Decreased Virulence ofCochliobolus victoriaeConidia After Incubation on Soils or on Leached Sand

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Abstract
Conidia of C. victoriae on membrane filters were aseptically incubated for 15-30 days on leached sand or on 5 soils. Virulence of conidia was assessed using susceptible, aseptic oat seedlings in glass vials inoculated with 10-104 incubated or nonincubated conidia per vial. Seedlings in vials inoculated with incubated conidia usually had significantly (P = 0.05) reduced disease indices and longer roots and coleoptiles than seedlings inoculated with nonincubated conidia. Similar results were obtained with seedlings in soils. Incubated conidia also showed an increased dependence on exogenous nutrients for germination, retarded rates of germination, and reduced viability. Incubated conidia at inoculum densities 2 or more times those of nonincubated conidia produced less disease than nonincubated conidia, suggesting that decreased viability after incubation was not solely responsible for decreased virulence. Prolonged exposure to soils can reduce the virulence of conidia of C. victoriae.