SPLASH DISPERSAL OF COLLETOTRICHUM-ACUTATUM AND PHYTOPHTHORA-CACTORUM BY SHORT-DURATION SIMULATED RAINS

The effect of short rain durations (less-than-or-equal-to 16 min) on splash dispersal of Colletotrichum acutatum, the cause of strawberry anthracnose, and Phytophthora cactorum, the cause of strawberry leather rot, was investigated using a rain simulator. Potted strawberry plants were held in two concentric circles, of radius 30 and 60 cm, respectively, surrounded by soil in an area that was uniformly exposed to simulated rain falling at 15 mm/h. Infected fruit with sporulating lesions (source fruit) were placed in the centre and healthy (target) fruit were placed in front of each circle of plants. Dispersal was measured as the proportion of fruit infected (disease incidence). Disease incidence increased with increasing rain duration. The incidence of anthracnose 30 cm from the inoculum source increased from c. 25% after 30s of rain to 99% after 16 min. The incidence of leather rot increased from 1% after 2 min to 26% after 16 min. In other experiments, source fruit and the entire target area were first exposed to 4 or 8 min of rain (pre-rain treatments), source fruit were then removed, and target fruit were placed on the soil and exposed to 4 or 16 min of rain. Splash dispersal still occurred, although there was a significant effect on disease incidence of a pre-rain on the source fruit. The incidence of C. acutatum infection was 68 and 98% for the 4- and 8-min pre-rain treatments, respectively, after 16 min of rain at 30 cm from the source. The incidence of P. cactorum infection was 15 and 36% for the 4- and 8-min pre-rain treatments, respectively, after 16 min of rain at 30 cm from the source. High disease incidence with pre-rain treatments was probably due to the rapid removal of spores by rain. Between 44 and 50% of spores of both pathogens were removed from the source fruit after 4 min of rain. When plants and fruit were arranged on a slope of 7-degrees inclination, there was greater dispersal of C. acutatum downhill than uphill. These results indicate that very short rains can result in high levels of disease if subsequent conditions are favourable for infection.