[Extracted from the Proceedings of the Philadelphia Academy of Natural Sciences; Sept. 28,
1886, pp. 322-341. The original paging of the volume is placed at the
bottom of the pages.]

HISTORY AND BIOLOGY OF: PEAR BLIGHT.

By J. CG. ARTHOUR.

(A thesis presented to the Faculty of Cornell University for the Doctorate in Science,
June, 1886.,

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HISTORY AND BIOLOGY OF PEAR BLIGHT.
BY J. C. ARTHUR.

To the American orchardist or nurseryman the name of pear
blight, or fire blight, as it is often called, brings to mind a serious
malady of fruit trees, which has been the theme of incessant dis-
cussion by horticultural writers and speakers since the earliest
days of fruit culture in this country. The most marked features
of the disease were admirably characterized by William Coxe! at
the beginning of the present century, in the following words:
“That species of blight which is sometimes called the fire blight,
frequently destroys trees in the fullest apparent vigour and health,
in a few hours, turning the leaves suddenly brown, as if they had
passed through a hot flame, and causing a morbid matter to exude
from the pores of the bark, of a black ferruginous appearance ;
this happens through the whole course of the warm season, more
frequently in weather both hot and moist.’’ The disease occurs
from Canada and Minnesota on the north, to Georgia and Louis-
iana on the south, and from the eastern limit of the Rocky
Mountains to the Atlantic ocean. No part of this vast extent of
country is exempt, although it does not appear with the same
frequency and power in all localities, and is usually rare in the
immediate vicinity of the sea-coast.

So far as at present known, it is exclusively confined to this
part of North America. This is partly inferred from the absence
of any distinct mention of such a disease in the horticultural
literature of other regions, and partly from direct testimony.
Prof. Dwinelle, late of the University of California, has told the
writer that it does not occur on the Pacific coast. Dr. De Bary,
whose word carries great weight, says, after giving a brief
description of the disease, ‘this phenomenon is not to my knowl-
edge known in Europe.” A long account of the disease has been
published by Dr. Wakker,’ in a gardening journal of Holland, in
order to learn if it occurs in that country, but up to the present

1 Cultivation of Fruit Trees, Philadelphia, 1817, p. 174.
? Voriesungen tiber Bacterien, 1885, p. 137.
3 Nederlandsche Tuinbouwblad, II (Jan. 9, 1886), p. 9.

(322)

6 HISTORY AND BIOLOGY

time no one has intimated any knowledge of it. In a recent
letter Dr. Masters, editor of the Gardeners’ Chronicle of England,
says that no such disease has been recognized in the British Isles.
The testimony of one of our own horticulturists, Prof. Budd,! of
Iowa, who is familiar with the disease in this country, and has
inspected the orchards of the old world far into Russia, is especi-
ally valuable; he says ‘no trace of blight of pear- or apple-trees
can be seen in Europe.” From these statements, and the infer-
ences to be drawn from other sources, it appears highly probable
that the disease does not extend to Europe. An account of the
principal diseases of fruit-trees of New Zealand, by Prof. T. Kirk,?
has been published, which describes a disease of the pear known
in that country as fire blight, due to a fungus, and another of the
apple, the American blight, due to an insect. No true pear blight,
as recognized in the United States, is mentioned, and in a recent
communication the author has definitely stated that it is not
known in the colony. Whether it occurs in other parts of the
world is not yet ascertained, if some slight testimony regarding
its absence in Japan be excepted.

It is only within a year or so that European writers have become
aware of its existence, and this only through American authors.
It is remarkable that a disease of such virulence and so easily
transported should not have found its way across the ocean, when
one remembers the number of destructive plant maladies that
America has already involuntarily foisted upon European culti-
vators. It will not be profitable to speculate much at this time
upon the reasons for this, but we may suppose that the small
exportation of American fruit-trees, or of scions,? has been a
factor in keeping it in check. The influence of climate, and some
less evident factors, need not be discussed in this connection.

Amount of Loss.—It has already been intimated that pear
blight is a frequent and destructive affection ; it will tend to give
a fairer appreciation of the subject if it be stated how frequent
and how destructive it is. Coxe,‘ as early as 1817, in the oldest
pomological work by an American author, says it “ frequently

' Trans. Minn. Hort. Soc. for 1883, p. 281.

? Fruit Blights and Diseases of Fruit-trees in New Zealand, 1885.

* For an account of the destruction of stored scions by blight, see Rep.
Hort. Soc. of Mich. for 1881, p. 137. + Tker pera.

(323)

OF PEAR BLIGHT. “(

destroys trees in the fullest apparent vigour and health, in a few
hours; I have in twenty years lost upwards of fifty trees.”” The
years 1826 and 1832 were notable in horticultural circles for the
increased prevalence of the disease; but it was in 1844 that the
most widespread and fatal epidemic, that the country has yet
known, occurred. Few, if any, pear-orchards escaped at that
time without the partial or total loss of many trees, and some
orchards, even large ones, were quite destroyed. The following
year the epidemic was much lighter, and had fully disappeared
by 1846. Although it had subsided as an epidemic, it still
occurred in localities here and there, and has continued to do so
until the present time. Judging from the communications in the
horticultural press, the whole country, or various sections of it
independently, have been subject at various times to epidemic
visitations, but none have equaled in severity that of the memor-
able year of 1844.

