Effects of surface topography and rain intensity on splash dispersal of Colletotrichum acutatum

Effects of ground cover, plant canopy density, and rain intensity on the splash dispersal of Colletotrichum acutatum were studied using a rain simulator. In one experiment, three ground covers (soil, straw, and plastic) and two rain intensities (15 and 30 mm/hr) were evaluated by collecting splash droplets with conidia in sheltered gravity samplers consisting of petri plates with a selective medium for C. acutatum. Ground covers were characterized by random roughness, the standard deviation of surface elevation. Infected fruits with sporulating lesions were clustered on the ground to serve as the inoculum source. Sampling plates were positioned 20, 40, 60, 80, 100, and 120 cm from the source and were exposed to rain for 1-min periods (every 5 min) for a total duration of 46 min. Ground cover had a major effect on splash dispersal, as measured by colonies growing in selective medium. The estimated total number of colonies over time and space (N) was inversely proportional to roughness; i.e., straw had the largest random roughness but the lowest N, and plastic the opposite. Differences in N among ground covers were due to differences in steepness of the dispersal gradients (i.e., straw had the steepest gradient and plastic the shallowest), not to the release rate of spores at the source. Total number of colonies increased with rain intensity, but the effect was due to the release rate, as measured by the intercept parameter of a gradient model. Rain intensity did not influence gradient steepness. The effect of plant canopy density on splash dispersal of C. acutatum was evaluated in a second experiment using a soil ground cover. Leaf area index (LAI) was inversely related to N. Cross-row dispersal of spores was reduced by 90% or more for two rows of plants with LAI greater than or equal to 2.7 compared to no plant rows. Results support the hypothesis that surface topography (including ground cover and plant canopy) is a major factor controlling splash dispersal due to its effect on splash droplet trajectories and loss of inoculum.