THE RELATIONSHIP BETWEEN THE AMOUNT OF VERTICILLIUM-DAHLIAE IN SOIL AND THE INCIDENCE OF STRAWBERRY WILT AS A BASIS FOR DISEASE RISK PREDICTION

The incidence of wilt was recorded in runner and fruiting crops of 13 strawberry cultivars at 72 locations in southern England in 1989 and 1 990, and soil samples from the sites were analysed for Verticillium da hliae. Linear regressions of wilt incidence on inoculum concentration in soil for runner crops of the susceptible cv. Elsanta in both years were significant whilst that for runner crops of the susceptible cv. H apil in 1989 approached significance; the regression for cv. Elsanta f ruiting crops in 1990 was not significant. The inclusion of sand conte nt of soil in the regression model improved the fit for the cv. Hapil data but not for the cv. Elsanta data; neither clay nor silt content o f soil significantly improved the fit of the models for any data set. There were insufficient data in either year for regression analysis fo r other cultivars, but the levels of wilt generally corresponded with the degree of soil infestation and broadly reflected known field resis tance. The data were used to estimate an inoculum concentration which corresponds to 5% wilt incidence (IC5) for cv. Elsanta. It is suggeste d that this could be used as a yardstick for determining the risk of u nacceptable levels of wilt in susceptible cultivars on the basis of pr e-planting soil analysis. For the 44 sites where the cropping history over the 15 years prior to soil analysis was available there was no cl ear association between any crop and soil infestation levels at or abo ve the IC5. However, V. dahliae was more common at sites with a histor y of vegetatively propagated crops than at sites which had only suppor ted crops grown from true seed.