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Ecology and Epidemiology

Effect of Water Potential on Survival of Sclerotia of Sclerotinia minor in Two California Soils. G. S. Abawi, Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell University, Geneva 14456; R. G. Grogan(2), and J. M. Duniway(3). (2)(3)Department of Plant Pathology, University of California, Davis 95616. Phytopathology 75:217-221. Accepted for publication 17 September 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-217.

Survival of sclerotia of Sclerotinia minor was studied in two Salinas Valley soils, one with (soil A) and one without (soil B) a previous history of high incidence of lettuce drop. The soil matric water potentials (φm) tested were 0, - 0.1, - 0.3, - 1, - 2, - 5, - 10, and - 15 bars. Both soils were also maintained in an air-dried state (- 400 bars) or were watered once a week and allowed to dry to give a fluctuating water regime (0 to - 430 bars). All sclerotia either had disintegrated or failed to germinate after 8 wk of incubation at φm = 0 in both soils. In contrast, no significant (P = 0.05) reduction in sclerotial germination occurred in either soil subjected to the fluctuating water regime after 52 wk of incubation, the longest period tested. At all other φm values tested, viability of sclerotia in soil A progressively decreased with time and with increasing φm, and only the fluctuating water treatment contained viable sclerotia after 52 wk of incubation. In soil B, most sclerotia were still viable after 52 wk of incubation at φm values of - 0.1 to - 15 bars, but only 10% of those in the air-dried treatment were viable. When the φm of soil B was allowed to fluctuate, nearly 100% of the sclerotia remained viable for 52 wk. The two soils differed in gas composition, water content, texture, organic matter, electrical conductivity, cropping history, and microorganisms isolated from sclerotia. However, the data obtained suggest that the differences in the biological activities of the two soils probably account for the differential survival of sclerotia in these soils. Trichoderma spp. represented 41 and 1% of total organisms recovered from sclerotia buried in soil A and B, respectively, using a sclerotium agar medium.