BLACK ROT OF THE POTATO I TYPE 



255 





wilt, blacken, shrivel, and fall over 

 (Fig. 193), and often the tubers decay. 

 The disease is readily inoculable into 

 the soft, upper part of shoots (Fig. 194) 

 and may run out on the petioles in 

 black lines exactly as in case of Bacter- 

 ium solanacearum (No. IV) or of Bacil- 

 lus amylovorus (No. XII). It is also 

 readily inoculable into the base of 

 potato shoots when not too old (Figs. 

 195, 196) and then progresses in the 

 same w-ay as the natural infections. 

 Old shoots are less susceptible than 

 young ones (Figs. 197, base, and 198) 

 and very often when the basal parts 

 of stems are inoculated they rot across 

 and break over without much down- 

 ward movement of the bacteria (Fig. 

 198), the underground parts sending 

 up new dwarfed shoots to take the 

 place of those destroyed (Fig. 199). 

 Prior to shriveling, especially as the 

 season advances, all the green parts 

 of the potato (stems, leaves, flow r er 

 stalks) may show black spots due 

 to the organism and from these 

 pure cultures of it may often be ob- 

 tained (Appel). The bacteria neither 

 show 7 any special tendency to multiply 

 in the vascular bundles, although some- 

 times found there (Fig. 200), nor, in FlG . 193. Potato plant 



inoculated by needle pricks 



on the base of the shoot (at X) and wilted by Bacillus phytophthorus (Appel I). 

 Plant inoculated January 30, 1915 from a 16-day culture on a steamed potato. 

 Photographed on the 7th day, > natural size. Of 9 shoots on 6 plants all of 

 which were inoculated only one failed to become diseased, and this was one of 4 

 from the same tuber (perhaps it was older than the other 3 shoots). The disease 

 progressed much faster upward than downward as shown by the tiny shoot which 

 is still unaffected though coming out of the base of the blackened stem at the 

 earth's surface. Earlier stages of the disease resembled Figs. 191 and 192. 



