[Extracted from the Proceedings of the Philadelphia Academy of Natural Sciences, Sept. SS, 
 
 1886, pp. SSSSU1. The original paging of the volume is placed at the 
 
 bottom of the pages.] 
 
 History and Biology of Pear Blight. 
 
 By J. C. ABTHUE, 
 
 i A thesis presented to the Faculty of Cornell University for the Doctorate in Science, 
 
 June, 1886., 
 
HISTOEY AND BIOLOGY 
 
 PEAR BLIGHT. 
 
 By J.0 OS ARTHUR. 
 
.« 
 
 
 ell Uuiv. Lin. JBxojtiaiig: 
 

 \0 
 
 CONTENTS. 
 
 PAGE. 
 
 Pear blight and its distribution, 5 (322) 
 
 Amount of loss, 6 (323) 
 
 Early records, 9 (326) 
 
 Conjectures regarding its cause, 9 (326) 
 
 Beginning of experimental research, 12 (329) 
 
 Description of Micrococcus amylovorus Bur., . . . .14 (331) 
 
 Formation of zooglcea, . .15 (332) 
 
 Cultivation in fluid media, 16 (333) 
 
 Cultivation in solid media, 18 (335) 
 
 Behavior toward staining fluids, 20 (337) 
 
 Chemical products, 20 (337) 
 
 Action of the organism in the living plant, . . . .22 (339) 
 
 Explanation of plate, 24 (341) 
 
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 b} r 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 1 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 an}^ distinct mention of such a disease in the horticultural 
 literature of other regions, and partly from direct testimony. 
 Prof. Dwindle, late of the University of California, has told the 
 writer that it does not occur on the Pacific coast. Dr. De Bary, 2 
 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, 3 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. 
 
 2 Yorlesungen tiber Bacterien, 1885, p. 137. 
 
 3 Nederlandsche Tuinbouwblad, II (Jan. 9, 1886), p. 9. 
 
 (322) 
 
b 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, 1 of 
 Iowa, who is familiar with the disease in this county, 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, 2 
 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, 3 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, 4 as early as 1817, in the oldest 
 pomological work by an American author, sa}-s it " frequently 
 
 1 Trans. Minn. Hort. Soc. for 1883, p. 281. 
 
 2 Fruit Blights and Diseases of Fruit-trees in New Zealand, 1885. 
 
 3 For an account of the destruction of stored scions by blight, see Rep. 
 Hort. Soc. of Mich, for 1881, p. 137. * L. c, p. 174. 
 
 (323) 
 
OF PEAR BLIGHT. f 
 
 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 destroj r ed. 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, 1 called it the 
 "monstrous malady of the pear." Chas. R. Baker 2 says it is 
 " the worst malady with which the cultivator of the pear-tree has 
 to contend." In southern Pennsylvania u the pear is so generally 
 destroyed by the blight," according to J. B. Garber 3 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. 
 
 2 Practical and Scientific Fruit Culture, Boston, 1866, p. 476. 
 
 3 U. S. Patent Oflace Report for 1850, pt. ii, p. 418. 
 
 (324) 
 
8 HISTORY AND BIOLOGY 
 
 Thomas Meehan. 1 T. T. Lyon, 2 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, 3 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, 4 speaking for southern Illinois, says, " pears have failed, 
 utterly failed, so that none are now cultivated for market, the 
 blight has destroyed the trees —branch and root," and S. G. 
 Minkler, 5 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, 6 of the 
 same State, is led to " doubt if one-tenth of the pear-trees that 
 are set, live ten yeai'S," 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 
 3-ear." 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. E. H. S. Dartt 8 
 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, 9 up to 
 1876, had lost one thousand Tetofsky apple-trees by it. F. G. 
 Gould 10 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. 
 
