1rk [Vol. 10 



1U ANNALS OF THE MISSOURI BOTANICAL GARDEN 



and below ground. There is, furthermore, a noticeable tendency 

 for the upper leaves in particular to manifest a somewhat xero- 

 phytic texture and a starkness of growth-habit, the total effect 

 being to give the tops of affected plants a narrowed or contracted 

 appearance. Besides being light-colored, the upper leaves are 

 often rendered more conspicuous by the presence of a metallic 

 luster. Plants affected with blackleg, as a rule, offer little re- 

 sistance to removal from the soil, and not infrequently break 

 off when one attempts to pull them. If upon removal from the 

 earth dark-colored cortical lesions are found on the underground 

 portions of the stem, there is little doubt but that the plant is 

 attacked by the blackleg organism. When upon splitting the 

 stem longitudinally the pith is found blackened and more or less 

 disintegrated the case is quite clear (pi. 1, figs. 2 and 4). Fur- 

 ther confirmation is afforded if the seed piece has disintegrated. 

 Aerial tuber development due to attack by the blackleg para- 

 site is not commonly seen. The production of aerial tubers does 

 take place when practically all the tuber-bearing stolons on 

 well-developed plants are destroyed by the pathogen, thus pre- 

 cluding normal tuber development. Rhizoctonia more frequently 

 causes such abnormalities of tuber development, and the under- 

 ground lesions due to this fungus may be confused with those due 

 to Bacillus atrosepticus. Certain characteristics of the Fusarium 

 wilt disease may also be confused with the yellowing and wilting 

 of potato vines caused by the blackleg parasite. 



As a rule, plants affected with blackleg occur scattered pro- 

 miscuously over the field. The occurrence and spread of the dis- 

 ease appear to bear a definite relation to environmental condi- 

 tions, and the number of diseased plants is likely to be greater 

 during a cold wet spring than in a warm dry one. Not infre- 

 quently there are more diseased plants in the low, poorly drained 

 spots in a field. 



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Vine" Symptoms.— The primary symptoms of the disease 



they occur on the potato vines are most striking. These _„_ 

 first, as a rule, on the lower parts of the stem both below "and 

 above ground, and are characterized by dark brown to black le- 

 sions or cankers, hence the origin of the German name "Schwarz- 

 beinigkeit," or "blackleggedness." The first cankers are likely to 

 develop below ground and not infrequently have their beginning 

 at the lowermost part of the stem. Under favorable circum- 

 stances the disease works rapidly upward. On young succulent 



