SB 608
. CSSESZ
Copy 1

-_ Calif.-30

"SOME RELATIONS OF TEMPERATURE TO GROWTH
: AND INFECTION IN THE CITRUS SCAB
FUNGUS CLADOSPORIUM CITRI

BY

Rise he :
H. S> FAWCETT

Reprinted from JOURNAL OF AGRICULTURAL RESEARCH
Vol. XXI, No.4 =: =: >: :. Washington, D. C., May 16, 1921

PUBLISHED BY AUTHORITY OF THE SECRETARY OF AGRICULTURE, WITH
THE COOPERATION OF THE ASSOCIATION OF EAND-GRANT COLLEGES

WASHINGTON ; GOVERNMENT PRINTING OFFICE : 1921

Se bt Av : 4

SOME RELATIONS OF TEMPERATURE TO GROWTH
AND INFECTION IN THE CITRUS SCAB FUNGUS
CLADOSPORIUM CITRI' ,

By H. S. Fawcett

Professor of Plant Pathology, Citrus Experiment Station, College of Agriculture,
University of California

INTRODUCTION

In some previous work by the author (3)? it was found that the results
of different inoculation tests on rapidly growing sour-orange trees (Cttrus
aurantium L,. (13) with Cladosporium citri Massee were variable. It had
previously been observed in the citrus orchards of Florida that abundant
infections from scab did not inevitably follow the presence of abundant
moisture on rapidly growing tissue, although these two conditions were
usually present when abundant infection did occur. Seasons were
encountered when scarcely any infection from scab occurred, even though
conditions of moisture and growth appeared to be ideal for an outbreak.
The author was led to suspect that temperature was also an important
factor in infection.

The experiments which form the basis of this paper were planned to
determine what influence different temperatures might have, first, upon
infection by the fungus when the two other conditions previously men-
tioned, abundant moisture on the leaves and rapid growth were main-
tained, and second, on the growth and spore formation of the causal
fungus on culture media. These experiments with the scab fungus
carried on during intervals in a more extended temperature investigation
with other citrus fungi reported elsewhere (4) are seen to be somewhat
incomplete, but they appear to throw considerable light on the possible
relation of temperature to the occurrence of citrus scab and to offer a
more complete explanation for the differences in the occurrence of scab
from year to year or from one season to another. It is, of course, realized
that in the orchard, under natural conditions, temperature is fluctuating
and not constantly maintained as in these experiments. Nevertheless,
the experiments indicate at least the comparatively narrow range of
temperature within which infection of a very susceptible host is possible
under the presumably ideal conditions.

1 Paper No. 72, University of California, Graduate School of Tropical Agriculture and Citrus Experiment

Station, Riverside, Calif.
2 Reference is made by number (italic) to ‘‘ Literature cited,” p. 253.

Journal of Agricultural Research, (243) Vol. XXI, No. 4
Washington, D. C. May 16, 1921

xv Key No. Calif.-30

244 Journal of Agricultural Research Vol. XXI, No. 4

Citrus scab has been described and illustrated in considerable detail
in previous publications by Scribner (8, 9), Swingle and Webber (14),
Fawcett (2), and others. Its main characteristics are corky wart-like
projections on leaves, fruit, and small twigs, due to the attack of the
fungus on tender, rapidly growing tissue. The portion of the leaves
not attacked usually maintains a healthy green color close up to the edge
of these warts or scabs.

Considerable confusion has resulted, however, in the identity of the
causal organism, Cladosporium citri (1, 2). It should be mentioned that
the fungus is an unusual Caldosporium and very different from the
ordinary type of Cladosporium such as Cladosporium herbarum Lk., for
example. This causal relation of Cladosporium citr, to scab was at
one time questioned by Grossenbacher (5), probably because he was
working with another Cladosporium as indicated by the characteristics
mentioned in his paper. The author’s identification of Clodosporium
citri as the causal organism has been confirmed by Hesler (6), Stevens (z2),
and Horne (3), as well as by his own later work (3). Moreover, Cladospo-
rium citri has been found by Stevens (zo) to be the cause of a similar
disease known as avocado scab in Florida. Sour-orange (Citrus awrantium)
(73) and lemon (Citrus limonia Osbeck) trees appear to be the most sus-
ceptible hosts to citrus scab. Satsuma orange (Cztrus nobilis var. unshiu
Swingle), trifoliata orange (Poncirus trifoliata Raf.), and pomelo
(Citrus grandis Osbeck) are also attacked. Sweet orange (Citrus sinensis
Osbeck) is nearly immune. In most of the infection experiments here
reported, the sour orange was employed because it was one of the most
susceptible species.

