Aggregation of Colletotrichum acutatum in Response to Simulated Rain Episodes

Abstract

AbstractThe influence of rain splash dispersal on the aggregation and variability of spores of Colletotrichum acutatum, the causal agent of strawberry anthracnose, was studied with simulated rain over a soil surface. In the first experiment, rains with intensities of 15 and 30 mm/h were generated and sampling plates, with a selective medium for Colletotrichum, were positioned under rainshields at both 20 and 60 cm from an inoculum point source (five infected fruit) for 1‐min exposure periods. Number of colonies growing in plates were used to measure spore density. For each distance, time, rain intensity, and replication, the mean (m), variance (v), and Lloyd's index of patchiness (LIP) were calculated. Estimated (LIP) exceeded 1, indicating the high variability and clustering of colonies, and hence spores, resulting from splash dispersal. LIP increased with distance from the source and with rain intensity at the greater distance. Aggregation was further quantified by Taylor's power law describing the linear relation between ln (v) and ln (m). The slope (b) of the relation for all data points was 1·67, which was significantly (P < 0·01) greater than 1, a further indication of clustering. Distance form the source had a slight positive effect on b. Colony means (Y) were transformed to Y*=Y1/5 based on an approximate b of 1·6. Analysis of Y* indicated an expected significant main effect of rain duration and a significant interaction of distance and rain intensity. In another experiment, six infected fruit and the sampling plates were uniformly placed over the surface. LIP again exceeded 1, and Taylor's b equaled 1·63. In a final study, the inoculum source and sampling plates were raised 46 cm above the soil surface. There was a drastic reduction in Y* compared to surface values, supporting the prior hypothesis that surface topography strongly affects splash dispersal. Results indicate that the physical process of splash dispersal produces a degree of aggregation similar to that produced by population dynamic and other ecological processes.