 3 Trans. 111. Hort. Society for 1868, p. 42. 
 
 4 Same for 1882, p. 118. 
 
 5 Same for 1882, p. 30. 
 
 6 Same for 1880, p. 03. 
 
 7 Proc. Mo. Hort. Soc. for 1870, p. 18. 
 
 8 Trans. Minn. Hort. Soc. for 1874, p. 22. 
 
 9 Same for 1876, p. 73. 
 10 Same for 1884, p. 127. 
 
 (325) 
 
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 3'ears makes it reasonably 
 certain that he had observed the disease as early as the opening 
 of the century. The earliest notice, however, which has } r et come 
 to hand, is in a letter written by Wm. Denning, 1 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 chronologh-alty the 
 various notices found in different publications, for without excep- 
 tion the}^ 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 agenc} 7 , 
 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. 
 (326) 
 
10 HISTORY AND BIOLOGY 
 
 Coxe, 1 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 varieties, 
 of which the St. Germain and Seckel were respectively conspicu- 
 ous instances. 
 
 The insect theoiy, 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 Scolytus pyri 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 of a 
 tree being ascribed to a poison which the insects were supposed 
 to emit. Among the prominent supporters of this view was the 
 " Genesee Farmer," 2 published at Rochester, X. 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 w r hich the moving currents of sap the next spring and 
 summer distributed, causing the death of the parts. It was first 
 published in 1844 hy Rev. H. W. Beecher, 3 of Indiana, in a long 
 and able article in "Hovey's Magazine." In the following year 
 
 1 L. c, p. 175. 
 
 2 See Genesee Farmer, vol. vii, 1846, p. 217 ; vol. vlii, 1847, pp. 122, 
 218, etc. 
 
 3 Magazine of Horticulture, vol. x, p. 441. 
 
 (327) 
 
OF PEAR BLIGHT. 11 
 
 it was independently elaborated by A. J. Downing 1 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, 2 who figured 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, 3 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, 4 by Mr. Median'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, 5 who in 1878 dis- 
 tinctly stated his bdief 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 6 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 Cbas. 
 Downing, p. 640. 
 
 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. 
 
 5 Trans. 111. Hort. Soc. for 1878, p. 80. 
 
 6 Proc. Amer. Assoc. Adv. Sci., vol. xxix, 1880, p. 583 ; Rep. of 111. 
 Industrial Univ. for 1880, p. 62 ; Trans. 111. Hort. Soc. for 1880, p. 157 ; 
 Amer. Naturalist, vol. xv, 1881, p. 527. 
 
 (328) 
 
12 HISTORY AND BIOLOGY 
 
 attention, of the scientific world. Although this was now the 
 popular hypothesis, it cannot he 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 l 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 variousl}' 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 2 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 to a 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 frequentty 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. 
 
 (320) 
 
OP PEAfi, 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, 1 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 haA r e 
 already referred) he was shown a thrift} 7 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 sa}-s : " 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, 2 opposes the opinion that " an epidemic 
 conveA'ed 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. 253. 
 
 (330) 
 
14 HISTORY AND BIOLOGY 
 
 Another interesting experiment was performed by Dr. E. S. 
 Hull, 1 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, 2 of Xew York, says in 1840. "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, 3 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, 4 
 but did not recognize them as such till the following year, 5 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. 6 
 
 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 
 b}- slight changes in the ratio between their length and breadth 
 
 1 Trans. 111. Hort. Soc. for 1870, p. 220. 
 
 2 U. S. Patent Office Rep. for 1849, pt. ii. p. 447. 
 
 3 Trans. 111. Hort. Soc. for 1878, p. 81. 
 * Same for 1877, page 114. 
 
 5 Same for 1878, page 79. 
 
 6 The Bacteria (a reprint from Rep. of 111. Industrial Univ. for 1882), 
 p. 42 ; Amer. Naturalist, vol. vii, 18S3, 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. 
 