The experiments were made in 1917 at the Labdeatory of Plant Physi-
ology, Johns Hopkins University, in surroundings entirely free from the
disease in question. The various temperatures employed were main-
tained for the most part by means of the apparatus described by Living-
ston and Fawcett (7).

TEMPERATURES AT WHICH YOUNG CITRUS LEAVES WERE INFECTED

A preliminary test on sour-orange seedlings was made on May 16, 1917,
with 8-months-old plants, which had been potted about three weeks and
were beginning to develop two or three new leaves each. These plants
were placed for three days in the different temperatures indicated below
and were inoculated by drawing over the surface of the leaves and shoots
a camel’s-hair brush moistened with spores and bits of mycelium of the
fungus in distilled water.

The average temperatures of the nine different chambers were 12°, 16°,
19°, 23°, 26.5°, 31°, 34.5°, 38.5°, and 42.5° C., with a fluctuation of 1°
to 2°. One plant was exposed in each chamber for a period of three
days in a saturated atmosphere and then taken out and the surface water

1 The author wishes to acknowledge the aid rendered by Dr. B. E. Livingston in these experiments.

CHET
Author
FOR 2° 1322

May 16, x21 Temperature Relations of Cladosporium citri 245

allowed to dry. The plants that had been inoculated at 167,19", and 23°
subsequently became diseased. The plant from the 16° chamber had
scabs just visible 7 days from date of inoculation. Those from the 19°
and 23° chambers were not seen to be diseased until 10 days later. The
plants that had been inoculated at 16° and 19° had many scabs. The
plant that had been inoculated at 23° was only slightly diseased even on
the seventeenth day after inoculation.

Controls which were not inoculated, but on which distilled water was
drawn over the leaves, were subjected to 16° and 26.5° C. and kept under
the same conditions as the others, but no scab developed upon them.

A second test on sour-orange seedlings just like the first was made on
June 5, 1917, except that different temperatures were maintained in
some of the chambers and two plants were left in each temperature, and
they remained for 54 hours instead of 3 days. Control plants not inocu-
lated were used at 20° and 27.5° C. The average temperatures in this
test were 14°, 20°, 24.5°, 27.5°, 92:5°,'36°, 40°: and AAS.

In this test one of the plants inoculated at 20° C. developed scabs on its
leaves. These were first noted two weeks after inoculation. None of the
other plants developed scabs. As will be seen in Table I, the chamber at
20° is the only one that lies in the range at which infection occurred in the
other two tests, namely, 16° to 23°. The chambers at 14> and’ 24.5%,
therefore, appeared to be outside, one below and the other above, the
range for infection.

On September 24, 1917, a third inoculation test was made with sout-
orange trees in pots, about one year from seed, on which small new leaves
were developing. The plants were surrounded by cylinders of blotting:
paper kept moist by absorption of water from below. Another piece of
blotting paper was placed over the top of the cylinder. The different
temperature chambers (7) in which the plants were left for 3 days
were maintained nearly constant within a fluctuation of about o. emer
At the end of this 3-day period when they were taken out, the moist
cylinders of blotting paper were removed, and the plants were set in the
greenhouse. Jn about 14 hour the moisture had evaporated from the
surfaces of the plants. At the same time that these were taken out, two
other plants were inoculated with the fungus as before and were set in the
Same greenhouse and under the same conditions as the others. These
were not surrounded by blotting paper but were allowed to dry like the
others, after water with spores had been drawn over their leaves.

The temperatures at which the plants remained during the 3-day
period in the saturated atmosphere of the dark chambers were 1 B50. TGS,
LSS ge Sg fen linl MAla Wo3 Ig eeSoih O respectively.

