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 a ggregate sheath spot disease. Recovered sclerotia were compared with diseas e ratings at harvest for paddies subjected to various postharvest cultural straw management practices. Sclerotial inoculum. level was not always corre lated with subsequent disease incidence in rice aggregate sheath spot disea se, suggesting alternative sources of inoculum and other factors influencin g disease development. Straw management practices affected the inoculum loa d of Rhizoctonia oryzae-sativae, as determined by methods recovering sclero tia from California rice field soil samples. Traditional wet sieving of soi l samples was improved using a potassium carbonate solution to float heavie r sclerotia that normally remained in the sediment. The relationship of ino culum in the soil from seedbeds and the incidence of resulting disease meas ured just before harvest was investigated over 3 years at three sites. A li near dose-response was found at only one site where the number of floating sclerotia was more strongly correlated with disease incidence than total nu mber of sclerotia (floating plus nonfloating sclerotia). Floating sclerotia were more easily counted using a water floatation extraction (WFE) than we re total sclerotia by using a potassium carbonate floatation extraction (PC FE) or a combination of the two assays (WFE + PCFE), and the WFE more accur ately predicted disease incidence than did either PCFE or WFE + PCFE. All a ssays detected significant differences between inoculum levels as influence d by various straw residue management practices, with removal of straw resi due significantly reducing number of soil-borne sclerotia.