SPATIAL HETEROGENEITY OF THE INCIDENCE OF GRAPE DOWNY MILDEW

Heterogeneity of the incidence of downy mildew of grape, caused by Pla smopara viticola, was quantified in an experimental Ohio vineyard. The proportion of diseased leaves on each of 15 shoots (sampling units) p er plot was determined for 18 separate plots at two times (generally A ugust and September) during each of 3 yr. The binary data analogue of Taylor's power law, in which the logarithm of the observed variance is regressed on the logarithm of the theoretical variance for a binomial (random) distribution, provided strong and consistent evidence that d iseased leaves were aggregated. Year and assessment time did not affec t power law parameters. The estimate of the regression slope (b), an o verall measure of heterogeneity, was 1.30 (SE = 0.04). Heterogeneity i n individual plots was measured with variance ratio and C(alpha) tests and with the aggregation parameter (theta) of the beta-binomial distr ibution fitted to the data. Except for mean incidence less than 0.05, the majority of the plots had significant heterogeneity, and the data were better described by the beta-binomial than by the binomial distri bution. Estimates of theta were variable but were highest in the middl e range of disease incidence. Using the modified power law, sampling c urves were generated to precisely estimate disease incidence.