Influence of Soil on Inoculum Density-Disease Incidence Relationships ofRhizoctonia solani

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Abstract
A method was developed to study inoculum density-disease incidence (ID-DI) relationships of Rhizoctonia solani in soil under controlled temperature and moisture conditions. ID-DI relationships were assessed by estimating disease in radish seedlings fter 3 days in soil artificially infested with R. solani. There was a significant linear relationship (P = 0.01) between mean number of hypocotyl infections per plant and inoculum density for all soils tested except those in which disease incidence was very low. When 75 soils were compared, disease incidence in radish ranged in a continuum from 0 to 1.52 mean number of hypocotyl infections per plant (.hivin.-x = 0.51 .+-. 0.32). Analysis of variance indicated that sample mean differences were significantly different. Native inoculum densities of these soils ranged from 0 to 35.9 propagules per 100 g. There was no relationship between native inoculum density and DI values of artificially infested soils. A disease suppressive soil was identified, having a high native ID (30.0 propagules per 100 g) and a consistently low infection incidence. Four soils, when given heat treatments of 55.degree. C for 1 hr, had DI values that increased in response to the treatment, the effect being greater in soils with low DI values.