THE EFFECT OF THE INCORPORATION OF CERTAIN COVER CROPS ON THE MICROBIOLOGICAL BALANCE OF POTATO SCAB INFESTED SOIL

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Abstract
The decomposition of soybean, rye, and red clover in naturally infested potato scab soil resulted in marked quantitative and qualitative changes in the microflora, but only soybean reduced disease incidence. Soybean and red clover increased the numbers of bacteria, actinomycetes, and fungi, with rye having a similar effect except on actinomycetes. Tubers invariably supported higher numbers of organisms than corresponding soils. In the soybean soil amino-acid-requiring bacteria were preferentially stimulated, while those with more complex requirements were relatively suppressed. Amino-acid-requiring fungi also were slightly increased and there was a corresponding decrease in those with simple nutrient requirements. A similar effect occurred in the rye soil. When potatoes were grown in the soybean soil, the amino-acid-requiring bacteria were markedly increased on, and correspondingly decreased apart from, the tubers. In contrast, amino-acid-requiring fungi were completely suppressed in the soybean soil, still further increased in the rye soil, and preferentially stimulated only on the tubers in the control soil. The Bacterial Balance Index showed a relationship with degree of infection, the high index in the soybean soil being associated with least scab. This effect was particularly emphasized in soil on the tubers.