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Factors Affecting Survival of Sclerotia, and Effects of Inoculum Density, Relative Position, and Distance of Sclerotia from the Host on Infection of Lettuce by Sclerotinia minor. E. D. Imolehin, Research assistant, Department of Plant Pathology, University of California, Davis 95616; R. G. Grogan, professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 70:1162-1167. Accepted for publication 19 May 1980. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-1162.

Inoculum densities of Sclerotinia minor found in surveys of lettuce (Lactuca sativa) fields in Salinas Valley, California, during 1978 and 1979 ranged from zero to three sclerotia per 70 ml (100 g) sample of soil. Sclerotia capable of eruptive germination and thus potentially able to cause infection (competent), ranged from 0 to 0.80 per 70 ml of soil. Within each field no significant differences were found among inoculum densities for different samples or sampling periods within a single crop season. Incidence of drop ranged from 0.5 to 18.5% and was proportional to both inoculum density of competent sclerotia (r2 = 0.80) and total sclerotia (r2 = 0.76). Sclerotia developed at all depths on newly infected lettuce tissue buried at 0 to 30 cm, but fewer were formed at greater depths than at or near the soil surface. Numbers of recoverable sclerotia formed on buried infected tissue and their percent germination decreased progressively with time of burial. Sclerotia survived better in dryer than in moist soil and better at shallower than at greater depths; washing and drying prior to burial had no effect on survival. Sclerotia survived better in a field with suppressive soil (without history of drop) than in a field with nonsuppressive soil (with a history of drop). Trichoderma spp. were isolated most frequently from retrieved ungerminated sclerotia and most isolates were parasitic on S. minor, but there was variability among isolates. Sclerotia in contact with the main lettuce stem on the soil surface caused the highest percent of infection; if located more than 1 cm from the plant, no infection resulted in most instances. Sclerotia in contact with the main root at greater depths were less effective in causing drop than those at shallower depths.

Additional keywords: epidemiology.