Not infrequently in the Prairie Provinces, the grain crop has been saved from
extreme stem-rust damage by a spell of very hot dry weather that checked the
disease and hastened the maturing of the crop.

RELATION OF SPORE ABUNDANCE TO EPIDEMICS

The rapidity with which an epidemic of stem rust can sweep over a whole
countryside for hundreds of miles, and even for greater distances, has always
been, and is yet, an amazing phenomenon. It is little wonder that in earlier
times, and not so far back even in modern times, a severe outbreak of stem
rust was regarded as a visitation of Divine displeasure. How else, it was
imagined, could destruction be so swift and sure? Like so many other mysteries,
this one, too, has been solved by careful and prolonged investigation.

An epidemic of the disease is the result of countless numbers of infections.
In order, therefore, that an epidemic may develop, an enormous number of
spores must be produced. Each pustule of stem rust arises from an infection
by at least one spore. Under epidemic conditions, plants may be almost com-
pletely covered with pustules. The fact that millions and millions of acres may
be, and often are, thus affected, indicates how very great must be the number of
spores necessary to produce such a condition.

Not all the spores, by any means, succeed in causing infections. As they
are distributed by wind, great numbers of them fall to the ground, or lodge on
nonsusceptible crops, or are blown away to forest, lake, or ocean. The wastage
of spores is, therefore, very great, and hence the ones that chance to lodge on
susceptible crops make up only a part, perhaps only a small part, of the total
number produced.

Factors Influencing Spore Abundance

The production of such a vast number of spores requires time. The length
of time, of course, depends on the rapidity with which spore multiplication goes
on. If the rate of multiplication is too slow, no epidemic can develop. Weather
conditions, as already indicated, have a pronounced influence on the rate of
multiplication; but, in addition to the weather, certain other factors play a part.

One of these factors is the number of spores that spreads from the source.
This number, in turn, depends on the number of barberry bushes present and the
extent to which they are infected or on the amount of overwintering that has
occurred. Another factor is the earliness with which spores spread from the
source. In ordinary summer weather, an infection by an aeciospore or a uredio-
spore takes approximately one week to produce a new generation of spores.
It is obvious that, other things being equal, the earlier that spores begin to
spread from the source, the greater is the number of spore generations that can
be produced in a season. Then, again, as it is a matter of chance where the
spores fall, it can readily be seen that the more extensively susceptible cereal
crops are grown the better are the chances of the spores lodging on plants that
they can infect.

31

CHARACTERISTICS OF SPREAD

Within Region of Source

The first stage in the spread of the disease occurs in the region where the
rust has its source. This region may be large or small. In Australia, for
example, it is large, for overwintering — actually "oversummering" — occurs to a
greater or less extent throughout the cereal-growing area. On the other hand,
in India, it is small in comparison with the area subsequently affected, for it
seems to be confined to restricted areas in the foothills of the Himalaya
Mountains, from which locality the disease spreads to the plains. In North
America the area where barberry becomes rusted is large, while that where
overwintering occurs is relatively small.

When spores begin to spread from an infected barberry or from a point
where the rust overwintered, more spores lodge in the immediate vicinity of
the barberry or the overwintering spot than at points farther distant. There
is a tendency, therefore, for infections to be most numerous around such centers
of infection and to diminish with increasing distance from them. For this
reason, the early stages of stem-rust spread are characterized by the occurrence
of comparatively small, more or less isolated rusted areas, the number of which
depends on the number of locations with infected barberry or the number of
points where the rust overwintered. At first, only a few infections may be
present in each center, but, as aeciospores or urediospores continue to be liberated,
and as each new infection gives rise to a new generation of spores, the severity of
the infection in each of the centers increases and the area involved becomes
larger. In the course of a few weeks, these localized outbreaks may produce
sufficient spores to cause a general infection of the whole region, and eventually
perhaps give rise to epidemic conditions.

Beyond Region of Source

While stem rust is increasing in the barberry or overwintering region, the
crops in that region are steadily progressing towards maturity, and when they
ripen no further increase of spores can take place. In the meantime, however,
spores are blown away by the wind in different directions. Some of these spores
may be carried to a neighboring area where no source of infection is present.
At first only relatively few spores may arrive, but as the disease makes headway
in the region of the source, the number of spores arriving in the neighboring
area increases. Distinct spore showers may occur when the wind blows from
the direction of the former region. If green susceptible crops are present and
weather conditions are favorable for stem-rust development, infections occur,
at first only a few, but later in greater numbers. In contrast, however, to the
early stages of spread in the region of the source, the infections are usually
more or less uniformly distributed, as would be expected of infections arising
from spores carried relatively far by winds.

In the course of a few weeks, stem rust runs its course in this second area,
but during that time spores produced on crops growing in it are blown away
to other areas. If in any of these areas there are green susceptible crops, the
disease may become established in them, and with favorable weather may
cause serious loss. Therefore, as long as there are green susceptible crops to
infect and weather conditions favorable for infection, an outbreak of stem
rust may advance a very long way from its source.

