Sampling and Extraction Procedures to Estimate Numbers, Spatial Pattern, and Temporal Distribution of Sclerotia of Sclerotium rolfsii in Soil. Z. K. Punja, Research Scientist, Campbell Soup Company, and Visiting Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. V. L. Smith, Graduate Research Assistant, C. Lee Campbell, Assistant Professor, and S. F. Jenkins, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Plant Dis. 69:469-474. Accepted for publication 11 December 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-469.

The mean inoculum density of Sclerotium rolfsii in six commercial fields sampled during 1983 and 1984 ranged from 0.3 to 53.7 sclerotia per 300 cm³ of soil. The range of sclerotial density in the least and most heavily infested field was 0–5 and 2–225, respectively. The means and ranges varied with time of sampling and were influenced by previous cropping history and cultural practices. The frequency distribution of sclerotia among soil samples from the six fields at all sampling dates was best described by the negative binomial probability distribution. Values of the dispersion parameter, k, ranged from 0.15 to 4.4 and variance-to-mean ratios were significantly greater than unity for all samples, indicating a clustering of inoculum. The range of sclerotial density, frequency distribution of sclerotia, and extent of variability among samples obtained from one field were influenced by sample probe size. Samples taken with a 10.2-cm-diameter probe inserted 7.5 cm deep had a wide and variable range of inoculum density, and the distribution of inoculum was described by the negative binomial. Bulked samples obtained with a 1.8-cm-diameter soil auger inserted 12 cm deep had a smaller and less variable range of inoculum density, and the distribution was best described by the Poisson. Samples taken along simulated diagonal, parallel, or diamond-shaped paths in each of three nonuniformly infested fields provided similar mean inoculum density values, all of which were within 5% of the estimated population mean determined by the quadrat method. Total numbers of samples required were considerably reduced with these sampling patterns, however. Maximum recovery of laboratory-or soil-produced sclerotia from artificially infested soil samples was high (91–95%) with a wet-sieving assay method and intermediate (77–86%) with a flotation-sieving method. A methanol assay procedure provided a low recovery (28–33%) of viable soil-produced sclerotia and intermediate recovery (57–59%) of viable laboratory-produced sclerotia.

Keyword(s): disease incidence, southern blight.