It is often maintained that a certain periodicity of occurrence
is observable, the periods usually being placed at five, ten, or
twenty years. <A careful examination of the literature of the
subject, however, gives little support to these views, and makes
it more probable that the intervals are irregular, and that they
vary for different sections of the country. The year of maximum
prevalence may or may not be preceded by one in which the
disease is noticeably common, but it is quite invariably followed
by a year or two of successive decadence.

In the absence of exact statistics, which it has not been prac-
ticable to obtain, something of the important nature of the disease
may be gathered from the statements of horticultural writers and
the phraseology which they employ in speaking of it.

The renowned horticulturist, A. J. Downing,! called it the
“monstrous malady of the pear.” Chas. R. Baker? says it is
“the worst malady with which the cultivator of the pear-tree has
to contend.” In southern Pennsylvania ‘‘ the pear is so generally
destroyed by the blight,” according to J. B. Garber? writing in
1850, “that very few trees are to be found.” At Philadelphia,
however, the disease has been rarely observed, according to

1 Horticulturist, vol. i, 1846, p. 62.
? Practical and Scientific Fruit Culture, Boston, 1866, p. 476.
3U. 8. Patent Office Report for 1850, pt. ii, p. 418.

(324)

8 HISTORY AND BIOLOGY

Thomas Meehan.! T. T. Lyon,? of Michigan, states as the
opinion of many cultivators in that State, that the pear-tree can-
not be grown with financial success on account of the blight.
Illinois has always been much subject to’ the disease, and Prof. J.
B. Turner,’ in 1868, gave expression to the general feeling of his
region by describing it as “that deadly Upas of the pear-tree
known par excellence as the pear-blight.” In 1882 Dr. J. L.
Hallam,’ speaking for southern [llinois, says, “ pears have failed,
utterly failed, so that none are now cultivated for market, the
blight has destroyed the trees—branch and root,” and 8. G.
Minkler,’ in the northern part of the State, observes that it is a
very uncommon thing to see pear-trees without dead branches or
other signs of the ravages of blight. Wm. A. Nourse,® of the
same State, is led to “doubt if one-tenth of the pear-trees that
are set, live ten years,” on account of this one destructive agent.
Geo. M. Dewey,’ of Missouri, says that ‘‘ with good cultivation
and rich soil the pear generally dies of blight before the eighth
year.’’ In Minnesota the severe climate has not permitted the
cultivation of pears, and almost the only apples grown for many
years were the hardy crab-apples. The latter have been rapidly
improved, and together with the hardier varieties of the common
apple would now furnish this part of the country with an abundant
supply of fruit, were it not for this same disease, which elsewhere
most conspicuously preys upon the pear-tree. KE. H.S. Dartt
held the opinion in 1874 that the severity of winter was not so
much to be dreaded as the ravages of blight. He had at that
time one or two thousand trees affected. Dr. P. A. Jewell,® up to
1876, had lost one thousand Tetofsky apple-trees by it. F. G.
Gould! says that “‘ only for this scourge every family living on a
farm in Minnesota could have a supply of apples.”

1 Rep. Penn. Fruit-Grower’s Soc. for 1877, p. 77.
2 Rep. Pomol. Soc. of Mich. for 1878, p. 368.

° Trans. Ill. Hort. Society for 1868, p. 42.

4 Same for 1882, p. 118.

5 Same for 1882, p. 30.

6 Same for 1880, p. 638.

™ Proc. Mo. Hort. Soc. for 1870, p. 18.

§ Trans. Minn. Hort. Soc. for 1874, p. 22.

® Same for 1876, p. 73.

10 Same for 1884, p. 127.

(825)

OF PEAR BLIGHT. 9

Citations enough have doubtless been given, although several
pages of equally strong ones might be added, to show that fruit-
growers, who have the best opportunities for observation, con-
sider it a disease greatly to be dreaded and one of special economic
importance. Other sections of the country, notably those of
Ohio, western New York and Georgia, could furnish equally im-
portant proof of these propositions. All that is desired in this
connection, however, is to give those not familiar with the subject
some idea of the disease and its effects as ordinarily observed.

. Early Records.—The oldest mention of the disease, that gives
a good and reasonably full description of it, is in Coxe’s work on
fruit-trees, bearing the date of 1817. The manner of the author
leaves no doubt that it was well known at that time, and the
reference to his losses during twenty years makes it reasonably
certain that he had observed the disease as early as the opening
of the century. The earliest notice, however, which has yet come
to hand, is in a letter written by Wm. Denning,’ describing the
disease in apples, pears, and quinces. He speaks of first observ-
ing it on the Highlands of the Hudson in 1780.

There is no interest, however, in tracing chronologically the
various notices found in different publications, for without excep-
tion they have the tone of treating a familiar theme, and show no
evidence that the disease in the first part of the century was in
any respect different from to-day.

Conjectures Regarding its Cause.—A brief treatment of this
topic will be all that is required for the purposes of this paper;
and only those hypotheses will be touched upon which received
such careful presentation as to attract the favorable attention of
the public.

