AGGREGATION OF COLLETOTRICHUM-ACUTATUM IN RESPONSE TO SIMULATED RAIN EPISODES

The influence of rain splash dispersal on the aggregation and variabil ity of spores of Colletotrichum acutatum, the causal agent of strawber ry anthracnose, was studied with simulated rain over a soil surface. I n the first experiment, rains with intensities of 15 and 30 mm/h were generated and sampling plates, with a selective medium for Colletotric hum, were positioned under rainshields at both 20 and 60 cm from an in oculum point source (five infected fruit) for 1-min exposure periods. Number of colonies growing in plates were used to measure spore densit y. For each distance, time, rain intensity, and replication, the mean (m), variance (v), and Lloyd's index of patchiness (LIP) were calculat ed. Estimated (LIP) exceeded 1, indicating the high variability and cl ustering of colonies, and hence spores, resulting from splash dispersa l. 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 In (m). The slope (b) of the relation for all data points was 1.67, which. wa s significantly (P < 0.01) greater than 1, a further indication of clu stering. Distance form the source had a slight positive effect on b. C olony means (Y) were transformed to Y(star) = Y1/5 based on an approxi mate b of 1.6. Analysis of Y(star) indicated an expected significant m ain 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 exce eded 1, and Taylor's b equaled 1.63. In a final study, the inoculum so urce and sampling plates were raised 46 cm above the soil surface. The re was a drastic reduction in Y(star) compared to surface values, supp orting the prior hypothesis that surface topography strongly affects s plash 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.