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Inoculum Density of Sclerotium cepivorum and the Incidence of White Rot of Onion and Garlic. F. J. Crowe, Formerly graduate research assistant, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Extension Plant Pathology, Kansas State University, Manhattan, KS 66506; D. H. Hall(2), A. S. Greathead(3), and K. G. Baghott(4). (2)Extension plant pathologist, University of California, Davis; (3)Farm advisor, Cooperative Extension Service, Monterey County, Salinas, CA 93901; (4)Farm advisor, Cooperative Extension Service, Modoc County, Tulelake, CA 96134. Phytopathology 70:64-69. Accepted for publication 24 July 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-64.

An improved soil assay for natural populations of Sclerotium cepivorum detected as few as 0.001 sclerotia per gram of soil. Survey data indicated that natural populations of sclerotia remained between 0.001 and 1.0 sclerotia per gram of soil for up to 8 yr, but decreased to between 0.001 and 0.01 sclerotia per gram of soil 1015 yr after Allium spp. crop plants were grown. No sclerotia were recovered from soil in two fields in which allium crops were grown 1720 yr prior to assay. Nearly all intact sclerotia recovered from infested soils germinated in soil when stimulated by garlic extract. Sclerotia which germinated were infective. A rapid, selective viability test was developed based on the characteristic growth of S. cepivorum on water agar. Disease incidence was dependent on pre-emergence inoculum density in uniform inoculum-density trials in different soils from several areas of California. Preemergence populations of ≤0.001, 0.0010.01, 0.010.1 and ≥0.1 sclerotia per gram of soil resulted in ≤10%, 1085%, 85100% and 100% incidence of disease in onion and garlic plants by harvest, respectively. At inoculum densities greater than 1.0 sclerotia per gram of soil most plants were killed soon after emergence. At lower inoculum densities, distinct clusters of plants became diseased as the pathogen spread from plant to plant. Disease loci appeared progressively later and with reduced frequency with decreasing inoculum density. Data from a naturally infested field were similar to those obtained from controlled experiments. Sclerotia populations declined, presumably due to germination, during the season in soil planted with onion or garlic. In plots infested at preemergence with as few as 0.00004 and as many as 10.0 sclerotia per gram of soil, between 0.4 and 1.0 sclerotia per gram of soil were recovered 6 mo after harvest; up to 9.2 sclerotia per gram of soil were recovered 6 mo after harvest from plots infested between 0.0010.01 sclerotia per gram of soil at preemergence.

Additional keywords: Allium cepa, A. sativum, yield loss, crop rotation, inoculum potential, fungistasis.