 (331) 
 
OF PEAR BLIGHT. 15 
 
 (PI. Ill, figs. 1, 2, 6). They are 1 to l£//. long, by | to f // broad, 
 and quite colorless. For the most part, the}' 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 specialty 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 entirety 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 Zooghea. — By far the most characteristic feature 
 in the life history of Micrococcus amylovorus is the formation of 
 zooglcea (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 the} T 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 
 
 (332) 
 
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 zoogloea 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 ver} r small; and although 
 they may reach 30 to 40 //. long by 20 to 30 [i wide, they rarely 
 lose their distinctness. When below 10 ;>. in length, their usual 
 form is oblong, varying to globular. The} 1- 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 fi 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 satisfactoiy 
 is an infusion of potato. This is prepared by paring a potato and 
 slicing it into three or four times its bulk of water. This is kept for a 
 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. If the 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 
 
 (333) 
 
OP PEAR BLIGHT. It 
 
 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 bine 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) meal in a similar 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 (C0 2 ) 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 whitisli 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 
 light 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 bactei'ia, 
 without a pellicle, turbidity, or zoogloea. A strong decoction of 
 old barnyard manure acted in the same manner. A solution of 
 
 2 (334) 
 
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 ^ per cent, of malic acid was added to the usual infu- 
 sion of potato. This prevented the formation of a pellicle, tur- 
 bidity or zoogloea, 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 zoogloea. 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 ia 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 
 
 (335) 
 
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 the}' 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 
 
 (336) 
 
20 HISTORY AND BIOLOGY 
 
 readity, 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 zooglcea 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, 1 
 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. 
 
 (337) 
 
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 see 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. 1 
 
 A cultivation in infusion of potato, giving about 200 cc. 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 
 b} r cooking a potato in just enough water to cover it, sterilizing, 
 and cultivating the bacteria in it as usual. In four da}*s from 
 beginning of the culture it was filtered ; the residue upon the 
 filter was treated with 100 cc. of distilled water, slightly acidulated 
 with hydrochloric acid, heated to 70°C. and filtered. The two 
 filtrates were united and evapoi-ated to a syrup. This was digested 
 in the cold with alcohol containing a little sodic h3'drate. 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. 
 
 1 Cf. Brieger, Ueber Ptomaine, 1885, p. 22, et seq. 
 
 (338) 
 
22 HISTORY AND BIOLOGY 
 
 These tests do not cover the possibility of the ptomaine being 
 volatile, which is really not very great. It is 3 T et necessary to 
 make tests of freshly blighted tissues from the tree, which can 
 011I3' 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 
 growth 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 hy 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 
 liquefied or changed into stony tissue, pass over into this viscous 
 product. 1 
 
 1 Rep. N. Y. Agric. Exper. Station for 1883, p. 248 ; Amer. Nat, vol. 
 six, 1885, p. 1181. 
 
 (339) 
 
OF PEAR BLIGHT. 23 
 
 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. 1 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 peai's 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. 
 
 (340) 
 
24 HISTORY AND BIOLOGY OF PEAR BLIGHT. 
 
 EXPLANATION OF PLATE III. 
 
 The drawings were made with a camera lucida and a Spencer's ^-objec- 
 tive, homogeneous immersion, of 125° balsam angle. They are uniformly 
 magnified 890 diameters. 
 
 Fig. 1. — Micrococcus amylovorus Bnr., grown upon a slice of boiled 
 
 potato, stained with Bismarck-brown and mour' n Canada balsam. 
 Fig. 2. — From a cultivation in hay infusion : a, separate bacteria ; b, zoo- 
 
 gloea. The large mass, only part of which is shown, is made up of 
 
 smaller masses more or less united. 
 Fig. 3.— Small zoogloea 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. 
 Fig. 5. — Three zoogloea from the same culture. 
 Fig. 6. — From another culture in hay infusion. 
 
 (341) 
 
PROC. ACAD. NAT. SCI. PHILA., 1886. 
 
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