INFLUENCE OF CROPS ON NUMBERS OF MICROSCLEROTIA OF VERTICILLIUM DAHLIAE IN SOILS AND THE DEVELOPMENT OF WILT IN SOUTHWESTERN ONTARIO

Abstract

Verticillium dahliae was studied in Harrow Sandy loam and Colwood loam using a root bioassay to determine microsclerotial numbers in soil. Plots established in the autumn of 1971 were sampled over a period of 12 mo to follow changes in populations under a susceptible crop, eggplant (Solanum melongena L.), indigenous weeds, spring-sown oats (Avena sativa L.), peas (Pisum sativum L.), snap bean (Phaseolus vulgaris L.) and bare fallow. At first, populations declined under all treatments, then increased in both soils under eggplant and in the weed plot on Harrow Sandy loam, which was heavily infested with velvet leaf (Abutilon theophrasti Medic). There was no increase in numbers of microsclerotia in the weed plot on Colwood loam, which was free from susceptible weeds. When populations in Harrow Sandy loam were reduced by fall fumigation with Vorlex (20% methyl isothiocyanate, 80% chlorinated C3 hydrocarbons) and Mylone (tetrahydro-3, 5-dimethyl-2H-1, 5-thiodiazine-2-thione), the rate of wilt development and fruit yields in eggplant were correlated with microsclerotial numbers in soil. The study showed that populations of 6–12 microsclerotia per gram of soil caused severe wilt in two field soils, and that the root bioassay was sufficiently sensitive to detect differences in numbers of microsclerotia ranging from 1–13 per gram of soil.