1969 Volume 35 Issue 4 Pages 286-293
Black scurf of Chinese yam, a disease of hitherto unknown etiology, occurred in the sand dune field of Tottori. In the early stages of the disease, black necrotic spots appear on the surface of tubers. As the disease progresses, the spots expand into black scurf-like lesions, and exhibit various degrees of blackening, corking and rifting. It is also characteristic of the disease that no symptom is recognizable on the above ground part.
Numerous bacteria and fungi isolated from the lesions of diseased tubers did not cause the disease in greenhouse and laboratory tests. Phytotoxic substances were detected in soils where black scurf is just prevalent but not from soils free from the disease. The results of the bioassay of soil toxins revealed a close correlation between the time or the depth of maximum toxicity of soil extracts and the time or the soil depth of the occurrence of the disease in the field. A number of carbonyl compounds which were toxic to radish seedlings were isolated from the water extracts of sick soil, and were identified as acetaldehyde, furfuraldehyde and iso-valeraldehyde.
Black scurf was particularly severe in the field treated previously with nematicides. Increased amount of soil toxins was also proved in these nematicide-treated fields.
Heavy applications of organic manures such as chicken manure, and abnormal rise of soil temperature in the growing season of tubers, one of predestined impediments in agriculturally utilizing sand dune, could be considered as the occasional cause of black scurf. Thus, an explanation of the etiology of the disease on the basis of volatile aldehydes formed during the anaerobic decomposition of manures and humus in the soil was suggested.
