S. G. Paine 483 



described by Appel, and they suggest that it is probably only a variety 

 of the same organism, but since it differs constantly in certain particulars 

 they rather reluctantly formed a new species B. melanogenes. The 

 disease investigated by F. C. Harrison (6) in Ontario resembles in outward 

 characteristics that of "Blackleg," as described by Appel, except that 

 while in the latter the woody bundles remain hard and strong enough 

 to support the diseased shoot so that these are conspicuous objects 

 standing erect in the field, in the former the vascular bundles are so 

 softened that the wilted and blackened shoots fall to the ground and 

 lie hidden so that on casual inspection the crop has the appearance 

 of being perfectly healthy. 



Mention must be made here also of a rotting disease of the potato 

 ascribed bv E. F. Smith (lO) to B. solanacearum since the final wilted 

 appearance of the shoots might be mistaken for the symptom of 

 "Blackleg." In this disease however the progression of the wilt is 

 from above downwards, the infection occurring in the leaves through 

 the agency of leaf-biting insects, while in "Blackleg" the lower leaves 

 are the first to show signs of wilting since the point of infection is sub- 

 terranean. Besides these a number of other organisms have from time 

 to time been described as producing rot in the potato accompanied 

 by the appearance of "blacklegged" shoots. Thus we find B. atro- 

 septicus described by van Hall (5), Micrococcus jjhytophthorus by Frank (3), 

 B. caulivorus by Prillieux and Delacroix (16) and B. solanincola by 

 Delacroix (2). 



Of these Micrococcus phytophthorus was possibly not the cause of 

 the disease which Frank had before him. Various workers have stated 

 that Micrococci appear abundantly on plate cultures from diseased 

 stems but that these prove to be saprophytic organisms. Frank 

 describes a special resistance to rotting exhibited by the potato in winter. 

 He states that his cultures during September and October produced 

 rapid rotting while those of December and January had no pathogenic 

 properties. While it is quite conceivable that physiological changes 

 in the tuber might result in such a resistance it seems more probable, 

 as Appel suggests (1), in view of the frequent occurrence of saprophytic 

 Micrococci, that Frank was dealing with mixed cultures probably con- 

 taining B. phijtophthorus in which a saprophytic Micrococcus predomi- 

 nated, and that by December when the "special resistance" made its 

 appearance the real parasite had been lost. B. caulivorus was con- 

 sidered by Laurent (12) and Griffon (4) to be identical with B. fluorescens 

 liquefaciens. Both hold that under certain conditions of the soil common 



Joum. of Asric. Sci, vm 32 