The temperatures recorded in the greenhouse for the 12 days after the
plants were taken out of the temperature chambers gave an average
minimum of 15° C., and an average maximum of 30°.

246 Journal of Agricultural Research Vol. XXI, No. 4

Only those subjected to 16°, 18.5°, and 21° C. for the 3 days after
inoculation developed any scab on their leaves after removal to the
greenhouse benches.

On plants inoculated at 16° C. scab was just visible in 6 days from the
time of inoculation, or 3 days from the time of removal from the temper-
ature chamber. In 2 weeks the scab lesions were well marked.

Plants inoculated at 18.5° C. showed a slight indication of scab 5 days
after inoculation, or 2 days after plants were removed from the temper-
ature chamber. In this case the scab lesions were well marked in 11 days
from time of inoculation.

Plants inoculated at 21° C. showed a slight indication of scab 5 days
after inoculation, as was true of those at 18.5°. In 7 days the diseased
spots were quite distinct, and in 11 days they had developed into definite
and typical scabs. On plants inoculated at 13.5°, 24.5°, 27.5°, and 32°,
however, no sign of lesions developed.

Control plants were also kept under the same conditions, but none
of these developed scab lesions at any time. The two plants which were
inoculated with the fungus at the time the others were taken out of the
temperature chambers and from whose surfaces the water was allowed at
once to evaporate also remained free from scab. ‘These were considered
to be additional controls. ‘The nonappearance of scab on these two plants
indicated that infection at exposures of 16°, 18.5°, and 21° C. developed
because the hyphae had penetrated while the plants were still in the
temperature chambers. ‘This conclusion is also supported by the fact
that the time from inoculation to the appearance of visible scabs agrees
well with that observed in previously reported tests (2, 3). The time
elapsing between the removal of the plants to the greenhouse bench and
_ the first appearance of scab appears generally to be too short to represent
the incubation period of the fungus within the tissues of the host.

On September 28, 1917, a set of young pomelo seedlings with leaves
just begining to unfold was inoculated in the same manner as that
described for the sour-orange seedlings of the third test, except that these
were left four days in the temperature chambers instead of three. With
the exception of one leaf on a plant inoculated at 18.5° C., which developed
a slight indication of disease but no distinct scab, there was no visible
sign of disease on any of the plants up to October 17, when the last
observation was recorded.

In all four tests just considered, scabs were always confined to rapidly
growing leaves. ‘The older leaves always remained free from the disease
in every test. Neither was there any development of scabs on any of the
large number of similar plants kept in the greenhouse and not inoculated.

May 16,1921 Temperature Relations of Cladosporium cttre 247

TABLE I.—Range of infection as indicated by the four inoculation tests on citrus seedlings '

A ‘ B wa
pisht Eid date of Results of inoculation at temperature (°C.) of

inoculation,
¥2 |13. 5] 14] 16 |18. 5] 19 | 20] 21 | 25 94. is la6y'slans 31 | 32 |32- 5/34- 5| 36 |38- 5] 40 142. 5]44- 5

Sour orange:

(1) May 16, 1917...| 0 }....|..- Shas. deals sleeta ee Hin@ulsbeeh iO: [epetee og Sees eS Hatori] Oslvaety

(2) June 5, 1917....|...|--+ oh es Pas AN Ie FE OeecrsiOl iercatanss ° ° ° o

(3) Sept. 24, r917..|... Ome Bali Sates Be Outre [uO Onl be beat clic aval bared owelta eine
Pomelo:

(4) Sept. 28, 1917..|...| o|...J oo} *]...]... Or eal LOpTAe Hale or Ts gall [oa Ehiietat Ae tee a lsryateratebe c's c
SOU A ape Sak Lu eee tS Ea) TE NOS ER SY TERE ao ee RETR A SP EE FS ee

1 * Indicates definite infection; o, no infection.
INOCULATION OF DETACHED LEAVES

