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Sclerotial Inoculum Density of Phymatotrichum omnivorum and Development of Phymatotrichum Root Rot in Cotton. Stuart D. Lyda, Associate Professor, Department of Plant Sciences, Texas A&M University, College Station 77843; Earl Burnett, Research Soil Scientist, USDA, ARS, Blackland Conservation Research Center, Temple, Texas 76501. Phytopathology 60:729-731. Accepted for publication 30 November 1969. DOI: 10.1094/Phyto-60-729.

Sclerotia of Phymatotrichum omnivorum were produced in sterile soil culture and recovered by wet sieving. They were air-dried and added to screened Houston Black clay to establish known inoculum densities. Maximum disease in cotton occurred with 125 to 625 sclerotia/kg of dry soil when the soil was maintained at 28 C. When the sclerotial inoculum was uniformly distributed throughout the soil, increasing densities to 3,125 and 15,625/kg of dry soil gave progressively less disease than 625/kg. Disease incidence was ascertained by number of dead plants. There was a delay in the rate of disease development when these same levels of sclerotia were centrally positioned within the soil; however, the final percentage of dead plants was the same as for 125 or 625 sclerotia/kg soil. Sufficient numbers of sclerotia survived air-drying and screening of the soil from the highest level of sclerotial infestation in the inoculum density tests to cause 100% plant kill when the soil was replanted to cotton.