As crops ripen successively later from the equator northward or southward,
it would be expected that the usual direction of spread in the northern hemisphere
would be from south to north, and, in the southern hemisphere, from north to
south. This is, as a matter of fact, the general direction of spread. For instance

32

in Eastern Asia, stem rust spreads northward from Manchuria into the Amur
region. In some years at least, stem rust seems to spread from Asia Minor to
the Balkan Peninsula and northward through Central Europe. Some of the
infection comes from infected barberry, so that the northward spread is not
always very clearly defined. In India, however, the direction of spread is
reversed, for stem rust spreads from the foothills (of higher elevation) in the
north to the plains (of lower elevation) lying to the south. In the southern
hemisphere, stem rust spreads from southern Brazil, wrhere it overwinters
southward to Uruguay and Argentina. In Australia, the disease first becomes
prevalent in the more northerly cereal-growing areas, and spores originating
there are blown southward and intensify the attack in the more southerly areas
where, in any case, some infection from the overwintered red stage would occur.
Southward-blown spores appear responsible for outbreaks of stem rust in the
Cape province in the Union of South Africa.

STEM RUST SPREAD IN NORTH AMERICA

To illustrate some of the observations made above, consideration may be
given to the spread of stem rust in North America. As cereal production is
carried on much more extensively in the Mississippi Valley in the United States
and in the Prairie Provinces of Canada than elsewhere on this continent, and,
as these two areas have suffered greater damage from the disease than have
any other areas, attention may first be given to them.

Mississippi Valley and Prairie Provinces

As far as the spread of stem rust is concerned, these two areas must be
regarded as one. They embrace an almost continuous cereal-growing belt,
extending from the Gulf of Mexico to the Peace River Valley in northern Alberta.
From south to north, crops mature successively later, so that, when those in the
extreme south are ready to harvest, those in the extreme north are being seeded
or are just beginning to show above ground. In these two respects, therefore,
this extensive cereal-producing belt is well adapted to the long-distance spread
of the disease. Furthermore, since an overwintering region for stem rust
comprises the southernmost portion of this belt, namely, central and southern
Texas, with suitable weather conditions during the course of the spring and
summer, an outbreak of the disease originating in the overwintering area might
spread throughout the whole length of the Mississippi Valley and on into Canada
as far north as cereal crops are grown.

Only in some years, however, is there very clear evidence of such a northward
spread from the overwintering region of the rust in the south. As a matter
of fact, the southern half of the Mississippi Valley has been less subject than
the northern half to outbreaks of stem rust. One of the principal reasons for
this is that, in the northern half, the disease spreads from infected barberry.
Outbreaks originating from this source, particularly those in the Upper Mississippi
Valley, spread on northward into the Prairie Provinces. Undoubtedly the
eradication of barberry, which has been in progress in the northern half of the
Mississippi Valley for 35 years or more, has reduced very appreciably the impor-
tance of barberry as a source of infection in this region. By reducing the number
of barberry bushes, the number of centers from which infection can spread is
reduced, and as a result there is less opportunity of a general infection developing,
with the consequent possibility of its attaining epidemic proportions.

The northern half of the Mississippi Valley is exposed to invasions of stem
rust from the south, and also to attacks arising locally from barberry. In
some years, therefore, infections arise both from northward-blown spores and
from locally-produced spores. If stem rust becomes prevalent and severe in

33

this area, more especially in the Upper Mississippi Valley, the chances of the
Prairie Provinces escaping a serious attack are very slight indeed. As a matter
of fact, when stem rust is heavy in the Upper Mississippi Valley, it is usually
heavy in the Prairie Provinces, at least in Manitoba and eastern Saskatchewan,
and, when stem rust is light in the former region, it is usually light in the latter.
Whether an outbreak of stem rust in the Prairie Provinces originates in the
extreme south (in the overwintering region) or in the northern part of the Missis-
sippi Valley, it always comes as a northward advance of the disease from the
Upper Mississippi Valley.

In the Prairie Provinces, infections almost invariably appear first in
Manitoba, then in Saskatchewan, and last in Alberta, usually a month or more
later than in Manitoba. In each province, wind-borne spores are usually present
one or two weeks, sometimes longer, before the first infections appear. Spores are
always more numerous in Manitoba than in Saskatchewan, and in Saskatchewan
than in Alberta. Western Saskatchewan and certainly Alberta seem to lie to
the west of the main northward drift of spores, whereas Manitoba, especially in
Red River Valley, and eastern Saskatchewan to a lesser extent are apparently
directly in it. Hence, outbreaks in Manitoba particularly, and in eastern
Saskatchewan, occur much more often and in much greater severity than in
western Saskatchewan and Alberta. Spread toward the west, of course, takes
place but it is usually more gradual than to the north, so that by the time the
disease gets fairly well established in Alberta, the bulk of the crop in that province
is mature. To a lesser extent, the same is true of western Saskatchewan.

There is one aspect of the spread of stem rust in the Mississippi Valley and
the Prairie Provinces that has not yet been referred to and about which there is
still some uncertainty. As already mentioned, cereal crops in Texas ripen in
late spring. The summers there are intensely hot, and conditions in the cereal-
producing area are such that stem rust would have great difficulty in persisting
through the summer. Cereals, except perhaps occasional volunteer plants in
protected situations, are absent, and grasses are largely withered by the heat.
The red stage, therefore, rarely persists through the summer. The black spores
are killed by the heat, and barberry thus escapes infection. Notwithstanding
these conditions, infections appear in autumn and early winter on fall-sown
grain.

There is little chance that the spores responsible for these infections come
from northern Mexico, unless the red stage of the rust can oversummer on some
of the higher plateaux. Though there is evidently some survival of rust during
the summer in Texas, Oklahoma, and other southern states, the probability
is that the spores come principally from those northern areas in which stem
rust builds up to enormous quantities during the summer. When there is a
late harvest in the Dakotas and the Prairie Provinces, as happened, for instance,
each year from 1950 to 1954, there is unquestionably a southward movement of
rust spores, some of which cause infection on fall-sown wheat in Texas and
adjacent states. Thus, there is not only a northward movement of the rust in
early summer but also a southward movement in autumn. This movement
and countermovement of the rust probably provides the reason for the persis-
tence, year after year, of certain rust races, once they have become established
in the Mississippi Valley area.

