Population Dynamics ofStreptomyces scabiesand Other Actinomycetes as Related to Common Scab of Potato

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Abstract
Effects of two potato cultivars on population dynamics of Streptomyces scabies, causal agent of common potato scab, and other actinomycetes were compared during 1985, 1986, and 1987. Population densities of total and melanin-producing actinomycetes were monitored in plots planted to the scab-susceptible cultivar Chippewa or the resistant cultivar Superior, and in fallow control plots. Colonies of S. scabies were identified by production of melanin, gray aerial mycelia, and spiral spore chains. Fewer than 6% of the isolates of S. scabies tested were pathogenic. Populations of actinomycetes in soils planted to potato, in potato rhizospheres, and on tuber surfaces generally increased during each growing season, but populations in fallow soil remained constant or decreased. Populations of total actinomycetes and S.scabies were greater on the surfaces of Chippewa than Superior tubers (P .ltoreq. 0.05), but populations in soil and the rhisosphere did not differ between cultivars. Scab severity was correlated with the population densities of total actinomycetes recovered from soil at 52 and 153 days after planting in 1986 and from the rhizosphere 88 days after planting in 1987, and with relative numbers of S. scabies on the tuber surface at harvest in 1987.