Few writers appear to ascribe the disease to a single agency,
but regard it as resulting from several causes, either acting to-
gether or brought about by dissimilar circumstances. Little dis-
crimination is made between predisposing conditions and active
agents. In fact sharply defined treatment could not be expected
when all was conjecture, and when the shrewdest observers did
not hesitate to avow that after years of loss under all kinds of
experimentation, and after interminable discussions, the cause
still lay shrouded in impenetrable obscurity.

1 Trans. Soc. for Prom. of Agric., pt. ii, 1794, p. 219.

10 HISTORY AND BIOLOGY

Coxe,! who has had many followers, thought that the hot rays
of the sun when acting through a misty or saturated atmosphere
deranged the vital activities of the plant and brought about the
disease. He considered old varieties more subject to it, on
account of having lower constitutional vigor, than new varicties,
of which the St. Germain and Seckel were respectively conspicu-
ous instances.

The insect theory, as it was called, was promulgated at this
time. It was started upon firm facts by the discovery of a small
brown beetle, about two millimeters long, which penetrated the
branch, and caused the part beyond to die. The beetle received
the name of Scolylus pyrt Peck, now changed to Xyleborus pyri
(Pk.), and is still known as the blight beetle. The effect of its
attack appears to the casual observer similar to that of the true
blight—the branch in June or July rapidly withers, and the leaves
and fruit turn black. The beetles being minute and inconspicuous
escape attention, and the fact that the branch does not die below
a definite point is sometimes overlooked. It is not difficult to
see how many persons came to connect this comparatively rare
affection with the common fire blight, and to believe that insects
of some sort were to be held accountable for all—their supposed
minuteness and wary habits being sufficient reasons for the failure
to find them, and the spread of the disease along the limbs ofa
tree being ascribed to a poison which the insects were supposed
to emit. Among the prominent supporters of this view was the
‘“‘ Genesee Farmer,’’? published at Rochester, N. Y., with Patrick
Barry as the horticultural editor. It has not, however, been so
strongly advocated for the last decade or two.

The next hypothesis that attracted general attention was known
as the frozen-sap theory. This was based upon the supposition
that the autumn or winter freezing of unripe wood produced a
poison which the moving currents of sap the next spring and
summer distributed, causing the death of the parts. It was first
published in 1844 by Rev. H. W. Beecher,’ of Indiana, in a long
and able article in ‘‘ Hovey’s Magazine.” In the following year

2 Tieec.s ps las

2See Genesee Farmer, vol. vii, 1846, p. 217; vol. viii, 1847, pp. 122,
218, etc.

8 Magazine of Horticulture, vol. x, p. 441.

(3217)

OF PEAR BLIGHT. ll

it was independently elaborated by A. J. Downing! in his work
on Fruits and Fruit-trees of America, who first called it the
frozen-sap theory, and who is usually spoken of as the author of
it. This view has probably had more firm adherents than any
other, as it explained many phenomena connected with the dis-
ease in a fairly satisfactory manner. It was especially well
received in the western States.

The next hypothesis which gained the attention of the public
was the fungus theory. Its first successful presentation was in
1863 by Dr. J. H. Salisbury,? who fizured the fungus which he
decided to be the specific cause of this kind of blight, and ven-
tured to give it a name, although he was sadly in error in most that
he did. Thomas Meehan,’ editor of the ‘‘ Gardener’s Monthly,”
has ably championed this explanation, and done much to keep it
in favor. In 1875, Dr. J. G. Hunt,* by Mr. Mcehan’s request,
undertook a microscopical examination of blighted pear-twigs,
and confirmed the opinion that it was due to a fungus, without,
however, deciding upon the specific character of it.

Blighted trees often attract attention immediately after a
thunder storm, and from this and other circumstances the belief
that the malady is due to electricity has gained many adherents,
but the argument has not had a full and connected presentation.

The last hypothesis of historical importance is the bacterial
theory. Although hinted at by a number of horticultural writers,
yet the credit of it is due to Prof. T. J. Burrill,® who in 1878 dis-
tinctly stated his belief that the cause resides with the bacteria
which he found in great abundance in the tissues of affected
branches, In 1880 he performed a series of experiments® by
inoculating healthy branches with the juices of diseased ones, the
results of which were presented to the American Association at
its Boston meeting, thus first bringing the subject clearly to the

1 Fruits and Fruit-trees of America, p. 324; same, 2d Revision by Chas.
Dowuing, p. 646.

2 Ohio Agric. Rep. for 1863, p. 450.

3 Proc. Amer. Pomol. Soc. for 1867, p. 59 ; and elsewhere.

* Gardener’s Monthly, vol. xvii, 1875, p. 245.

° Trans. Ill. Hort. Soc. for 1878, p. 80.

6 Proc. Amer. Assoc. Adv. Sci., vol. xxix, 1880, p. 583; Rep. of Il.
Industrial Univ. for 1880, p. 62; Trans. Ill. Hort. Soc. for 1880, p. 157;
Amer, Naturalist, vol. xv, 1881, p. 527.