Remainder of Continent

Although stem rust is of less importance in other parts of the continent
than in the Mississippi Valley and the Prairie Provinces, it is, nevertheless,
responsible for considerable damage elsewhere. In the semiarid Great Plains
area, which lies west of the Mississippi Valley, stem rust is not usually of much
importance, but occasionally, as in 1904 and 1935, a heavy infestation occurs.

34

In the southern part of this area, native species of barberry seldom become
infected and hence play little or no part in the spread of the disease. It would
appear that when outbreaks occur in this area, they are largely attributable to
spores blown in from the Mississippi Valley. The northern part lies in the
barberry-eradication area of the North-Central States, and where barberry
occurs there is the possibility of infection spreading from this source. Un-
doubtedly wind-borne spores from the south and east influence the amount of
infection in this part.

West of the Great Plains area of the United States and the Prairie Provinces
of Canada, the area suitable for cereal production is comparatively small and
much divided owing to the mountainous nature of the country. Overwintering
occurs in the Pacific coastal region, where outbreaks of the disease are usually
severe and of rather frequent occurrence. In some of the mountain valleys, stem
rust apparently overwinters occasionally, and there is the possibility, as happened
in 1904, of spores being carried in from the Mississippi Valley and the Great Plains
by wind. The Inland Empire, between the Rocky and the Cascade Mountains, is an
important agricultural area, but there stem rust does not appear to overwinter.
In some of the grain-growing areas barberry was of considerable importance as a
source of stem-rust infection before eradication was undertaken in 1944.

Farther north, in British Columbia, grain-growing is of minor importance,
and the occurrence of stem rust there has not been given much attention.
Barberry hedges are rather common in the lower Fraser River Valley, and
infection from them causes considerable damage to oats, the principal cereal
crop. In the interior valleys, barberry seems to be very scarce, and stem rust
is seldom of any consequence. A native species occurs, but it rarely rusts. The
possibility of stem rust overwintering has not been investigated, and nothing is
known concerning the importance of wind-borne spores.

In Mexico, stem rust persists throughout the year in certain areas such as
the southern plateau and parts of the west coast; but rust spores are probably
not blown in significant numbers from these far-away regions to Texas. It
seems likely that the red stage that overwinters in Texas and adjacent north-
eastern Mexico is the chief source of rust infection for the Mississippi Valley
region.

In eastern North America, little attempt has been made to eradicate
barberry, and as overwintering of the red stage of stem rust is not known to
occur, it is probable that localized outbreaks originate largely or entirely from
infected barberry. Undoubtedly much of the infection in the Eastern States
arises from this source. Localized outbreaks, for instance in the Allegheny
Mountain region, have been repeatedly traced to barberry. There seems, how-
ever, to be no good reason why wind-borne spores from the Mississippi Valley
may not be responsible for considerable infection in these states. In most years,
however, it appears that localized outbreaks occur rather than general infections.
Stem rust is present every year in Eastern Canada, and surveys, particularly in
Ontario, have demonstrated the occasional relation of barberry to local out-
breaks of rust. There is not much probability that overwintering occurs to
any considerable extent.

In some years, however, the distribution of the disease in Eastern Canada
seems to be much too general and uniform to be accounted for satisfactory
by spread from local sources. It would appear that these general outbreaks
are attributable to invasions of wind-borne spores from states lying to the
south and southwest. As a matter of fact, there seems to be a relationship
between the occurrence of stem-rust epidemics in the Mississippi Valley and
heavy general infections in Ontario — certainly in western Ontario — and possibly
in Quebec. Not enough is known about general outbreaks in the Maritime

35

Provinces to relate such outbreaks to epidemics in the Mississippi Valley or
Eastern States but it seems probable that these provinces are well within the
range of wind-blown spores from areas south and east of the Great Lakes.

Figure 24. — Seed of Marquis wheat grown in 1935. Left. — Seed from plants badly damaged by stem rust.
Right — Seed from plants uninjured by stem rust. About natural size. (Photograph by Mr. A. M. Brown)

EFFECT OF STEM RUST ON CEREAL PLANTS

A heavily rusted crop produces shriveled light-weight seed (Figure 24), so
that the yield is much reduced and the quality and grade of the grain are
lowered. How are these results brought about? It must be remembered that
the fungus causing stem rust is a parasite which infects green growing plants.
In order to develop, it must obtain its food and water from the plants that it
attacks. In cereal plants, therefore, the fungus uses the food that would other-
wise be carried up to the heads to fill the kernels. Besides, as the fungus develops
that is to say as the mycelium in the infections grows, it produces numerous
pustules, and these, breaking out, do great injury to the surface of the plants.
As the manufacture of plant food takes place largely in the cells at or near the
surface, this process is very seriously interfered with in rusted plants. In these
two ways, therefore, the fungus reduces the food supply within the plants. It
also reduces the water supply within the plants. It needs water as well as
food. The water, therefore, that is used by the fungus is lost to the plants.
There is a further loss of water, by evaporation, through the numerous ruptures
caused by the pustules.

