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

Abstract

The effect of patch scale on disease incidence and heterogeneity was assessed in a three-scaled spatial hierarchy using data collected from strawberry leaf blight epidemics and with Monte-Carlo simulation studies. Cluster sampling was used to collect disease incidence data from several strawberry fields at two commercial farms in Ohio over 2 years. Sampling units consisted of five leaves of three leaflets each. Leaflets represented the lowest scale of the hierarchy, leaves represented the intermediate scale, and the sampling units represented the upper scale. Results from data randomizations indicated that diseased leaflets were nearly maximally aggregated among leaves within sampling units, resulting in close to the minimum possible incidence and heterogeneity of disease among sampling units at the leaf scale. Based on Monte-Carlo simulations, incidence and heterogeneity of disease at the leaflet (i.e. lowest) scale had the largest effect on incidence and heterogeneity of disease at the leaf scale, respectively. However, analysis of variance of simulation results indicated that arrangement of diseased leaflets among leaves within sampling units, sampling unit size (=number of leaves or leaflets per sampling unit), leaf complexity (leaflets per leaf), and all interactions affected disease incidence and heterogeneity at the leaf scale. For a given level of intensity of leaflet disease incidence and heterogeneity, incidence and heterogeneity of disease at the leaf scale were most affected by sampling unit size when leaflets were arranged in an aggregated fashion among leaves; increasing the number of leaves per sampling unit was associated with decreasing disease incidence and increasing heterogeneity at the leaf scale. However, leaf complexity had the greater effect on leaf disease incidence and heterogeneity when diseased leaflets were arranged randomly or uniformly among leaves within sampling units; greater leaf complexity was associated with greater incidence and heterogeneity. Combined results indicated that incidence and heterogeneity of incidence of Phomopsis leaf blight varied between scales in a patch hierarchy. This variability was shown to be a function of lower-scale disease incidence and heterogeneity, sampling unit size, and leaf complexity.