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12 HISTORY AND BIOLOGY

attention, of the scientific world. Although this was now the
popular hypothesis, it cannot be said to have received more sub-
stantial credence than those which had gone before, either from
the horticulturists or the scientists. The experimental results
gained by Prof. Burrill were confirmed and extended by the
writer ! during 1884, by means of a similar series of inoculations.

Of the multitude of minor hypotheses which were put forth in
explanation of phenomena connected with pear-blight, and which
were variously received, and of all degrees of plausibility, it is
impossible to speak at this time without carrying this paper to
undue length.

Beginning of Experimental Research.—The question of the
cause of pear-blight was finally removed from the domain of
speculation to that of established fact by a series of crucial
experiments performed by the writer? a year ago, and recorded
in a paper before the American Association at the Ann Arbor
meeting. These consisted in showing that the bacteria when
removed from the tree and passed through a series of artificial
cultures would generate the disease when again introduced into
the tree, and that the juices accompanying blight when cleared
of bacteria by filtration will not produce the disease.

Having now come toa firm basis for scientific advancement, let
us look over the historical ground again to see if some one did
not hit-upon the true explanation of the disease, although he may
not have been aware of its significance. In a connection like
this, facts derived from experiment have greater weight than
statements of opinion; the latter acquire importance in propor-
tion as they are logically derived from correct and close observa-
tion. Bearing this in mind, we need not give much heed to the
not uncommon inference that pear-blight was in some way inti-
mately related to the epidemic diseases of man, e. g. cholera.
This view is said to have been quite frequently entertained in the
early part of the century, but was not sanctioned by the learned.
The use of such phrases as “ first cousin to the cholera,” “a spe-
cies of vegetable ferment,” etc., surely does not entitle the author
to any priority in way of discovery.

1 Rep. N. Y. Agric. Exper. Station for 1884, p. 357.
2 Proc. Am. Assoc. Adv. Sci., vol. xxxiv, 1885, p. 295 ; Bot. Gazette, vol.
x, 1885, p. 343 ; Gardeners’ Chronicle, vol. xxiv. 1885, p. 586.

(329)

OF PEAR BLIGHT. 13

We turn from these slight hints to the record of an experiment
in inoculation, made in 1845 and published the following year.
We are told by S. B. Gookins,! of Indiana, that visiting Mr.
Ragan (the same person who furnished Mr. Beecher with many
of the facts on which he founded the theory to which we have
already referred) he was shown a thrifty young pear-tree in the
nursery, which had been “ inoculated ” “by way of experiment”
with ‘the sap of a blighted tree,” “a few days previous.” ‘ He
made an incision about three feet from the ground, lifted the
bark as in the process of budding, and injected a small quantity
of the diseased sap.” ‘‘ We found the leaves of the patient chang-
ing color, and emitting that peculiar odor which is always present
in cases of blight, and upon applying the knife, the inner bark
was found to be black from the root to the top, while nothing of
the kind appeared elsewhere in the nursery.”

This admirable experiment was combined with a no less admir-
able interpretation of the cause of blight. The writer cites facts
to disprove the hypothesis of Mr. Beecher, and then says: “I
strongly incline to the belief, that the pear-blight is an epidemic,
that it prevails like other epidemics, and will pass off like them.
The atmosphere is, I believe, generally admitted to be the medium
by which they prevail, and are carried from place to place. What
that subtle principle may be, which pervades our atmosphere, by
which infection is retained and transmitted, human science has
not discovered; but that such a principle exists is sufficiently
obvious from its effects.”

This clearly conceived elucidation of the matter could only
have been improved by a knowledge of the germ theory of disease,
and when we remember the date at which it was uttered, we do
not feel that the writer was guilty of any lack of acuteness in not
perceiving the relation which we now know to exist between his
theory and his facts. He seems to have been a modest man, for
he only signs his initials, and does not defend his views when the
editor, A. J. Downing,? opposes the opinion that ‘an epidemic
conveyed by the atmosphere is too slightly supported by facts to
weigh at all against the observations of cultivators,” which
“strongly point to the freezing of the sap as the cause.”

1 Horticulturist, vol. i, 1846, p. 253.
2 Same, p. 255.

(330)

14 HISTORY AND BIOLOGY

Another interesting experiment was performed by Dr. E. S.
Hull,’ of Illinois, in 1870. Having received some blighted apple
twigs from a correspondent, he cut pieces from them with which
he “inoculated several succulent pear shoots by tying in the pieces
as in budding.” This was done the middle of June, and no ob-
servation taken for thirty-four days, when the blight was found
to have extended several inches into the healthy tissues. From
this he very justly concludes that the blight in apples and pears
is but one disease, but seems to take it for granted that in both
it is due to “ vitiated sap.”

The fact that the disease may be transferred to healthy trees
by the pruning knife has been observed by several persons. H.
Wendell,? of New York, says in 1849, “I am also careful that the
blade of the knife is perfectly clean, and that it has none of the
sap of a diseased tree adhering to it, because I have known many
valuable trees destroyed by having been inoculated in this man-
ner.” Prof. Turner,’ of Illinois, makes a similar statement: ‘“ I
found that this disease is exceedingly contagious, for if I used
my knife to prune a healthy tree after having used it in shaving
the diseased one, I communicated the disease to that tree.”

