Comparative spatial analysis of foliar epidemics on white clover caused by viruses, fungi, and a bacterium

Incidence and spatial patterns of eight foliar pathogens and diseases were monitored during two 6-wk growth periods per year from June to September in 1990 and 1991 on regularly spaced plants of white clover (Trifolium repens) planted in a tall fescue (Festuca arundinacea) sward in a 10-ha pasture in Wake County, North Carolina. Disease ratings were made in four plots on a total of 512 plants in eight square lattices of 64 plants each. Each plot comprised two proximal lattices of either the virus-susceptible white clover cultivar, Regal, or the Southern Regional Virus Resistant white clover germ plasm. Incidence of three viruses (alfalfa mosaic virus, clover yellow vein virus, and peanut stunt virus) was assessed by indirect enzyme-linked immunosorbent assay in June or July, August, and September. Per-plant incidence of the following leaf spot diseases was assessed on 16 dates during 1990-1991: black spot (Pseudomonas andropogonis), Cercospora leaf spot (Cercospora zebrina), Curvularia leaf spot (Curvularia trifolii), summer blight (Rhizoctonia solani), and Stagonospora leaf spot (Stagonospora meliloti). Two-dimensional distance class analysis was used to quantify spatial attributes of viral and leaf spot epidemics on the two white clover host populations. Virus-resistance altered the spatial pattern of the virus disease complex. Viral epidemics in populations of Regal were characterized by higher disease incidence, stronger aggregation of diseased individuals, more numerous and larger clusters of virus-infected plants, and stronger edge effects than for those in populations of the Southern Regional Virus Resistant germ plasm. Spatial patterns of the leaf spot diseases were similar in the virus-resistant and virus-susceptible host populations. In general, spatial patterns of the fungal and bacterial leaf spot diseases reflected the gradient and mode of pathogen dispersal. Significant edge effects and large, well-defined, expanding clusters were characteristic of the splash-dispersed pathogens, P. andropogonis and S. meliloti. Smaller clusters (in plot interiors rather than plot edges) were found for the aerially dispersed pathogens C. zebrina and Curvularia trifolii, which have shallower dispersal gradients. Defoliation throughout the epidemics was related to continual changes in disease incidence, strength of aggregation and cluster location, and morphology. Variability in spatial attributes between and within years, among plots, and between growth period