Splash dispersal of Colletotrichum acutatum and Phytophthora cactorum from strawberry fruit by single drop impactions

Dispersal of Colletotrichum acutatum conidia and Phytophthora cactorum sporangia by single drop impactions onto strawberry fruit was studied using a drop-generating system. Uniform water drops, 0.5-4 mm in diameter, were released from heights of 25-150 cm above infected target fruit with spores labeled with fluorescent tracer. Splash droplets were collected on water-sensitive paper. Size and distance from the source of each droplet trace were determined using an image analysis system, and number of spores contained in randomly chosen traces were counted by means of fluorescent microscopy. Size and fall height of impacting drops were shown to have a significant effect on number, mass, and travel distance of splash droplets, and on percentage of droplets with no conidia of C. acutatum. Only size of incident drops significantly affected droplet diameter. Number of spores per droplet generally was well described by the log-normal distribution for C. acutatum and the negative binomial distribution for P. cactorum. For C. acutatum, both size and fall height of impacting drops significantly affected (transformed) number of spores per droplet and total number of spores per impaction, but only drop size significantly affected spore entrainment with P. cactorum. Mean number of spores per droplet ranged from 6 to 134 for C. acutatum, and from 0.2 to 0.8 for P. cactorum, resulting in totals of 17-11,546 conidia or 9-56 sporangia dispersed by a single impaction. Transformed total spores per impaction for both pathogens were linearly and positively related to impact velocity of incident drops on a log scale. The percentage of splash droplets with no C. acutatum conidia was significantly correlated with ln(kinetic energy) of incident drops at impaction. Percentage of droplets without P. cactorum sporangia was unaffected by impacting drop size, fall height, or kinetic energy. A weak-positive relationship between spores per splash droplet and droplet diameter also was found for both C. acutatum and P. cactorum. Differences in dispersal between the two pathogens could be partially attributed to higher inoculum density of C. acutatum on the fruit surface compared to P. cactorum.