Seasonal variation and effects of wheat rotation on populations ofVerticillium dahliae Kleb. in Ohio potato field soils

Abstract

Soil population levels ofVerticillium dahliae in Ohio were monitored from May–October, 1982–1985, in 15 fields in potato-wheat rotation and two fields in potato monoculture. Population levels in fields in rotation ranged from 0 to 86 microsclerotia/10 g of air-dried soil with average values during each 6-month sampling period of 9.7, 12.9, 9.6 and 19.6 microsclerotia/10 g of soil for 1982, 1983, 1984 and 1985, respectively. In at least 2/3 of the fields sampled each year, soil populations ofV. dahliae peaked in either July or August, regardless of whether fields were under wheat rotation or potato monoculture. Of the 15 fields under potato-wheat rotation, 13 showed a general pattern of increased populations ofV. dahliae in one or both years following cropping to potato. Among those 13, significantly higher populations (P < 0.05) were observed in one wheat field in 1983 and in eight fields in 1985. In the two fields in potato monoculture, one consistently had population levels ofV. dahliae 3–4 times higher than any other field sampled in this study. Factors that may contribute to periodic changes inV. dahliae populations, implications of these changes in interpreting soil population data, and usefulness of a potato-wheat rotation in managingV. dahliae populations in Ohio are discussed.