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THE BACTERIAL DISEASES OF PLANTS:
A CRITICAL REVIEW OF THE PRESENT STATE OF
BY ERWIN F. SMITH.
I. It is scarcely fourteen years since Dr. Robert Hartig declared that there were no diseases of plants due to bacteria. Two years later Dr. Anton de Bary, unquestionably one of the most learned and critical botanists the world has ever known and the foremost student of cryptogamic plants, expressed the belief that bacterial diseases of plants were of rare occurrence, and suggested as a partial explanation the fact that the tissues of plants generally have an acid reaction.” In his Vorlesungen über Bacterien, published in 1885, he expresses much the same opinion, and cites only four diseases, viz., Wakker's hyacinth disease, Burrill's pear blight, Prillieux's rose red disease of wheat grains, and the wet rot of potatoes, described by Reinke and Berthold. Concerning the first of these four diseases he says: “Successful infection experiments and exact study of the life history of the bacterium are still wanting.” Respecting the second he contents himself with briefly summarizing the statements made by Prof. Burrill. Of Prillieux's micrococcus he says: “Its importance as a cause of disease cannot be determined with any certainty from the brief account. It may turn out to be only secondary, appearing as a saprophyte in consequence of injuries previously received." Concerning the wet rot of potatoes he states that ordinarily it is a secondary phenomenon following the attacks of the parasitic fungus Phytophthora infestans, but admits that exceptionally potato tubers may become wet rotten without the presence of Phytophthora, and that "the above named observers succeeded in producing the appearance of wet rot in sound potato tubers by inoculations with their bacteria ; in agreement with which stands a recent experiment of van Tieghem, who succeeded in totally destroying living potato tubers by means of Bacillus amylobacter when he introduced this into the interior of the tuber and maintained the same at a high temperature (35°).”
1" Für die Krankheitsprocesse der Pflanzen kommen sie durchaus nicht in Frage, etc.” Hartig : (1) Lehrbuch der Baumkrankheiten, 1882, p. 27.
2 . Bacteria parasitic on plants have scarcely ever been observed, a fact to wbich R. Hartig has already drawn attention. One reason for this may be that the parts of plants have usually an acid reaction.” De Bary: (2) Vergleichende Morphologie und Biologie der Pilze Mycetozæn und Bacterien, 1884, p. 520; English ed., p. 481.
S“ According to the present state of our knowledge parasitic bacteria are of but little importance as the contagia of plant diseases. Most of the contagia of the numerous infectious diseases of plants belong to other animal and plant groups, principally, as already noted, to the true fungi.” De Bary: (3) Vorlesungen ueber Bacterien, 1885, p. 136.
In the second edition of his Lehrbuch, published in 1889, Dr. Hartig modified his statements somewhat, expressing essentially the same opinions as de Bary. The yellow rot of hyacinths is recognized as a bacterial disease, although rather doubtfully in as much as it is said not to attack sound, well-ripened bulbs, under normal conditions, but only when they have received wounds or been attacked by fungi, especially by a hyphomycete which is said to be an almost constant accompaniment of the rot. The wet rot of potato tubers is admitted to the list, but with the statement that it is mostly a secondary matter, following the rot of stem and cells due to Phytophthora infestans. One other bacterial disease is mentioned, viz., pear and apple blight, with the suggestion, however, that it may have been erroneously attributed to bacteria, since the fungus Nectria ditissima produces in the bark numerous little bacterialike gonidia.
Such was the general opinion on this subject down to within less than a decade. Even to-day the majority of well educated botanists would find nothing to contradict in the statement that there are very few diseases of plants distinctly attributable to bacteria. As a matter of fact, however, there are in all probability as many bacterial diseases of plants as of animals.
Various explanations have been advanced to account for this freedom or supposed freedom of plants from bacterial parasitism. As we have already seen, de Bary was inclined to ascribe
it in good part to the acid reaction of vegetable tissue. Dr. Hartig's view is best expressed in his own words: “Whereas the processes of decay, and most of the infectious diseases of man and animals, may be traced to bacteria, the plant organism is protected against them by the peculiarity of its structure, and especially by the absence of circulatory channels for conducting the nutrient fluids which could serve to distribute any lowly organisms which might happen to be present in the food. It is only by means of the vessels and intercellular spaces that they can distribute themselves in any great numbers in the body of the plant, for in other cases they have to pass through the cellulose or woody cell walls, which offer great resistance to their attack. In addition to this, the vegetable juices, most of which show an acid reaction, are unfavorable to their growth. As a matter of fact, bacteria have hitherto been found only in the tissues of plants whose cells are parenchymatous in character and possessed of very delicate walls, as for instance, bulbs and tubers."
