Waksman — 166 — Actinomycetes 



the pox spots, which were said to be caused by a fungus. Manns and 

 Adams (4, 284) isolated from the sweet potato another organism which 

 they designated as Actinomyces ■pox and which they believed was the 

 cause of the pox. This organism was found to be distinct from A. fo- 

 olensis. 



A detailed study of the sweet potato disease was made by Person 

 and Martin (289, 336). In heavily infested soils the plants were 

 dwarfed and made ver}' poor growth. The leaves were small and pale 

 green to yellow. Many of the plants died before the end of the 

 season. The root system was poorly developed, most of the roots being 

 entirely decomposed and many breaking apart when the plant was 

 lifted from the soil. Small, elongated, dark-colored lesions also were 

 observed on the stem below the soil line. On mature potatoes the in- 

 fection produced pits or cavities, with irregular jagged or roughened 

 margins. These lesions varied from one-quarter of an inch to more 

 than an inch in diameter, sometimes coalescing and covering most of 

 the surface of the potato. At first, the lesions were covered by the 

 skin of the root, but when the latter broke away, the slightly sunken 

 pits were exposed. The new epidermal layer was covered with black, 

 granular remnants of the old dead tissue. The severity of rot on sweet 

 potatoes was influenced by the water content of the soil and its reac- 

 tion. A high water content favored more rapid growth of the roots 

 and enabled a diseased plant with a deficient root system to absorb 

 water and essential mineral salts more easily from the soil. Because 

 of this, rainy seasons enabled infected sweet potato plants to produce 

 vines and often to give satisfactory yields. 



The organism causing sweet potato rot was found to be a typical pro- 

 ducer of aerial mycelium and was described as A. ifomoea, belonging 

 to the genus Streptovtyces. 



Like potato scab, sweet potato rot does not develop in soils of 

 pH below 5.2. In soils of pH 5.8 to 6.2, however, the disease develops 

 readily. It can be eliminated by adding sufficient sulfur to lower the 

 pH to 5.0. 



Person and Martin concluded that sweet potato rot is more serious 

 in dry soils and in wet seasons, and it is found in soils with a pH above 

 5.2. The disease has been produced in the greenhouse and in the 

 field in inoculation experiments with pure cultures of S. ifovioea. In 

 cultures, the optimum temperature of growth for S. ifomoea is 32° C. 

 The optimum reaction for growth is pH 5.6 or above. 



Other Plants Infected by Scab Organisms.— In addition to potatoes, 

 sugar beets, and sweet potatoes, certain other plants are infected with 

 various forms of scab, which are apparently caused by actinomycetes. 

 These plants comprise turnips, rutabagas, occasionally radishes and 

 carrots, peppers, brazil nuts (406), and a variety of others (165, 178, 

 332, 360). According to Ken Knight (216), lesions caused by actino- 