The kernels are, therefore, poorly supplied with food and water and as
a result they shrivel. Chemical analyses have shown that badly rusted straw
has a higher food value for stock than has unrusted straw. Of course, the
higher food value of the rusted straw may be due, in part at least, to the food
contained in the fungus within the straw, rather than to the food in the straw
itself; but very probably some of the food that should have reached the kernels
is left in the straw because of insufficient water to carry it to the heads.

The belief has been rather commonly held that the fungus living in the
stems of the plants is able to draw away from the kernels food that had already
been stored in them. Grain was frequently cut "on the green side" to prevent

36

this withdrawal of food. There is no good evidence for this belief. Careful
experiments by different investigators at different times and in different places
go to show that rusted grain yields best when it is cut in the hard dough stage
that is, when the kernel cannot be readily crushed by pressure between the thumb
and forefinger. At this stage there may be less water in the kernels than there
was earlier, and on this account the kernels may look less plump, but the actual
weight per bushel of dry threshed grain is greater. It is not advisable, however,
to allow a rusted crop to stand until it is dead ripe. The straw becomes brittle,
and harvesting is difficult. Moreover, many heads may break off from the
brittle straw and thereby reduce the yield.

Figure 25. — Comparison of seedlings grown from seed of badly rusted and
of unrusted wheat. Left. — Seedlings from seed badly shriveled by stem
rust ('Feed' grade). Right. — Seedlings from seed uninjured by stem rust
(No. 1 Northern). Notice the slender delicate plants on the left in contrast
to the vigorous strong ones on the right. (Photograph by Mr.

A. M. Brown)

Another belief was that if a rusted crop is cut green, considerable filling of
the kernels takes place after the crop is cut. Experiments, however, have
shown that this belief is not well founded. The movement of food from the
straw into the kernels ceases soon after the crop is cut, so that little or no filling
can take place.

It has been known for many years that badly shriveled kernels make poor
seed grain. The germination of such kernels may be good, but the seedlings
produced by them are generally weak (Figure 25). These seedlings suffer
from partial starvation, as the kernels have an insufficient amount of food
stored in them to support vigorous seedling growth until the young plants can
get established and begin to manufacture food for themselves. If such seed is
sown rather deeply, the seedlings have difficulty in emerging and becoming
self-supporting. Moreover, in recent years, it has been found that weak seedlings
are much more likely than vigorous ones to be injured by root-rotting fungi
living in the soil. The effect of stem rust, therefore, finds expression not only
in the crop that is attacked but also in the crop arising from the seed of the crop
that it has injured.

CONTROL OF STEM RUST

Stem rust is a difficult disease to control. It is widely distributed through-
out the world, it is spread by wind, it increases very rapidly, and the crops that
it attacks are very extensively grown. It is not seed-borne, although, when a

37

crop is heavily rusted, kernels may become infected while still immature. When
seed thus affected is sown, it does not produce rusted plants. Seed treatments,
such as are used for the control of cereal smuts, are therefore of no value in
controlling stem rust. Infections only arise from spores that come into contact
with the above-ground parts of the plant. Owing to the extensive scale on
which cereals are grown, control measures must be practicable for relatively
large areas if rust losses are to be measurably reduced. Of course, the protection
of a single farm or field would be of decided benefit to its owner; but the problem
of stem-rust control must be regarded largely in its broader aspect. The more
permanent and widely effective the method, the more valuable must it be con-
sidered.

Destruction of Barberry

It has been pointed out that infected barberry may serve as a source of
infection. In some areas it is the chief or only source. Furthermore, races of
stem rust may hybridize on it and produce new races. On both counts, therefore,
it is a menace to cereal crops. Wherever in a cereal-growing area it is known
to rust, it should be destroyed. In so far as it is the source of infection, its
destruction is a permanent and widely effective means of control.

Early Maturity

Crops that mature early are, as a rule, considerably less damaged by stem
rust than are those that mature later. One reason for this is that the amount
of infection almost invariably increases as the season advances. Another
reason is that by the time the disease becomes very prevalent and severe, the
early crop is more fully developed than the later one. That is, more food has
been stored in the kernels of the early crop. The less mature a crop is when the
disease becomes severe on it, the less chance have the kernels of filling well.
A difference in maturity of one week often makes a pronounced difference in
the amount of injury. Early maturity does not confer resistance on a crop;
it only gives the crop a chance to escape damage.

Early maturity can be secured by sowing early varieties. Reward wheat,
for example, usually suffers considerably less damage than Marquis from stem-
rust, largely on account of its earliness, although actually it is not quite so
susceptible as Marquis. Garnet is an early but very susceptible variety of
wheat. In heavy-rust years, occasional fields of this variety have been known
to escape appreciable damage owing to the fact that they ripened before the
disease got well under way. Early seeding brings about the same result. The
crop is pretty well along to maturity before the disease becomes severe. Then,
again, the addition of phosphate and potash fertilizers to the soil tends to induce
early maturity. There are, of course, obvious limitations to the extent of
control that can be obtained by early maturity.

Chemicals

To be satisfactory a chemical must be effective in itself and, in addition, it
must be available in adequate amounts, methods of application must be practical,
and the cost must be commensurate with crop returns. At present no chemical
satisfies all requirements.

Zinc ethylene bis-dithiocarbamate (sold under trade names such as Zineb,
Parzate, and Dithane Z-78) and sulphur will control rust if several applications
are made. These fungicides are toxic to germinating rust spores and prevent
the entry of the rust into the plant so long as they are present on the leaf or
stem surface in sufficient concentration. They do not destroy the rust after it

38

has entered the plant. As they can be washed off the plant surface by heavy
rains, their effectiveness depends a good deal on the weather immediately
following application.