Young sour-orange leaves were detached and placed in Petri dishes,
one set containing distilled water on which the leaves were floated and
the other set containing cornmeal agar. Some of the fungus mycelium
was placed in contact with the leaves, and the preparations were allowed
to remain in the temperature chambers for 15 days. The temperatures
used were 13.5°, 16°, 18.5°, 21°, 24.5°, 27.5°, and 32° C., with a fluc-
tuation of about 0.5°. Infection took place in water at 16°, 18.5°,
21°, 24.5°, and 27.5°, but not at 13.5° nor at 32°; and in cornmeal agar
at 18.5° and 21°, but.not at 13.5°, 16°, 24.5°, 27.5°, mor 32°.
INFLUENCE OF TEMPERATURE ON GROWTH AND SPORE PRODUC:

TION OF CULTURES

In addition to the inoculation experiments described above, the rate
of growth ard the formation of spores were studied with different main-
tained temperatures. Distilled water, in which was floating a young
sour-orange leaf, and cornmeal agar were used as culture media.
Inoculation was accomplished by means of a disk (2.5 mm. in diameter)
of agar medium bearing the mycelial weft. The cultures were kept in
the dark, maintained temperature chambers for three or four days.
At the end of the period the radial extension of the hyphae from the
transferred disk, was determined. Observations were also made on the
general abundance of spores. The data obtained are brought together
in Table II. The growth rate is seen to have been very slow as com-
pared with that of many other citus fungi (4). For the time employed
and for the temperatures used the greatest extension of hyphae occurred
at 21°C. This growth rate was smaller, for each temperature, in corn-
meal agar than in distilled water with the leaf, except at the two higher
temperatures. At 32° no extension of hyphae was seen in water, while
in cornmeal agar the enlargement was about one-sixth of that obtained
at 21° for the same period.

Spores were observed on the marginal hyphae in from 24 to 48 hours in
all cases where growth was observed, except in the agar culture at 32° C.
The first examination for presence of spores was made after 48 hours in the
agar cultures. Spores were abundant at the first examination upon the
growing hyphae of all the test cultures at 21°, one of the temperatures

248 Journal of Agricultural Research Vol. XXI, No. 4

at which infection of rapidly growing sour-orange seedlings and detached
sour-orange leaves had been observed to be pronounced. Spores were
also abundant in the water preparations at 24.5° and 27.5° after 24 hours,
and in the preparations of agar at 24.5° after 48 hours; at these tempera-
tures no infection had occurred in the tests with growing plants, but a
slight infection had been observed after 10 days on detached leaves in water.

Spores that had fallen from the hyphae in certain ones of these cultures
were found to have fallen on the surface of the medium at some distance
laterally from the ends of the aerial hyphae on which they had been
borne. Apparently these spores had been ejected with considerable
force from the ends of the hyphae. In one case at 16° C. a fringe of spores
was observed, most of them at least 210 microns distant from the ends of
the outermost hyphae. This feature of spore dispersal in cultures had
previously been seen but not recorded. It was noted as most frequent
after four days in cultures growing at the edges of leaves in cornmeal
agar at 16°, 18°, 21.5°, and 24.5°.

In the 16° preparation with water and a sour-orange leaf, spores were
observed to be mostly formed on the tip ends of the outwardly extending
aerial hyphae at some distance from the floating leaf. They were nearly
hyaline at first, subsequently becoming slightly dusky. Most of them
were 1-celled, but a few were 2-celled. Detached spores were germi-
nating from the ends of spores in line with the longest axis. Hyphae were
hyaline when viewed singly, but were pinkish or flesh-colored in mass.
On cornmeal agar under the same conditions spores were forming with
from two to four in a chain, and the ejected spores were germinating
on the surface of the medium.

The influence of temperature on the vegetative hyphae was marked.
At 27.5° and 32° C. in the cornmeal agar preparations the hyphae were
broad and straight, 8 to 12 microns or more in diameter. At 23° and 21°,
on the other hand, the hyphal diameter was only one-half as great and
the hyphae were more bent and tortuous. At 18.5°, 16°, and 13.5°,
the hyphae were broader, much as at the higher temperatures.