Prof. Burrill first observed the bacteria of ‘blight in 1877,
but did not recognize them as such till the following year,> when
he avowed his belief that they were the cause of the disease.
His first inoculation experiments were made in 1880, as already
stated. In 1882 he characterized the organism under the name
of Micrococcus amylovorus.®

Description of Micrococcus amylovorus Bur.—The form of
this species of bacteria is very constant, under all conditions.
The single cells are from oval to roundish-ovoid, and only vary
by slight changes in the ratio between their length and breadth

1 Trans. Ill. Hort. Soc. for 1870, p. 220.

*U. S. Patent Office Rep. for 1849, pt. ii, p. 447.

3 Trans. Ill. Hort. Soc. for 1878, p. 81.

* Same for 1877, page 114.

® Same for 1878, page 79.

6 The Bacteria (a reprint from Rep. of Ill. Industrial Univ. for 1882),
p. 42; Amer. Naturalist, vol. vii, 1883, p. 319. In the last publication,
by a typographical error, the name was made to read M. amylivorus, a
mistake which has been copied into other works—see Grove’s Bacteria
and Yeast Fungi, London, 1884, p. 10.

(381)

OF PEAR BLIGHT. 15

(Pl. ITT, figs. 1, 2,6). They are 1 to 1} long, by 4 to 3» broad,
and quite colorless. For the most part, they exist as single inde-
pendent cells, but may often be found in pairs, especially when
still multiplying, and in rare instances are united into a series of
four or even more, but never extend into chains.

During rapid vegetation, in rich nutritive media, the move-
ments reach a stage of extreme activity. The appearance is
what is termed swarming, in which the bacteria move rapidly
back and forth, in and out among each other, but never in a
straight line to any distance. As the rate of growth becomes
less from any cause the movements are retarded. Taken directly
from the tissues of a blighting tree, the movements of transla-
tion are usually sluggish or imperceptible, although the universal
Brownian movement is likely to give a misleading appearance of
activity. Under specially favorable conditions, as when grown
during hot weather in very succulent shoots, or from artificial
inoculation in unripe fruit, the movements are much increased
and may become quite rapid. When taken from the tree in
winter, or when grown in solutions that are too acid or too alka-
line, or which are deficient in the proper nutritive substances,
there is no perceptible locomotion.

When in active growth, the cells present a uniformly dull
appearance. By conditions which are unfavorable to normal
growth, yet do not entirely check it, such as strongly acid or
alkaline solutions, deficient nutriment, or exhaustion by keeping
the cultures several months, the cells become highly refractive,
and to some extent take on the appearance of the spores of
other species of bacteria. Whether in this state they possess
any of the characteristic powers of resistance which belong to
spores, has not been ascertained.

Formation of Zooglea.—By far the most characteristic feature
in the life history of Micrococcus amylovorus is the formation of
zooglwa (figs. 2,3,5). These have never been observed in the
tissues of the tree under any conditions, or in or upon any sort
of solid media, but they occur with much regularity in fluid
cultures, when placed under favorable conditions for rapid
growth.

They are produced to some extent throughout the fluid, but
are’ most abundant in the thin pellicle which forms upon the
surface, appearing within forty-eight hours from the beginning

(382)

16 HISTORY AND BIOLOGY

of the culture. The substance of the pellicle consists of a color-
less matrix uniformly filled with motionless bacteria, and against
this the zooglea are sharply defined. They are often brought out
yet more distinctly by being surrounded by a colorless layer, free
of bacteria, which is doubtless an extension of the ground sub-
stance of the zoogloea mass (fig. 4).

The masses are far more dense than the pellicle, and are com-
pactly filled with refractive bacteria. They possess a definite
outline, and are recognizable when very small; and although
they may reach 30 to 40 » long by 20 to 30 » wide, they rarely
lose their distinctness. When below 10 » in length, their usual
form is oblong, varying to globular. They occur singly, or united
more or less intimately end to end in pairs, and sometimes several
form a short chain. At this stage they possess a uniformly even
and unbroken surface, which now becomes uneven and wrinkled,
and is finally thrown into folds, giving some resemblance to the
external aspect of the brain. Zoogloea more than 20 » in length
have the folds of somewhat unequal height, and the sinuses deeper,
giving a stronger cerebric look, or when the folds are small and
circular, they are better described as mulberry-like. The elon-
gated forms, which at some stage of growth might doubtless have
been composed of two or more distinct masses, often take on a
vermiform appearance. But whatever the variations may be, the
distinctness of outline, the general form, and the cerebric surface
are unfailing characters, which so far as my knowledge extends,
are not found in any other species of bacteria.

Cultivation in Fluid Media.—The range of substances which
may serve as culture media for this organism is very wide. An
infusion of almost any vegetable substance containing a fair
amount of soluble carbohydrates is likely to be sufficient to enable
growth to take place, even if not very luxuriantly.