For several years Ph. van Tieghem experimented with one or more, probably several, bacteria, called by him Bacillus Amylobacter and believed to be the specific agent in the decomposition of cellulose. In 1879," he stated that all the cells of all plants are equally dissolved by it in the meristematic stage but that as soon as the tissues have become differentiated profound differences are noticeable. The cellulose of many plants is dissolved by it but that of mosses, sphagnums, hepatics, lycopods, fern leaves, and stems and leaves of phanerogamous aquatics proved resistant. This behaviour of water plants is "une nécessité d'existence." In 1884, he made a number of additional similar statements. The tubers of the potato, the seeds of beans (first swelled in water and then inoculated directly into the substance of the cotyledons), and the fruits of cucumbers and melons rotted quickly when infected with this organism. Inoculated leaves of Crassulaceæ and stems of Cac
• Hartig: Lehrbuch. 2nd. Edition. English translation, p. 37. • Van Tieghem : (4) Sur la Fermentation de la Cellulose. Bull. de la Soc. Bot. de France, 1879, pp. 25 to 30.
6 Van Tieghem: (5) Développement de l'Amylobacter dans les plantes à l'état de vie normale. Ibid., 1884, pp. 283-287.
taceæ resisted until plunged under oil when they decayed quickly. Aquatics resisted : “By means of a Pravaz syringe I have injected a drop full of the spores of Amylobacter into the lacunary system of several submerged aquatics (Vallisneria, Helodea, Ceratophyllum) but always without result. The plant remained healthy in all its parts.”
These papers of van Tieghem are often cited, but they have little substantial value. Undoubtedly he believed that he was experimenting with pure cultures, or, at least, that the results obtained were due to Bacillus amylobacter, but such is, to say the least, very improbable. B. amylobacter is now believed to be strictly anaerobic, and incapable of any action on cellulose.
More recently Julius Wiesner has divided all plants into two classes, ombrophobic and ombrophylic plants, according as they are or are not readily injured by prolonged rains or exposure to stagnant fluids. His experiments show that the aerial parts of some plants rotted very quickly when exposed to continuous artificial spray while similar parts of other plants proved very resistant, remaining sound for weeks (62 days in case of Tradescantia guianensis). The same contrast was observed when leaves of the two sorts of plants were placed in stagnant water, the former lost their turgor and rotted in a few days, the latter proved much more resistant. Many land plants have this power of resistance and all water plants, also all underground parts, even the roots of plants having very susceptible foliage. As additional confirmation Wiesner states that when meat infusions are left to themselves they always decay much sooner than when fragments of ombrophylic plants are placed therein. Ombrophobic plants in water or meat infusion also decay less rapidly when mixed with fragments of om brophylic plants than when left to themselves. This decay is more rapid in the dark than in light, especially
Prazmowski: (6) Untersuchungen ueber die Entwickelungsgeschichte und Fermentwirkung einiger Bacterien-Arten. Leipzig, 1880, pp. 23-37.
* Wiesner :(7) Veber ombrophile und ombrophobe Pflanzenorgane, Sitzungsb. K. Ak. d. Wissenschaften, Math.–Naturw. Classe. Wien., 1893, Bd. 102. Abt. I, pp. 503–521. See also Wiesner: (8) Pflanzenphysiologische Mittheilung aus Buitenzorg (III). Ueber den vorherrschend ombrophilen charakter des Laubes der Tropengewächse. Ibid., 1894, Bd., 103, pp. 169-191.
bright light. The foliage of ombrophylic plants is easily wetted; that of ombrophobic plants is as a rule not readily wetted, being usually protected by bloom or some other device for warding off water. When ombrophobic plants are not protected in some such manner, decay is remarkably rapid. In general if the leaves of a plant are readily wetted, it may
be assumed that they are ombrophylic, but there are exceptions, e. g. the potato and tomato. Roots of all plants are extraordinarily resistant. In most plants middle aged leaves are least susceptible to decay but in the potato the youngest leaves resist best. Old leaves lose this power of resistance. Sometimes this resisting power is variable in different individuals of the same species, depending on the conditions under which they have been grown. Curiously, all plants of shady, damp places are ombrophobic, if they possess leaves which are not readily wetted, e. g. Impatiens. The more the parts of a leaf are divided the quicker the decay. The green parts of the following plants are mentioned as particularly susceptible to bacterial decay: Solanum tuberosum, Lycopersicum esculentum, Xeranthemum annuum, Impatiens nolitangere, Chenopodium album, Veronica buxbaumii, Viola arvensis, and Taraxacum officinale (from sunny, dryplaces) Mimosa pudica, Pisonia alba. The following plants were found to be very resistant: Ranunculus aquatilis, Lemna minor, Lysimachia nummularia, Begonia magnifica, Tradescantia zebrina, T. guianensis, Selaginella sp. (from green house), and Scolopendrium officinarum. Among underground organs the roots of the yellow beet proved most resistant. The author's general conclusion from these experiments is best expressed in his own words: “It can now be stated as highly probable that the power of ombrophilous organs to resist rain for months is referable chiefly to the fact that antiseptic substances are produced in the tissues of the organs." These experiments are interesting but seem to have been performed in a rather crude way. The relative rapidity of decay was determined by appearance and the sense of smell and the organisms inducing this decay were undetermined. These experiments should be repeated and extended by Dr. Wiesner