178 BACTERIA IN RELATION TO PLANT DISEASES. 



entire tuber will be sound and only the rhizome affected this through the whole length of 

 its vascular system or only at that end farthest from the tuber. On the other hand, when 

 plants are attacked late in the growing season, the tubers are more numerous and larger, 

 and the majority of them, especially if borne on long rhizomes, may escape infection or 

 be diseased only slightly at the stem end when dug. In tubers of this sort the rot is apt 

 to continue after digging, but exceptionally it might, perhaps, remain dormant, or nearly so, 

 until the season for planting. In plants attacked by this disease the juice of the stems will 

 be found to have an alkaline reaction to litmus paper and, if examined microscopically, will 

 be seen to be swarming with bacteria, which are very easily cultivated. 



On tomato shoots this organism causes the early development of great numbers of 

 incipient roots in the form of small nodules. Hunger was the first to point out this sign and 

 his statements have been confirmed repeatedly by the writer (see pi. 27). Sometimes 

 swelling of the inoculated parts of the tomato stem and the appearance of these incipient 

 roots, with dwarfing, and the changed position of the leaves are the only external signs of 

 disease in inoculated plants (pi. 28). These nodules are visible in the first figure of an inocu- 

 lated tomato published by the writer (in 1896) and were observed repeatedly in the field in 

 1895, but inasmuch as they develop naturally on sound stems under very moist conditions, 

 e. g., when a stem lies for some time on the earth, the influence of this disease in stimulating 

 their premature development was overlooked. Anyone may satisfy himself easily, how- 

 ever, that Hunger's statements are correct. For instance, if he will select a young, rapidly 

 growing tomato plant, the stem of which is entirely free from such developments, and will 

 inoculate one of two nearly equal branches, that one which was inoculated will promptly 

 develop the roots, while the other will remain free (pis. 26, 27) or will develop them only 

 much later as the result of a general infection of the stem. This is true even when the 

 younger of the two shoots has been the one purposely selected for inoculation. Moreover, 

 these roots always appear first in parts of the inoculated stem nearest the punctures. 

 Wounds made by a sterile needle do not cause them to appear. 



ETIOLOGY. 



The cause of this disease is a dirty white or brownish-white schizomycete which the 

 writer named Bacillus solanacearum in 1896. Further studies have shown that it is usually 

 motile by means of one polar flagellum. It should be classified therefore as Bacterium sola- 

 nacearum or Pseudomonas solanacearum, if one follows Migula's system. This organism 

 was first isolated and described by the writer, unless Burrill's statements relate to this 

 species (see History and Literature), and the statements here given rest chiefly upon his 

 own observations. The disease is sometimes very readily induced in susceptible species 

 by simple needle-punctures, without hypodermic injection. At other times the writer has 

 experienced many failures, using in some instances, at least, what appeared to be equally 

 genuine material.* 



In recent years the irregular behavior of plants inoculated with this organism has been 

 a source of much perplexity. Some have contracted the disease with great rapidity ; others 

 not at all; others have shown some signs of the disease and then recovered, or have been 

 seriously injured only in the inoculated shoots, or only after a great length of time (pi. 29). 

 The reason for these marked differences has not been made out clearly. It seems to depend 

 partly, at least, on the amount of water in the stem. Frequently the punctured areas have 

 been separated from the rest of the plant by the development of a protective cork-layer. 

 In general, infections have been more successful in young, rapidly growing, soft, watery 

 plants than in slow-growing, woody ones or in mature ones; infections have been more suc- 



*One labors under difficulties in the isolation of this organism. On agar plates its colonies are best distinguished 

 from other while colonies by waiting a week or two for the appearance of a brown stain in the colony, and yet this 

 treatment seems to weaken the virulence of the organism. 



