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Evaluation of Three Quantitative Assays for Sclerotinia minor. K. V. Subbarao,Department of Plant Pathology, University of California, Davis, c/o U.S. Agricultural Research Station, 1636 E. Alisal Street, Salinas 93905; S. Dacuyan(2), S. T. Koike(3), and L. E. Jackson(4). (2)(3)UC Cooperative Extension, Salinas, 93901; (4)Department of Vegetable Crops, University of California, Davis, c/o U.S. Agricultural Research Station, 1636 E. Alisal Street, Salinas 93905. Phytopathology 84:1471-1475. Accepted for publication 23 September 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1471.

Three techniques to quantitatively assay field soil for Sclerotinia minor were compared for precision, bias, and time required to assay a sample. The techniques compared were wet sieving, wet sieving with Calgon, and hydropneumatic root elutriation. Precision was measured by the standard error of the mean for repeated assays of 40 100-g samples each of clay, loam, and sand soils artificially infested with 15 viable sclerotia. Bias was measured by the deviation between the number of sclerotia recovered with each of the three techniques and the true number of S. minor sclerotia introduced into the above autoclaved soil samples. Regardless of the soil type, the root elutriation technique was the most precise and unbiased method to assay soil for S. minor. The efficiency of the hydropneumatic root elutriation technique was higher for clay soil and was comparable with the other techniques for loam and sand. In general, 9295% of the sclerotia added to soils were recovered with the root elutriation technique. Wet sieving was the least precise, most biased, least efficient, and gave the lowest recovery of sclerotia from a naturally infested clay soil. Efficiency of the wet sieving technique was primarily dependent on soil type. Adding Calgon to soil samples before wet sieving increased the precision, reduced bias, and improved the efficiency of wet sieving for all soil types except clay. Relative net precision, encompassing the bias, precision, and cost in time of assaying a sample, was also highest for the hydropneumatic root elutriation technique regardless of the soil type. Viability of recovered sclerotia was influenced by the assay technique employed and soil type used.

Additional keywords: lettuce drop, quantitative epidemiology.