198 The Potato 



The fungus causing the disease gains entrance into the 

 eyes, which become black and swollen. "In more ad- 

 vanced stages one or more nodules, varying in size from 

 that of a wrinkled pea to a lump as large or larger than 

 the tuber itself, may be found. These are green where 

 they project from the ground and white below, turning 

 as they grow old to dark brown or almost black. They 

 have a wrinkled coral-like appearance like the head of a 

 cauliflower. 'A still more advanced stage occurs when 

 the fungus has utilized every particle of food stored in 

 the tuber and has reduced it to a brownish black soft 

 mass, giving off a very unpleasant putrefactive odor.' 

 Such potatoes cannot be harvested whole. The black 

 pulpy mass breaks up, liberating millions of sporangia 

 (spore-sacs) which live for years in the soil. These 

 sporangia have been known to infect potato crops after 

 an eight-year interval." ^ 



Common scab 



Common scab, caused by a soil bacterium, Actinomyces 

 chromogenus, is too well known to require detailed de- 

 scriptions. The organism affects only the outer layers of 

 cells, though there are deep scabs supposed by some to 

 be caused by the same organism. It has been shown 

 that gnats and possibly other insects may be responsil)le 

 for the original deeper injury, though the scab may also 

 be present. 



The scab organism winters over on the scabby tuber, but 

 may also live from year to year in the soil. Badly scabbed 

 tubers should not be planted. Clean tubers, as well as 

 those not badly scabbed, after being treated may be 



» U. S. D. A.. Farmers' Bui. 489. 