Since 1950, there has been considerable search for a systemic fungicide, a
chemical that would be absorbed into the plant and would then be sufficiently
antagonistic to the rust to prevent further infection for a period of several
weeks. No satisfactory chemical of this type has been found thus far. In
some experiments, though not in all, calcium sulfamate has given fair control of
rust. This chemical, however, has a tendency to reduce severely the germina-
tion of the seed produced by treated plants, and according to some reports it
reduces the quality of the grain. For these reasons, calcium sulfamate cannot
be recommended; moreover, it is not yet on the market in quantity.

If producers of a chemical to be used could be assured of a constant market,
it would be profitable for them to maintain supplies to meet the demand. Rust,
however, does not strike every year, and when it does strike, the period during
which control measures can be effective is short — at most only a month or six
weeks. It might be difficult therefore to provide adequate supplies of the
chemical, particularly if the demand were great.

Finding a suitable method of applying a chemical is another problem.
Few farmers have the machinery necessary for the application, in dust form,
of fungicides such as sulphur or zinc ethylene bis-dithiocarbamate. The latter
chemical, however, is available in liquid form and will give control of rust equal
to that of sulphur, at a lower cost, if applied in the form of spray. The purchase
of machinery is an additional expense and, moreover, the use of ground machinery
damages standing grain. Application by airplane might solve this problem
but is likely to be expensive, especially in the case of substances such as sulphur,
which have to be used in high dosage, a requirement that would involve frequent
landings and reloadings. The ideal chemical for airplane application would be
one in liquid form that could be applied in low dosages and that would require
only a single application for control. No such chemical is yet available.

The final and most important consideration in control of rust by chemicals
is the cost. Costs must be relatively low if this means is to be feasible. Sulphur,
for example, would probably require three applications of about 40 pounds
each per acre to give even moderate control of rust. The total cost is likely to be
from 8 to 10 dollars per acre, varying with the year and the locality. If dithio-
carbamates were used, as sprays, the cost of materials would be considerably
lower.

At present, chemical control, though not impossible, is expensive and
uncertain and cannot be adopted over areas involving many millions of acres.
However, more effective chemicals at reduced cost are being sought and the
situation may improve within a relatively short time.

Plant Breeding

The simplest and best method of controlling stem rust is to grow resistant
varieties. A variety, however, may be resistant to stem rust but wholly
undesirable in other respects. As a rule, high resistance to stem rust occurs
naturally in varieties that are of little or no commercial value. To obtain a
satisfactory resistant variety, it is necessary, by plant-breeding methods, to
combine high rust resistance and desirable agronomic and commercial characters
in the same variety. Even in the simplest case, this may be much more difficult
to accomplish than might be expected.

In securing such a combination of characters, the plant breeder meets
with two sources of difficulty, one relating to the cereals and one to the fungus.
With the cereals, the chief difficulty lies in the fact that a large number of

39

desirable characters is required in addition to high resistance. For instance, a
suitable bread wheat variety must be satisfactory in agronomic characters, such
as earliness, strength of straw, yield, weight per bushel, kernel color and shape,
and in milling and baking characters, such as flour yield, flour color, gluten
quantity, gluten quality, loaf volume and crumb texture. Besides, it must not
be unduly susceptible to leaf rust, loose smut, bunt, rootrot, and other diseases.
Unfortunately resistance to stem rust is frequently associated with some un-
desirable character, so that rust resistance and the undesirable character tend
to be inherited together, as if they constituted a single character.

With the fungus, the chief difficulty lies in the fact that there are different
races or strains of stem rust. A variety of wheat, for example, may be highly
resistant to certain races of wheat stem rust, but completely susceptible to
other races. A second variety may be highly resistant to some of the races to
which the first variety is susceptible. A third variety may be resistant to still
other races. If no variety can be found that is resistant to all races, the resis-
tance of two or more of such partially-resistant varieties must be combined.
If, on the other hand, a variety can be found that possesses high resistance to all
races, and if that resistance is rather simply inherited so that it can be com-
pletely transferred to another variety, the breeding problem would be simplified.
In this case, the building up of resistance would not be necessary, and hence
fewer crosses would be required to accomplish the same result: Care would
have to be taken, however, in both cases to get rid of any undesirable character
that may be associated with rust resistance.

Success in producing rust-resistant varieties of wheat has been achieved by
combining the resistance of two or more varieties in a new variety. For example,
Thatcher, originated at the University of Minnesota, was produced in this way.
It arose from a cross that is usually referred to as a double cross. One of its
parents is a selection from a cross between Marquis and Iumillo (a resistant
durum variety), the other a selection from a cross between Marquis and Kanred
(a partially resistant winter wheat). Part of the resistance of Thatcher, there-
fore, came from Iumillo and part from Kanred.

Many of the wheats grown in Canada and the United States in the period
from 1937 to 1950 possessed a type of resistance known as "mature-plant resis-
tance". These varieties (for example, Regent, Redman, Mida, Cadet), as they
grow towards maturity, gradually develop a type of resistance that, until the
advent of race 15B, proved effective against all stem-rust races prevalent in the
wheat-growing areas of North America. This type of resistance, which proved
so valuable for many years, was derived from an emmer wheat, known as Yaroslav
Emmer. By crossing this variety with Marquis, Mr. E. S. McFadden, at
Brookings, South Dakota, succeeded in transferring its resistance to two new
varieties, Hope and H-44-24. Neither of these wheats proved to be commercially
valuable, but most of the rust-resistant North American wheats grown before
1950 derived their rust resistance from them.