TABLE II1.—Growth and spore formation in Petri-dish cultures of Cladosporium citri at
different maintained temperatures

Fungus floating in water with tender leaf, | In cornmeal agar.
Temperature.| Radial Spore formation. Radial Spore formation.
growth in growth in
3 days. In x day. In 3 days. 4 days. In 2 days. In 4 days.
fC: Microns. Microns.
ig: 5 02.) OWT: n'5\oT)> * yy ee a Bibel Hew i mips Few.
16 EVO Mes AO. stan Abundant.. rien As COMBINE Do.
18. 5 B85 ter. dor. i 2: Bew. 0/02/21, B16 [U.N dong Oa. Do.
21 655 | Abundant..| Abundant. 370 | Abundant!.| Abundant.
24. 5 Ae Ms MEG ULahcts Abundant.. CCTM fo iG eae ae Do.
27. vi ah icacdaey 6 LCA a Abundant!. 197'| Bewls, 22! Few.
32 oj: None. 6.0. None fers. 62 | None.. None

ot Spee found some distance laterally from ends of the outermost hyphae as if ejected with considera-
e force.

May x6,1921 Temperature Relations of Cladosporium citrt 249

TABLE III.—Number of days from inoculation to first indication of disease at different
temperatures }

Days after inoculation at temperature (°C.) of—

Sour-orange seedlings:

tr} MAY16; TOLPHIS. sha 7 nN i 7 il thie _ — —
2) Juries} LON] .gvasiis ean: 7 —bo}hiwa| — | — — — —
C2) Sent. 2A. TORT eo. cient sen 6 si) — | .51— _ — —
Detached leaves:
(4) In cornmeal agar.......| — § bono tlt | oe — _ —
CSNY WHERE Ma. oes ca ete eat 6 41— 3h 10 — IO

1 A dash indicates that no scabs developed on plant placed at the respective temperatures. A blank
space indicates that no plants were used in a given test at these respective temperatures.

DISCUSSION

Infection of sour-orange seedlings was limited to a range of about 8°C.,
16° to 23°, inclusive, in these experiments. This range could not have
been over 10° in extent under the same conditions even if more tempera-
tures had been investigated, since plants inoculated at 14° and at 24.5°
failed to become diseased. ‘This range is seen to include the temperature
(21°) at which the fungus was found to have the most rapid growth
in water and in cornmeal agar medium. The temperatures just outside
the range on the lower side (14° and below) are those at which the fungus
grew somewhat more slowly than it did at the temperatures just outside
the range on the upper side (24.5° and above). Jf, however, we consider
the experiment with detached leaves in water (Table III) in which an
exposure of 15 days was given at the different temperatures, the range
for infection, though not extended downward, was extended upward to
27.5°, a temperature at which the hyphal growth in water was approxi-
mately the same as that at the lower limit of the range, 16° (Table IT).
This may indicate that the rate of extension of the hyphae must be above
a certain minimum before infection of the host can occur, a point that
may have some significance. The rate of extension of the hyphae also
appears to have a relation to the time between inoculation and first
appearance of scab as is shown, in a general way, in Table III. This
time period, with a single exception, is shortest with temperature at or
near 21°, the temperature for maximum growth if the tests on different
dates be considered separately.

It is, of course, not to be definitely concluded that infection can not
occur with temperatures outside the range within which it was con-
fined in these experiments, but it appears probable that scab is not to
be expected with temperatures that remain outside this range. Of
course, orchard temperatures may vary so as to subject the trees to
temperatures within the range of infection for a sufficient time period to
allow the penetration of the fungus. No attempt was made to determine

250 Journal of Agricultural Research Vol. XXI, No. 4

the minimum time necessary for the fungus to become established in
the host tissue at the different temperatures listed in order to produce
scab subsequently.

The interrelation of the host and parasite also demands consideration.
Temperature, of course, influences the host as well as the parasite, and
the lack of infection in these experiments at certain temperatures may
have been due to special physiological conditions or states either in the
host or in the parasite, or even in both. It may be suggested that there
was not enough time for infection in those cases where it failed to occur.
From the experiments in which leaves floating in water were left for a
long time in contact with spores of the fungus at different maintained
temperatures, it appears that, even with long periods of time, there are
certain temperatures, not fatal to either parasite or host, at which in-
fection does not take place. At some temperatures the fungus even
grew fairly well on the surface of the leaf without producing infections
leading to scab.