The substance which on the whole has proved most satisfactory
is an infusion of potato. This is prepared by paring a potato and
slicing it into three or four times its bulk of water. Thisis kept fora
couple of hours at about 70° C., by placing it over a water-bath,
during which time it is occasionally stirred. It is then filtered,
and is ready to be placed in the culture vessels for sterilizing and
use. Ifthe heat is allowed to rise much above 70°, the starch is
gelatinized, and it is only with difficulty that the solution can be
filtered. The resulting liquid is clear and watery, but is often

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OF PEAR BLIGHT. 17

- light brown from coloring matter contained in the potato, which
does not, however, materially interfere with observations on the
growth of bacteria in it. Iodine gives a blue coloration to this
liquid, showing that it contains starch, probably in the form of
amylon. Another equally good culture fluid is made by treating
corn (maize) mealinasimilar manner. The solution is colorless,
but it is very apt to throw down a troublesome sediment, which
makes it less desirable to use than the potato solution.

Test-tube or flask cultures with these liquids, when kept at a
temperature of 25° to 30°C., usually show some turbidity in
twenty-four hours after being infected, and if the growth is very
rapid, bubbles of gas (CO,) will be given off, which collect at the
surface into a slight froth. In forty-eight hours the liquid has
become thoroughly turbid. By this time a thin whitish pellicle
has formed on the surface, which does not increase much in thick-
ness up to the end of active growth, and rarely becomes wrinkled.
With the formation of the pellicle, a sediment gathers at the bot-
tom of the liquid, often a centimeter in depth, but which is so
lignt that it only apparently differs from the liquid above by being
whiter. In the. course of some weeks this sediment will mostly
gather upon the bottom of the vessel. No difference has been
observed in the appearance of the bacteria taken from different
parts of the culture. Those imbedded in the pellicle are not
arranged in any recognizable order.

In proportion as liquids are less suitable to the growth of
the organism, the visible changes are less. The pellicle may not
be formed, and there may be no turbidity, but if any growth at
all takes place there will be some evidence of it by formation of
a slight sediment. But the occurrence of a precipitate does not
necessarily imply growth, for it not infrequently separates from
a liquid containing organic matter, although remaining perfectly
sterile.

An infusion of hay, and also of dead, partly decomposed
grass from a marsh, gave nearly a normal growth of blight
bacteria, but the cells were considerably more refractive than
usual.

A solution of starch, having one part of starch to fifty of
water, gave but a slight growth of highly refractive bacteria,
without a pellicle, turbidity, or zooglea. A strong decoction of
old barnyard manure acted in the same manner. A solution of

2 (384)

18 HISTORY AND BIOLOGY

one part of glucose to fifty of water gave no growth of the
bacteria.

In testing the effect of acids upon the development of blight
bacteria, a 4 per cent. of malic acid was added to the usual infu-
sion of potato. This prevented the formation of a pellicle, tur-
bidity or zooglea, but gave a very considerable cloudy sediment,
largely made up of loosely aggregated groups of blight bacteria,
which were brilliantly refractive. A similar solution with 2 per
cent. of malic acid gave a slightly less abundant sediment, but with
otherwise the same results. Some of the latter was transferred
to a corn-meal solution, producing the characteristics of a pear-
blight culture, except the formation of zooglea. After some
days this was introduced into a pear tree, which in due course of
time gave the true blight, showing that the bacteria of the acid
solution were really blight bacteria. Attempts to grow them in
a nutrient 5 per cent. solution of citric and tartaric acids have
not been successful.

Testing the nature of the bacteria in cultures producing
limited growth, by inoculating directly into the tree, has not, as a
rule, proved successful, as for some reason they seem unable to
gain a footing in the living tissues. It is therefore necessary to
transfer them first to richly nutrient cultures, from which, after
a time, they may be introduced into the tree, and, if the blight
bacteria are present, will start the disease.

Cultivation in Solid Media.—In test-tube cultures with nutrient
gelatine the most characteristic results have been obtained by
adding a drop containing blight bacteria to the gelatine while
liquid, and thoroughly distributing the germs by shaking the
tube. In from two to three days the gelatine contains numerous
small white dots, which, upon examination under the microscope,
prove to be a mass of bacteria of the usual appearance. The
dots are globular or oval, and increase to about .5 mm. in
diameter. No further growth or change takes place, and in this
condition they remain for weeks, without liquefying or otherwise
affecting the gelatine.

When sown upon the surface of the gelatine by drawing a
needle or glass rod over it, or by placing a drop on it, the growth
is feeble and does not amount to more than a slight shining
appearance of the surface.

A nutrient solution made from an unripe pear, in which blight

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OF PEAR BLIGHT. 19

germs were well distributed by shaking and then left undis-
turbed for two days, gave the same isolated white dots as in
gelatine; but they dropped to the bottom of the liquid upon
being jarred. The bacteria were evidently prevented from
moving freely by the jelly, which was not, however, thick
enough to keep the groups in place when its cohesion was
once disturbed. Fruit jellies, doubtless, may be found to be
convenient media for the cultivation of this species of bac-
teria.

No success has been attained in the use of agar agar, but
whether due to a want of adaptability in the substance, or to
wrong manipulation, must be left to future experiments to
determine.

The opaque solid cultures proving most successful have been
conducted upon freshly gathered unripe pears. Slices of these
are placed under a moist bell-jar, and infected by touching with
a needle that has been dipped in some substance containing the
bacteria. In two or three days fine milky drops, like beads of
dew, will appear scattered over the surface for 5 mm. or more
about the infected spot. These will become somewhat larger
after a time, while the spot which received the infection will turn
slightly brown, the tissues gradually wasting away and forming
a small depression.

