Population and distribution of sclerotia of Rhizoctonia solani kühn in sugar beet field soil

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The population and distribution of sclerotia of Rhizoctonia solani Kühn in two sugar beet field soils was determined at harvest by a sieving-flotation method. In rhizosphere soil (RS) and non-rhizosphere soil (NRS) from the most heavily infected roots of sugar beets, 1.43–2.5 and 0.83–1.0 sclerotia g −1 dry soil were detected, respectively. In the soil around healthy sugar beet, these values were 0.04–0.12 and 0.03–0.04 sclerotia g −1 dry soil. More sclerotia were always obtained from RS than from NRS. More than 80% of the sclerotia were in the upper 10 cm of soil and within 10 cm of diseased roots. Therefore, there is a non-uniform distribution of sclerotia of R. solani in soil. The sclerotial population in soil increased significantly with disease severity and a good correlation was obtained between the number of sclerotia and the disease severity on infected plants. Most of the sclerotia collected from the field soil ranged in size from 0.5 to 2.0 mm diameter. Viability of sclerotia increased as severity of crown rot increased and as the size of the sclerotia increased. Conversely, there was a progressive decrease in sclerotial germination with increasing depth in soil and increasing distance from the infected root.