The relation of temperature to infection by the scab fungus, as here
brought out, seems to suggest a satisfactory explanation for some differ-
ences in the results of inoculations previously recorded by the author (2).
A set of inoculations made in August, 1906, failed to produce scab,
while a similar set made in January, 1909, was successful. An exam-
ination of the weather records for the locality where the experiments
were made shows that the mean daily temperatures for the two weeks
following the August experiment fluctuated between a minimum of 23°
and a maximum of 30° C., while the corresponding temperature range
for the January experiment lay between a minimum of 11° and a maxi-
mum of 19°. In the former case, the daily means were above the range
for infection of seedlings in the experiments here reported except at its
minimum, 23°, while the other case shows an overlapping of the range
of daily means with the range for infection, to 19°. Since the tempera-
tures in the greenhouse where the 1909 experiments were performed
were probably higher than those outside, the successful infections in
January were almost certainly due to the fact that the temperatures
fell for the most part entirely within the range that is favorable to
infection.

The temperature relation here emphasized is probably important in
determining the occurrence of scab in the orchard and in the explana-
tion of scab epidemics. But temperature is only one of a number of
conditions, such as stage of growth, humidity, time period, etc., that
must be fulfilled for scab to occur.

Among the various conditions discussed by Grossenbacher (5) as favor-
ing the development of scab, the following general observations seem to
bear on this temperature relation.

If the air is fairly dry and the weather mostly warm and bright during the develop-
ment of the first spring flush, scab may failto develop. On the other hand, the disease

May 16,1921 Temperature Relations of Cladosporium citrt 251

often becomes very severe in groves if the weather is cold and wet during the develop-
ment of the first spring growth.

Again:

Some trees in scabby groves under observation during the past two years have
retained the late starting habit and remained practically free from the disease.

Again:

During the early spring of rors the air was very moist and cold while the first growth
was in its early stages and as a result sour scab developed in great abundance even
on high sandy land if growth was early and vigorous.

Stevens (zo) says of citrus scab:

If cool wet weather prevails at the time the new growth is putting out or at the time
the fruit has set, the disease is apt to be severe . . . in groves where scab has become
established.

In the light of these temperature experiments, the greater severity
of scab at the low spring temperatures of Florida seems to be due largely
to the fact that these temperatures fall mainly within the range for
infection, while the later spring and summer temperatures are usually
too high for infection to take place, even though other conditions are
favorable. An examination of the temperature data for Tampa, Fla.,
given in Table IV suggests that this last statement may be translated
into terms of mean temperatures, to the effect that severe infection will
usually occur when the mean temperatures are well within the infection
range and that infection will be unlikely to take place when the mean
temperatures are outside this range.

TaBLE I1V.—Mean temperatures at Tampa, Fla.

[In degrees centigrade]

Month. 1909 IgIo IQII I9I2 1913 1914 1915 1916 I9I7 1918
February....... TOS (ots. Sql ie hel Cee £e. ee ee eee ree gS
Mares cof2 36.3 EQ; §))| £6 20 20:5, | 22 10.5 | 14.5 | 27 20: §° {22
Apr Aa 23 20.5 | 23 23 2I 22 20.5 | 20.5 | 22 22

Mean....| 19.5 | 18.5 | 20 19 20 18 17 18 20 2I
Mads! ssicnuly sia 24.5.) 24. 5 1.24..5.)' 25.53 24 25 26 25 24 24.5
1 ESTES ES RNA fei 26.5 | 27 26 26 27 28 26 26.) 5 uhe27,
Paly AO. 26.5 | 26.5 | 27 28 27 27 28.5 | 28 28 28

Mean.....| 26 26 26 2O0N SH 25- Sy 200 5 toere st lens) 4 20 26. 5

The average of the monthly mean temperatures for February, March,
and April for this 10-year period was between 17° and 21° C., while the
average of the monthly mean temperatures for May, June, and July was
between 25.5° and 26.5°. All the monthly mean temperatures for the
first period were well within the infection range for scab, while all the

252 Journal of Agricultural Research Vol. XXI, No. 4

monthly means for the second period are well above the infection range
as determined by the inoculation experiments.

