Effect of scale on plant disease incidence and heterogeneity in a spatial hierarchy

The effect of patch scale on disease incidence and heterogeneity was assess ed in a three-scaled spatial hierarchy using data collected from strawberry leaf blight epidemics and with Monte-Carlo simulation studies. Cluster sam pling was used to collect disease incidence data from several strawberry fi elds at two commercial farms in Ohio over 2 years. Sampling units consisted of five leaves of three leaflets each. Leaflets represented the lowest sca le of the hierarchy, leaves represented the intermediate scale, and the sam pling units represented the upper scale. Results from data randomizations i ndicated that diseased leaflets were nearly maximally aggregated among leav es within sampling units, resulting in close to the minimum possible incide nce and heterogeneity of disease among sampling units at the leaf scale. Ba sed on Monte-Carlo simulations, incidence and heterogeneity of disease at t he leaflet (i.e. lowest) scale had the largest effect on incidence and hete rogeneity of disease at the leaf scale, respectively. However, analysis of variance of simulation results indicated that arrangement of diseased leafl ets among leaves within sampling units, sampling unit size ( = number of le aves or leaflets per sampling unit), leaf complexity (leaflets per leaf), a nd all interactions affected disease incidence and heterogeneity at the lea f scale. For a given level of intensity of leaflet disease incidence and he terogeneity, incidence and heterogeneity of disease at the leaf scale were most affected by sampling unit size when leaflets were arranged in an aggre gated fashion among leaves; increasing the number of leaves per sampling un it was associated with decreasing disease incidence and increasing heteroge neity at the leaf scale. However, leaf complexity had the greater effect on leaf disease incidence and heterogeneity when diseased leaflets were arran ged randomly or uniformly among leaves within sampling units; greater leaf complexity was associated with greater incidence and heterogeneity. Combine d results indicated that incidence and heterogeneity of incidence of Phomop sis leaf blight varied between scales in a patch hierarchy. This variabilit y was shown to be a function of lower-scale disease incidence and heterogen eity, sampling unit size, and leaf complexity. (C) 2001 Elsevier Science B. V. All rights reserved.