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Effects of Rain Intensity on Splash Dispersal of Colletotrichum acutatum. L. V. Madden, Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center (OARDC), Wooster 44691; Xiusheng Yang(2), and L. L. Wilson(3). (2)Department of Natural Resources Management and Engineering, University of Connecticut, Storrs 06269; (3)Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center (OARDC), Wooster 44691. Phytopathology 86:864-874. Accepted for publication 25 April 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-864.

A rain simulator was used to determine the effects of intensity (millimeters per hour or milliliters per square centimeter per hour) on splash dispersal of Colletotrichum acutatum conidia from a point source consisting of infected strawberry fruit. Seven wide-angle spray nozzles reproduced the cumulative volume distribution of natural rains with intensities of 2 to 60 mm h-1. Dispersal over a uniform soil surface was assessed in separate studies by either (i) collecting splash droplets with conidia in sheltered gravity samplers, consisting of petri plates with a selective medium for Colletotrichum spp. and counting colonies formed or (ii) exposing healthy immature strawberry fruit to incident and splashed water and determining the proportion of infected fruit after a 7-day incubation period. Colony density declined with distance from the source, increased over time to a maximum, and then declined. Estimated total colonies over time and space increased in a linear fashion with rain intensity. Proportion of infected fruit increased with time and decreased with distance. However, the proportion generally increased with intensity to a maximum, corresponding to about 15 to 30 mm h-1, and then declined. Transport of splash droplets across the soil surface was assessed by the intensity of water splashed into gravity samplers; from 0.6 to 6.3% of incident rain was splashed, the percent increasing with intensity and time during generated rain episodes. In other studies, the proportion of conidia removed from source fruit and the rate of removal both increased with rain intensity. Wash-off of conidia from healthy fruit also was estimated by placing a fixed number of conidia on fruit, exposing the fruit to rain, and determining the proportion of fruit infected. The multiple-infection transformation (MIT) of disease incidence, a measure of the number of infections per fruit, declined with intensity, and the rate of change in MIT increased with intensity, although this rate of wash-off was less than the rate of spore removal from the source fruit. By considering essential components of splash dispersal--removal of spores from the source, transport of spores in splash droplets, and wash-off of spores from potential infection sites--it was possible to relate dispersal to the physical properties of rain and explain the previously noted variable or surprising effects of rain intensity on pathogen dissemination.

Additional keywords: anthracnose, disease spread, Fragaria ananassa, models, quantitative epidemiology.