STREPTOMYCES 



367 



An important disease of potatoes is due to several species of Strep- 

 tomyces. Potato scab is a disease of the tubers characterized by the 

 production of warty excrescences or "scabs" on the surface, which 

 eventually break down and slough off. For a more complete discus- 

 sion of this disease the reader is referred to any standard book on 

 plant pathology, e.g., Heald, or to an earlier but excellent work on 

 potato scab by Lutman and Cunningham.^^ 



There are several species now known to cause this disease. Differ- 

 ent species differ in their invasive ability and they may produce 



Fig. 134. Potato scab. 



different types of scab on the same variety. There is considerable 

 varietal difference in potatoes and some varieties of the host are 

 more seriously parasitized by certain species or strains of the species 

 of the parasite, hence not only rotation of crops but also of varieties 

 is indicated.^' ^"^ In general, in addition to rotation, treatment of 

 seed potatoes with germicide, and use of certified scab-free seed 

 potatoes, control consists in making the soil acid by the addition to 

 the soil of sulphur or ammonium sulphate which is transformed to 

 mineral acids by autotrophic bacteria. Fortunately the potato plant 

 is more tolerant of acid than are the actinomycetes. This acidity 

 helps control the disease but is not an absolute preventive. The best- 

 known species, Streptomyces scabies {Actinomyces scabies), is some- 

 times used for all the potato scab actinomycetes. The specific name 

 chromogenus is inadmissible for reasons given earlier. 



Other Streptomyces have been isolated, causing spoilage of nuts 

 in storage and earthy flavors in various food and dairy products. 



