Effects of Soil Temperature, Moisture, and Depth on Survival and Activity of Sclerotinia minor, Sclerotium cepivorum, and Sporidesmium sclerotivorum

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Abstract
High soil temperature and low soil moisture reduced survival and activity of sclerotia of Sclerotium cepivorum and Sclerotinia minor and macroconidia of Sporidesmium sclerotivorum. Fifty percent of more of the propagules of the three fungi in the soil were killed within 39 hr at 40.degree. C, 6 hr at 45 C, or 2 hr at 50 C. When soil containing the propagules of these fungi was dried to a moisture level of -1,516 bars or lower for 7 days and remoistened to -0.2 bar, survival of the sclerotia and the viability of the macroconidia declined over a period of 6 wk. In the field, numbers of sclerotia of S. minor at the soil surface and at depths of 0-2 cm declined during the summer months, whereas numbers of sclerotia at depths of 2-8 and 8-14 cm increased sightly and then remained constant. Decline in numbers of sclerotia on the soil surface and at depths of 0-2 cm was assumed to be due to low soil moisture levels during the summer months. Apothecial initials (stipes) were formed on sclerotia during April through June at low levels (1-6%) at all soil depths. Natural infection of sclerotia of S. minor by Sporidesmium sclerotivorum in the field occurred irregularly and at low levels at the soil surface and at 0-2 cm. Infection of sclerotia was consistent and high at 2-8 and 8-14 cm. The implications of these findings for control of Allium white rot and lettuce drop are discussed.