The discovery and subsequent widespread outbreak of Race 15B and the
measures taken in breeding a bread-wheat variety resistant to it, provide a
good example of how plant breeding functions as a means of rust control. This
race was discovered about 1940. Soon thereafter studies on its infective abilities
made it apparent that, if it ever became widely distributed, it could severely
damage all the most useful bread-wheat and durum-wheat varieties grown at
that time in North America.

Before plant breeding could even be attempted, it was necessary to discover
a source of resistance against Race 15B. Such resistance was found in the variety
McMurachy (named after the Manitoba farmer who selected it). This wheat

40

did not have the agronomic, milling, and baking qualities required for a com-
mercial wheat, nor the necessary resistance to leaf rust, but it did have resis-
tance to Race 15B and the various other stem-rust races then known to occur in
North America. Since it was desired to produce a commercial wheat resistant
to leaf rust as well as stem rust, McMurachy was crossed with the leaf-rust-
resistant variety Exchange. The resulting hybrid lines had the necessary stem-
rust and leaf-rust resistance but, because both parents were poor in quality,
the hybrids lacked the quality required in a commercial wheat. This quality
was obtained by crossing the hybrids repeatedly with the variety Redman and
retaining only those new hybrid lines that had the highest quality combined
with the necessary rust resistance. The ultimate product of this series of
crosses was the variety Selkirk, which was released to farmers in the spring of
1954.

When breeding for rust resistance in oats was undertaken in Canada, no
varieties were known that had resistance to all races of oat stem rust in North
America. The best that could be done at the time was to produce varieties
resistant to the most common races of the rust (Races 1, 2, and 5),
which accounted for about 95 per cent of oat stem rust present in Canada each
year. The varieties Vanguard and Ajax, released in 1937 and 1942, respectively,
were resistant to the common races and could, therefore, be expected to remain
practically free from rust so long as rarely occurring races, such as Races 6 and 8,
did not increase. The first notable increase of a race that could damage these
varieties took place in 1943, when Race 8 began to appear rather commonly in
the Prairie Provinces. Since then this race gradually increased until, five years
later, it made up almost half of the oat stem rust present in Canada. The
increased prevalence of Race 8 has caused Vanguard, Ajax, and other varieties
with similar resistance to rust considerably, but these varieties suffer less from
rust than older varieties susceptible to all races.

To cope with the possible increase of Race 8, plant breeders in the United
States produced such varieties as Clinton, Mindo, and Bonda, which possess
resistance to this race and to the common races as well. These varieties, how-
ever, lacked resistance to Races 6 and 7. Consequently, an increase of these
races would endanger this type of variety. A sudden increase of Race 7 in 1950
showed that the resistance of these varieties could not be depended on. There-
fore, the objective of producing an oat variety resistant to all known oat stem
rust races became imperative. This objective was achieved at the Cereal
Breeding Laboratory, Winnipeg, by the production of the variety Garry, which
is not only resistant to the known races of oat stem rust but has resistance also
to many races of crown rust.

It is clear from the foregoing discussion that the rust-resistant cereal
varieties produced from time to time are not likely to remain rust-resistant
permanently. It is possible that a variety with permanent rust resistance may
be produced, but it is not possible to predict, at the time of its distribution,
that its resistance will be permanent.

The difficulty of producing varieties with permanent disease resistance is
a consequence of the tremendous variability that exists in most disease-pro-
ducing organisms. Owing to this variability most microorganisms, including
the rusts, are made up of numerous strains or races. When a new cereal variety
is produced, the most that the rust investigator can say for it is that it is resistant
to all known races of a rust. That is not to say that it is resistant to all existing
races, because some races, unknown to the investigator, are likely to exist in
small quantities and may be able to attack the new variety. If these races
exist in, or are introduced into, the area in which the new variety is grown, they
are likely to increase greatly in a few years.

41

The most important function of the investigator of the rusts is to discover
the presence of new and dangerous rust races before they become widely dis-
tributed and so be able to warn the plant breeder that danger lies ahead. Breed-
ing can then be undertaken against the threatening race in order that the pro-
duction of resistant varieties will have progressed as far as possible before the
new race becomes sufficiently prevalent to be destructive to the varieties then
being grown. Breeding of oats resistant to Races 8 and 7 of oat stem rust was
attempted while these races were still scarce. Breeding of a wheat resistant to
Race 15B was undertaken five years before that race became prevalent in the
great wheat-growing areas.

As races of rust can sometimes increase very rapidly and, as it requires
about 10 years to produce a new cereal variety, the plant breeder may occasion-
ally lose out in the race with the rust. But if the surveys for the presence of
new rust races are performed effectively, and if resistant breeding material can
be found quickly enough, the farmer should never, for long, remain without a
rust-resistant variety.