If, however, the slices, having freshly cut surfaces both above
and below, are laid upon a plate with a little water, and placed
under a bell-jar, the result is not the same. The dew-like drops
appear within forty-eight hours, as in the other case, but increase
rapidly in size, while a drop is also formed at the point of infec-
tion. Drops finally appear over the whole surface of the slice.
They remain more or less distinct, and soon become as large as
a pea, retaining the globular or rounded form to a remarkable
degree. Microscopically they are composed of the usual form
of blight bacteria, suspended in a colorless fluid. After about a
week, the drops coalesce and the tissues of the pear begin to
break down. This sort of culture requires no precautions of
sterilizing, as no other bacteria can multiply upon it till after the
cells of the pear begin to die.

When blight bacteria are sown upon slices of baked or boiled
potato, they spread out over the surface in a thin, slightly moist
layer, which is usually somewhat yellowish, but do not grow

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20 HISTORY AND BIOLOGY

readily, or produce a characteristic appearance. Under the
miscrope the cells are strongly refractive.

A boiled potato was infected by thrusting a platinum wire,
smeared with blight bacteria, into one end. After sixteen days
it was cut open. No external change had taken place, and, to
the unaided eye, no internal change either; the odor and texture
were still those of a freshly boiled potato. The microscope, how-
ever, revealed the blight bacteria in every part of the potato, in
irregular motionless masses, and with more than the usual’
refractiveness.

These opaque solid cultures have brought out one fact very
distinctly, which is, that Micrococcus amylovorus requires a
large supply of water for its best development—a fact which has
an economic bearing.

Behavior toward Staining Fluids.—So far as trial has been
made, nothing especially characteristic has been detected to dis-
tinguish this form of bacteria from the majority of micrococci.
The most successful results have been obtained with a watery
solution of Bismarck-brown, especially in cover-glass prepara-
tions. These make excellent specimens when mounted in
Canada balsam.

The zooglea are inclined to be too deeply stained by this pro-
cess, and for most purposes they are best studied unstained.
They may be well preserved by mounting in glycerine.

Hematoxylin has also given good results, but has not been
found particularly useful.

Chemical Products.—The chemical changes brought about by
the activity of the blight bacteria have not yet been fully and
carefully worked out. The most obvious product is carbon
dioxide, which often passes off so freely from a cultivation as to
produce a slow effervescence. Butyric acid and alcohol are
formed in very small quantities, if at all. The tests by which
these facts have been determined have already been published,!
and need not be repeated here. Vigorous cultures of the bacteria
in infusion of potato give no reaction for glucose with Fehling’s
solution; and blighting tissues from the tree give no indication
by the same test of more than the normal amount of glucose to
be found in healthy tissues. On the other hand a quantitative

1 Rep. N. Y. Agric. Exper. Station for 1885, p. 247; and less fully in
Amer. Nat., vol. xix, 1885, p. 1181.

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OF PEAR BLIGHT. 21

determination of sound and blighting pears, taken from the tree
at the same time, shows considerable less sugar in the latter.

A favorite explanation with horticulturists of the action of fire-
blight upon the pear-tree, has been to say that the sap is poisoned.
This poison was supposed to be introduced by insects, or to be
due to some disorganization of the tissues. Although it is now
known that specific bacteria are directly answerable for the dis-
ease, it is yet worth while to sce if the old idea of a poison has
not some foundation in fact.

It has been ascertained that certain bacteria produce during
their growth, characteristic poisons which are classed under the
name of ptomaines. Most of the ptomaines are non-volatile, and
readily soluble in water or alcohol. The chemical tests which
are applied for their detection cannot be considered conclusive
except when taken collectively. The tests tried below are among
the most satisfactory known at present.!

A cultivation in infusion of potato, giving about 200 ce. of liquid
was filtered, and the filtrate evaporated to a syrup. This was
treated with alcohol, and the solution tested with the most char-
acteristic test for ptomaines—the reduction of potassic ferric-
cyanide. Other portions of the solution were successively tested
with phospho-molybdic acid, potassio-mercuric iodide, and iodine
in potassic iodide, all of which failed to give any distinctive
reactions.

Another trial was made with about 200 cc. of material prepared
by cooking a potato in just enough water to cover it, sterilizing,
and cultivating the bacteria in it as usual. In four days from
beginning of the culture it was filtered; the residue upon the
filter was treated with 100 ce. of distilled water, slightly acidulated
with hydrochloric acid, heated to 70°C. and filtered. The two
filtrates were united and evaporated to a syrup. This was digested
in the cold with alcohol containing a little sodie hydrate. This
solution was tested as before, and also with platinic chloride and
concentrated sulphuric acid, and all with no distinctive reactions.

A third trial was made with a boiled potato, which had been
permeated with the blight. The extract was made by the Stas-
Otto method, and the same reagents used as in the last case, with
equally negative results.

e

1 Cf. Brieger, Ueber Ptomaine, 1885, p. 22, et seq.
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99 HISTORY AND BIOLOGY

These tests do not cover the possibility of the ptomaine being
volatile, which is really not very great. It is yet necessary to
make tests of freshly blighted tissues from the tree, which can
only be done during the hot months.

