212 



BACTERIA IN RELATION TO PLANT DISEASES. 



of the stem or main root; (2) A sudden wilt due to the filling of the vascular bundles with 

 fungi of the form-genus Fusarium; and (3) A wilt due to the rotting off of the main stem at 

 the surface of the earth. This disease may be distinguished readily enough by the facts 

 that fungi are not present and that there is no stem-injury or root -injury of the kinds just 

 described, and also by the further fact of the invariable presence of large numbers of white, 

 sticky bacteria in the vascular system. These are so abundant and usually so viscid that if 

 the tip of the finger be pressed against the cross-section of a diseased stem at once, or better, 

 some minutes after cutting, and then gently removed, the bacteria will remain attached to 

 the finger and string out in numerous delicate threads (fig. 52) resembling cobwebs. For 

 the microscopic structure of these threads consult Vol. I (fig. 14). If a little time is allowed 

 the bacteria also ooze from the cut surface (cross-section) of such stems in milk-white drops, 

 especially if the stems are cut a second time and the basal end put into water or moist air. 

 The wilting and shriveling of the leaf blades always precede the destruction of the leaf- 

 stalks and of the stem by a considerable period, so that it is common to find plants which 

 have lost all or nearly all of their foliage while still retaining a green and normal looking 

 stem (plate i, fig. 2), the vessels of which, however, for long distances will be found to be 



more or less fully occupied by the bacillus (fig. 6). In the end, 

 petioles and stems shrivel and die, but the organism does not 

 make its appearance on the surface of the plants and there is 

 nothing resembling a soft wet-rot, not even in the fruits. In rather 

 resistant plants, e. g., certain squashes, the foliage may wilt dur- 

 ing warm, dry days and partially recover at night or during cool, 

 moist days, to wilt again when the demands of transpiration are 

 greater. In such plants there is dwarfing (fig. 53) accompanied, 

 in some instances at least, by an excessive blossoming and branch- 

 ing. In the cucumber and muskmelon the disease is, on the con- 

 trary, quite speedily destructive, a few weeks after the close of the 

 period of incubation being generally sufficient to destroy the 

 plants. There is in these species much less tendency to recover 

 from the wilt temporarily during cool or^wet weather than there 

 is in the squash, and the writer has not observed any prolifera- 

 tion of shoots or flowers. When the disease is active there is 

 seldom any yellowing of the foliage in advance of the wilt. The 

 loss of turgor and change of color (from bright green to dull 

 green) are sudden. The characteristic signs are well exhibited 

 in the accompanying illustrations. 



The disease generally starts in the center of a hill, i.e., on the blades of the basal leaves 

 which soon shrivel. In this stage of the disease in that part of the main axis near the pri- 

 mary infections the vascular bundles, and especially the spiral vessels, are gorged with the 

 bacteria and there are usually many bacterial cavities in the primary vessel-parenchyma 

 (%- 54)- 



ETIOLOGY. 



The cause of this disease is a white peritrichiate schizomycete named by the writer 

 Bacillus tracheiphilus from its special fondness for the vessels of the plant. This organism 

 was first isolated and described by the writer (1893-95), an d most of the statements here 

 given rest upon his own observations and experiments, which now cover a period of 18 years. 

 The disease is very readily induced by needle-punctures without hypodermic injection. All 

 that is necessary to produce the disease is to dip the end of a sterile steel needle into a recent 



Fig. 52.* 



*Fio. 52. Viscid threads of B. tracheiphilus stringing from cut end of a cucumber-stem. Plant from field, autumn 

 of 1904. Threads slightly diagrammatic, i. e., not sufficiently cobwebby. 



