Soil Sampling Techniques for Determining the Effect of Cultural Practices on Rhizoctonia oryzae-sativae Inoculum in Rice Field Soils

Methods were evaluated to quantify sclerotia in rice fields affected with aggregate sheath spot disease. Recovered sclerotia were compared with disease ratings at harvest for paddies subjected to various postharvest cultural straw management practices. Sclerotial inoculum level was not always correlated with subsequent disease incidence in rice aggregate sheath spot disease, suggesting alternative sources of inoculum and other factors influencing disease development. Straw management practices affected the inoculum load of Rhizoctonia oryzae-sativae, as determined by methods recovering sclerotia from California rice field soil samples. Traditional wet sieving of soil samples was improved using a potassium carbonate solution to float heavier sclerotia that normally remained in the sediment. The relationship of inoculum in the soil from seedbeds and the incidence of resulting disease measured just before harvest was investigated over 3 years at three sites. A linear dose-response was found at only one site where the number of floating sclerotia was more strongly correlated with disease incidence than total number of sclerotia (floating plus nonfloating sclerotia). Floating sclerotia were more easily counted using a water floatation extraction (WFE) than were total sclerotia by using a potassium carbonate floatation extraction (PCFE) or a combination of the two assays (WFE + PCFE), and the WFE more accurately predicted disease incidence than did either PCFE or WFE + PCFE. All assays detected significant differences between inoculum levels as influenced by various straw residue management practices, with removal of straw residue significantly reducing number of soil-borne sclerotia.