Action of the Organism in the Living Plant.—The bacteria of
blight have the power of growth and multiplication in the pres-
ence of the living cells of the pear, and in this one important
respect differ essentially from other species of bacteria. By arti-
ficial inoculation into growing unripe pears, which give most
marked and certain results, it is found that other bacteria are
entirely innocuous, at once disappearing without having made
any growth or induced any changes in the tissue of the pear. If
blight bacteria in active condition are intermixed with the other
forms, they penetrate the cells, multiply, and finally bring about
the disorganization and death of the tissues which marks the pro-
gress of the disease, but the associated forms disappear the same
as when introduced alone, and the product is a mass of practically
pure blight bacteria.

This result is rendered possible on account of the fact already
stated, that the blight bacteria penetrate the tissues, and main-
tain their normal growth for some time (days or weeks), before
the life of the cells is sufficiently interfered with to permit the
crowth of other forms. The bacteria always extend beyond the
visible location of the disease—in small branches, often to the
distance of a third of a meter or more.

One of the properties which enables this species to successfully
penetrate the pear-tree is evidently its unusual indifference to
acids, which prevents most other forms from making any
growth; the juices of the pear give a strong acid reaction with
test paper.

What chemical changes are brought about by its activity in
the plant cannot be definitely stated, further than to say that a
mucilage or gum, which is soluble in water, is produced in abun-
dance, with the disengagement of carbon dioxide. The contents
of the cells, together with the cell-walls which have not been
liquetied or changed into stony tissue, pass over into this viscous
product.!

1 Rep. N. Y. Agric. Exper. Station for 1885, p. 248; Amer. Nat., vol.
xix, 1885, p. 1181.

OF PEAR BLIGHT. 93

It was early observed by cultivators, being recorded by Coxe,
that succulent shoots blight the most readily, and any process of
cultivation which as far as possible prevents succulency has
always been considered an aid in keeping the disease in check.
The avidity of the blight bacteria for water has been well demon-
strated in the cultures on slices of pears. There seems to be
some connection between these facts and the well known fact
that the disease shows different degrees of virulence in different
varieties of fruit trees, especially in different varieties of the
pear. The variation, or at least part of it, to be observed in
pears, apples, quinces, hawthorns, etc., may be due to some inhe-
rent difference in the nature of the host, not readily formulated ;
for we find that the blight bacteria will grow to only a slight
extent in the succulent peach shoots, and not at all in most
other plants.! But in varieties of the same fruit it may reason-
ably be inferred that to a considerable extent the difference in
the progress of the disease is due to physical causes.

To determine what relation the hydration of the tissues holds
to this question, a series of determinations of the percentage of
water in the parts of the tree most subject to blight has been
begun. These are yet incomplete, and can only now be referred
to briefly.

The Bartlett and Seckel pears very well represent the extremes,
the first being most affected by the disease and the second least.
Twigs taken from the tree in February were found to contain 50-2
per cent. and 50°85 per cent. of water respectively. ‘Twigs taken
in the same way April 30, bearing flower buds, but with the
leaves removed, gave 68°7 per cent. and 67'3 per cent. of water.
The half-grown fruit, taken the first week in July, gave 79°3 per
cent. of water for the Bartlett and 77 per cent. for the Seckel.
According to these figures the amount of water in the Bartlett
and Seckel twigs during the winter is practically the same, but
during growth both the twigs and fruit of the Bartlett contain
more water than those of the Seckel. These numbers give some
support to the view that succulency and the strength of the dis-
ease are directly related, but the data are yet too incomplete to
warrant a definite statement.

1 Rep. N. Y. Agric. Exper. Station for 1884, pp. 362, 377; Amer. Nat.,
vol. xix, 1885, p. 1182.

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D4 HISTORY AND BIOLOGY OF PEAR BLIGHT.

EXPLANATION OF PLATE IIL.

The drawings were made with a camera lucida and a Spencer’s ;1)-objec-
tive, homogeneous immersion, of 125° balsam angle. They are uniformly
magnified 890 diameters.

Fie. 1.— Micrococcus amylovorus Bur., grown upon a slice of boiled
potato, stained with Bismarck-brown and moun’ —_n Canada balsam. _

Fic, 2.—From a cultivation in hay infusion : a, separate bacteria ; b, zoo-
glea. The large mass, only part of whichis shown, is made up of
smaller masses more or less united.

Fic. 3.—Small zooglea from a potato infusion, drawn from a preparation
in Canada balsam, stained with Bismarck-brown. |

Fig. 4.—Portion of a zoogloea mass from the same culture, showing an
envelope free from bacteria. Drawn from an unstained preparation
mounted in glycerine.

Fie. 5.—Three zoogloea from the same culture.

Fie. 6.—From another culture in hay infusion.

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PLATE Ill.

PROC. ACAD. NAT. SCI. PHILA., 1886.

J,C-ARTHUR Del

BUR,

MICROCOCCUS AMYLOVORUS

MMe RE Oye
ey r ie
‘ the by ba ea
Net 4h Ne
Peery ae,

wih