Influence of Crop Rotation on Inoculum Density ofRhizoctonia solaniand Sheath Blight Incidence in Rice

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Abstract
A linear correlation (P = 0.01; r2 = 0.85) was observed between the number of sclerotia of R. solani recovered from soil and sheath blight incidence a the panicle initiation growth stage of rice. Cropping systems for 34 fields had a significant effect (P = 0.05) on the preplant inoculum density of R. solani and the percentage of sheath blight incidence. Mean numbers of sclerotia recovered (per kilogram of soil) were 4.02, 1.43, and 0.07 with an average disease incidence of 5.4, 2.7, and 0.4% for rice-[Oryzea sativa]-soybean[Glycine max]-rice, soybean-soybean-rice, and pasture-pasture-rice cropping systems, respectively. Significantly higher (P = 0.05) inoculum densities and disease incidence were found in alternate year rotations out of rice than in 2-yr rotations out of rice. Spatial patterns of inoculum and disease were best described as aggregated in seven fields where individual soil core processing allowed for analysis of the population distribution. Inoculum was spatially autocorrelated in these fields, whereas disease was not. In matrix surveys conducted at two sites in 1984, inoculum density ranged from 0 to 44 sclerotia recovered per 440-cm3 soil core, and disease incidence ranged from 0 to 26 diseased tillers per 50-tiller sample. Inoculum and disease were aggregated in the quadreat sampled sites at large (> 1 m) sample spacings and random at smaller sample spacings. A rapid and reliable method of quantifying sclerotia of R. solani from field soil was developed.