Perhaps the most encouraging development in rust research work in recent
years is the international co-operation set in motion after the outbreak of Race
15B of wheat stem rust in 1950. Before that time there had been a certain
amount of co-operation of an informal nature between individuals or institutions
in several countries. At a meeting held at St. Paul, Minnesota, in November,
1950, co-operation in rust research, at least for North and South America, was
placed on an organized basis. In consequence, "rust nurseries," that is, experi-
mental plots for testing the resistance of cereal varieties under severe rust
conditions, have been established in Canada, the United States, Mexico, and
several South American countries. The great World Collection of more than
12,000 wheat varieties maintained at Washington, D.C., is gradually being
tested in these rust nurseries and in the greenhouses of a number of laboratories
in order to discover the varieties with most rust resistance. Hybrid wheats
from plant-breeding laboratories in Canada and other countries are tested in
these nurseries to determine their rust resistance, and are also tested in certain
laboratories for their reaction to known rust races. Attempts are being made to
introduce into wheat varieties additional rust resistance from rye and from
certain grasses that are x related to wheat. When any Canadian hybrid wheats
are found to show promise as possible new varieties, they are increased as rapidly
as possible, in the summer in Canada and in the winter in southern California.
For the first time in history, the available resources of science are being fully
used to provide rust-resistant varieties of cereals.

42

SUMMARY

Stem rust is a destructive and widespread disease of cereals. It is caused
by a fungus with a rather complex life cycle. Two stages of this cycle, the red
and the black, occur on cereals and grasses, a third stage occurs on barberry.

Spores of the red stage are readily distributed by wind. They germinate
when moisture conditions are favorable, and infect cereals and grasses. Each
infection gives rise to a pustule containing numerous spores. Under favorable
conditions, a new generation of red spores is produced each week, so that the
disease increases very rapidly. Only in mild climates is the red stage able to
persist from one season to the next.

When infected cereal or grass plants begin to ripen, the fungus ceases to
produce red spores and, in their place produces black (winter) spores. These
spores remain attached to the plants on which they are produced and are not,
therefore, readily distributed by winds. They cannot infect cereals or grasses.
They can withstand severe cold, but they are less successful in withstanding
prolonged exposure to high temperatures. They require about a six-month period
of dormancy before they will germinate. The black spores are, therefore,
adapted to survive winter conditions.

After their dormancy period, that is to say, in the spring, the black spores
readily germinate whenever temperature and moisture conditions are suitable.
On germination, they give rise to spores of a totally different type, called sporidia.
The sporidia are distributed by wind. They are not able to infect cereals or
grasses, but, if barberry is growing in the vicinity where they are produced,
they infect its young leaves and twigs. Such infections give rise on barberry to
the cluster-cup stage of the fungus. In the cluster cups (aecia) another type
of spore called aeciospore is produced. These spores, too, are distributed by
winds. If they lodge on green susceptible cereal or grass plants, they infect
them, and thus complete the life cycle of the fungus.

There are several varieties of stem rust. They are very similar in
appearance, and are distinguished from one another by the host plants that they
are able to infect. For example, one variety infects wheat and barley but not
oats, another infects oats but not wheat or barley. Three varieties — wheat stem
rust, oat stem rust, and rye stem rust — are known to comprise a number of
different races. The races of wheat stem rust are distinguished by their ability
to attack certain wheat varieties; those of oat stem rust, by their ability to
attack certain oat varieties; and those of rye stem rust, by their ability to attack
certain varieties of rye.

The fungus has a sexual stage, which begins when the black spores germinate.
The sporidia produced are of two sexes. The sexual stage is completed on
barberry. It is possible, therefore, for varieties and races of stem rust to
hybridize on barberry and produce new races.

To control stem rust, the most effective method is to grow resistant varie-
ties. Other methods consist in the destruction of barberry, the sowing of early-
maturing varieties, the securing of early maturity by cultural practices, and the
use of chemicals.

43

USEFUL REFERENCES TO STEM RUST LITERATURE

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2. Arthur, J. C. Progress of rust studies. Phytopathology 18: 659-674. 1928.

3. Bailey, D. L. Physiologic specialization in Puccinia graminis avenae Erikss. & Henn.

Minn. Agr. Exp. Sta. Tech. Bull. 35, 1925.

4. Bailey, D. L. Studies in cereal diseases. IV. Stem rust in Western Canada. Dom.

Dept. Agr. Bull. 106-N.S., 1928.

5. Craigie, J. H. An experimental investigation of sex in the rust fungi. Phytopathology. 21:

1001-1040. 1931.

6 Craigie, J. H. Increase in production and value of the wheat crop in Manitoba and Eastern
Saskatchewan as a result of the introduction of rust resistant wheat varieties. Sci.
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7. Craigie, J. H. Epidemiology of stem rust in Western Canada. Sci. Agr. 25: 285-401. 1945.

8. Ellis, J. H. Observations on rust control. Man. Dept. Agr. and Immigration, Extension

Bull. 41, 1919.

9. Freeman, E. M. and E. C. Johnson. The rusts of grains in the United States. U.S.

Dept. Agr. Bur. Plant Ind. Bull. 216. 1911.

0. Gordon, W. L. and J. N. Welsh. Oat stem investigations in Canada. Sci. Agr. 13: 228-235,

1932.

1. Goulden, C. H. Breeding rust-resistant varieties of wheat — fundamental aspects of the

problem. Sci. Agr. 10: 258-267. 1929.

2. Goulden, C. H. and K. W. Neatby. Breeding rust-resistant varieties of spring wheat.

J. Amer. Soc. Agron. 23: 859-870. 1931.

3. Greaney, F. J. The influence on yield and grade of harvesting rusted wheat at different

stages of maturity. Sci. Agr. 11: 492-511. 1931.

4. Greaney, F. J. Method of estimating losses from cereal rusts. Proc. World's Grain

Exhibition and Conference, Regina, Canada 2: 224-236. 1933.

5. Greaney, F. J. Studies in cereal diseases. XI. The prevention of cereal rusts by the

use of fungicidal dusts. Dom. Dept. Agr. Bull. 171-N.S., 1934.