It is of interest to note also that the lowest average for February,
March, and April was 17° C. in 1915, the year mentioned by Grossen-
bacher (5) as one in which scab developed in great abundance on the
growth starting in early spring. These data taken in connection with
the maintained temperature experiments seem to explain the frequent
absence of scab on tender growth of the second or third cycles even in
the moist periods of summer. The spores may be present, the moisture
and growth conditions may be favorable, but the temperature may be
too high.

The occurrence of severe late scab in some seasons is probably due to
a decided drop in the temperature, bringing it well within the infection
range for a sufficient length of time for infection.

SUMMARY

(1) Experiments to determine the influence of temperature on scab
infection on young sour-orange (Citrus aurantiwm) leaves and on the
growth and spore formation of the causal organism (Cladosporium citrt)
are reported.

(2) The inoculation temperatures resulting in infections of growing
plants under conditions of rapid growth and abundant moisture were
16°, 18.5°, 19°, 20°, 21°, and 23° C. No infections were obtained under
the same conditions on plants inoculated at 12°, 13.5°, 14°, 24.5°, 26.5°,
27.5. 1) B14 Bs 82-5) B45; 30°) O- ay AO wae Sea Aa 57,

(3) Detached leaves floated in water with the scab fungus were infected
at\16°,.18.5°, 2B?;.2aig?) andsamig hs

(4) The temperature at which the greatest extension of hyphae of
the causal organism in cultures was observed was 21° C. The highest
temperature at which extension was observed in water was 27.5°, and
in cornmeal agar 32°.

(5) Spores were observed in 48 hours or sooner in all the temperatures
at which growth took place except 32° C. At certain temperatures the
spores appeared to be ejected with considerable force from the ends of
the hyphae.

(6) The temperature at which the time was shortest between inocu-
lation and first observance of signs of disease was usually 21° C. in the
different tests. This time increased toward the upper and lower limits
of the infection range.

(7) This limited range of temperature at which infection of a sus-
ceptible host took place under the presumably favorable conditions of
the experiment appears to explain the great differences Observed in
the occurrence and severity of scab from year to year and from season
to season in citrus orchards. It also explains the differences in results
of previous inoculation experiments not hitherto understood.

May 16,1921 Temperature Relations of Cladosporium citri 253

(8) The conditions necessary for scab infection indicated by these
experiments are (1) viable spores of the fungus, (2) young citrus leaves
of a susceptible species, (3) moisture, and (4) temperatures between 16°
and'23°)C,

LITERATURE CITED
(x) Fawcert, H. S.
IgI0. CLADOSPORIUM CITRI MASS. AND C. ELEGANS PENZ. CONFUSED. In Myco-
logia, v. 2, no. 5, p. 245-246.

(2)
1912. CITRUS scaB. Fla. Agr. Exp. Sta. Bul. 109, p. 51-[60], fig. 24-31.
References, p. [60].
(3)
1916. CITRUS scAB. In Phytopathology, v. 6, no. 6, p. 442-445.
(4)

THE TEMPERATURE RELATIONS OF GROWTH IN CERTAIN PARASITIC FUNGI.
Univ. Cal. Pub. Agr. Sci. (In press.)
(5) GROSSENBACHER, J. G.
1916. SOUR SCAB OF CITRUS IN FLORIDA, AND ITS PREVENTION. In Phytopath-
ology, v. 6, no. 2, p. 127-142, 4 fig.
(6) HesLER, L. R.
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(7) Livincston, B. E., and Fawcett, H. S.
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1918. FLORIDA CITRUS DISEASES. Fla. Agr. Exp. Sta. Bul. 150, 110 p., 54 fig.

1919. REPORT OF PLANT PATHOLOGIST. In Fla. Agr. Exp. Sta. Ann. Rpt.
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and WEBBER, Herbert J.
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