Spatial Pattern, Inoculum Density-Disease Incidence Relationship, and Population Dynamics of Sclerotium rolfsii on Apple Rootstock. S. F. Tomasino, Former Graduate Research Assistant, Department of Plant Pathology, Oklahoma State University, Stillwater 74078. K. E. Conway, Associate Professor, Department of Plant Pathology, Oklahoma State University, Stillwater 74078. Plant Dis. 71:719-724. Accepted for publication 18 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0719.

Nursery plots of apple at Tahlequah and Stillwater, OK, showing southern blight (Sclerotium rolfsii) symptoms were divided into 54 (2.75 × 2.75 m) and 30 (2.75 × 2.05 m) quadrats, respectively, and randomly sampled for sclerotia. Average preplant densities of sclerotia of S. rolfsii as determined by a wet-sieving/methanol assay technique ranged from 3.6 to 4.7 viable sclerotia per 1,000 g of dry soil at the apple nurseries. Numbers of sclerotia from individual quadrats ranged from 0 to 7. The spatial pattern of sclerotia extracted from bulked samples from a naturally and artificially infested field plot was random, according to Fisher's variance-to-mean ratio, frequency distribution analysis, and the k dispersion parameter of the negative binomial distribution. A mean density of 44 sclerotia per 525 g of dry soil was recovered from soil samples taken adjacent to dead trees in studies performed in microplots. These densities declined with increase in time between tree death and sampling. Densities of sclerotia adjacent to and at various distances from dead trees were best described by a polynomial regression equation. A positive linear relationship was observed between sclerotial densities in soil and disease incidence. Five sclerotia per 1,000 g of dry soil resulted in 19, 5, and 35% disease in container, microplot, and field studies, respectively. Placing sclerotia 3 cm from apple rootstock, 0.5 cm deep, grown in containers in the field, resulted in lower disease incidence than placing sclerotia in contact with the rootstock.