6. Greaney, F. J. Cereal rust losses in Western Canada. Sci. Agr. 16: 608-613. 1936.

7. Hart, H. Morphologic and physiologic studies on stem rust resistance in cereals. U.S.

Dept. Agr. Tech. Bull. 266. * 1931.

8. Hayes, H. K. The genetics of stem rust resistance in wheat. Proc. 6th Intern. Congr.

Genetics, Ithaca, N.Y. 1932. p. 81-83. 1932.

9. Hayes, H. K. and E. C. Stakman. Wheat stem rust from the standpoint of plant breeding

Proc. 2nd Ann. Meeting West. Canada Soc. Agron. 1921, p. 22-35.

20. Humphrey, H. B. The cereal rusts and their control. Sci. Agr. 10: 225-231. 1929.

21. Hungerford, C. W. Rust in seed wheat and its relation to seedling infection. J. Agr

Res. 19: 257-277. 1920.

22. Hutton, L. D. Barberry eradication reducing stem rust losses in wide area. U.S. Dept

Agr. Yearbook 1927. p. 114-118. 1928.

23. Jackson, V. W., W. P. Fraser, and D. L. Bailey. The present status of the barberry

eradication campaign in Western Canada. Sci. Agr. 5: 375-378. 1925.

24. Johnson, E. C. Cardinal temperatures for the germination of uredospores of cereal rusts.

(Abstract) Phytopathology 2: 47-48. 1912.

25. Johnson, T. Studies in cereal diseases. VI. A study of the effect of environmental

factors on the variability of physiologic forms of Puccinia graminis tritici Erikss. & Henn.
Dom. Dept. Agr. Bull. 140-N.S., 1931.

26. Johnson, T. Variation in the rusts of cereals. Biol. Rev. 28: 105-157. 1953.

27. Johnson, T. and Margaret Newton. Specialization, hybridization, and mutation, in the

cereal rusts. Bot. Rev. 12: 337-392. 1946.

44

28. Johnson, T., Margaret Newton, and A. M. Brown. Further studies of the inheritance of

spore color and pathogenicity in crosses between physiologic forms of Puccinia graminis
tritici. Sci. Agr. 14: 360-373. 1934.

29. Lambert, E. B. The relation of weather to the development of stem rust in the Mississippi

Valley. Phytopathology 19: 1-71. 1929.

30. McAlpine, D. The rusts of Australia. Melbourne, Australia, 1906. 349 p.

31. Metha, K. C. Observations and experiments on cereal rusts in the neighbourhood

of Cambridge, with special reference to their annual recurrence. Trans. Brit. Mycol.
Soc. 8: 142-176. 1923.

32. Metha, K. C. Annual outbreaks of rusts of wheat and barley in the plains of India. Indian

J. Agr. Sci. 1:297-301. 1931.

33. Metha, K. C. The cereal rust problem in India. Indian J. Agr. Sci. 1 : 302-305. 1931.

34. Melander, L. W. The effect of temperature and light on the development of the uredial

stage of Puccinia graminis Pers. J. Agr. Res. 50: 861-880. 1935.

35. Newton, Margaret. The cereal rusts in Canada. Emp. J. Exp. Agr. 6: 125-140. 1938.

36. Newton, Margaret and T. Johnson. Studies in cereal diseases. VIII. Specialization

and hybridization of wheat stem rust, Puccinia graminis tritici, in Canada. Dom. Dept.
Agr. Bull. 160-N.S., 1932.

37. Newton, Margaret, T. Johnson, and A. M. Brown. A preliminary study of the hybridiza-

tion of physiologic forms of Puccinia graminis tritici. Sci. Agr. 10: 721-731. 1930.

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color and pathogenicity in crosses between physiologic forms of Puccinia graminis tritici.
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39. Peltier, G. L. A study of the environmental conditions influencing the development

of stem rust in the absence of an alternate host.

I. The viability of the urediniospores of Puccinia graminis tritici form III. Nebr. Agr.
Exp. Sta. Res. Bull. 22. 1922.

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Exp. Sta. Res. Bull. 25. 1923.
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Nebr. Agr. Exp. Sta. Res. Bull. 34. 1925.

40. Peltier, G. L. Relation of weather to the prevalence of wheat stem rust in Nebraska.

J. Agr. Res. 46: 59-73. 1933.

41. Shutt, F. T. The effect of rust on the straw and grain of wheat. Dom. Dept. Agr. Rept.

Exp. Farms, 1904. p. 189-191. 1905.

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1918. p. 75-100. 1919.

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Plant Science, Ithaca, N.Y., 1926. 2: 1312-1330. 1929.

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and Vancouver, Canada, 1933. 4: 3177-3184. 1934.

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graminis on Triticum spp. Min. Agr. Exp. Sta. Tech. Bull. 8, 1922.

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51. Stakman, E. C, L. W. Melander, and D. G. Fletcher. Barberry eradication pays.

Minn. Dept. Agr. Bull. 55, 1930.

52. Stakman, E. C. and F. J. Piemeisel. Biologic forms of Puccinia graminis on cereals and

grasses. J. Agr. Res. 10: 429-495. 1917.

45

53. Stoa, T. E. The early harvest of rusted Marquis wheat. J. Amer. Soc. Agron. 16: 41-47.

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1937. Dom. Dept. Agr., Exp. Farms.

55. Verwoerd, L. A review of the blank stem rust {Puccinia graminis Pers.) situation with

special reference to the experimental methods applied in rust research in the United
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