Ecology and Epidemiology
Characteristics of splash dispersal of Phytophthora cactorum from infected strawberry fruit were studied with an integrated drop-generating and photographic system that was developed previously. Drops of 1- to 4-mm diameter were released from heights of 10, 20, and 40 cm above fruit bearing sporangia that had been labeled with a fluorescent tracer. Splash droplets produced by the impact of the incident drops were collected on sheets of water-sensitive paper. Drops with diameters of 1 and 2 mm failed to produce any splash droplets when released from heights of 20 and 10 cm, respectively. Multivariate regression analysis was used to examine the relationship between a set of intercorrelated responses and attributes of the incident drop. Responses evaluated were 1) the total number of droplets collected, 2) the average droplet diameter, 3) the total droplet volume, 4) the mean distance of droplet travel, 5) the number of droplets bearing at least one sporangium, and 6) the total number of sporangia among droplets with at least one sporangium. Incident drop attributes were drop mass or weight (W) and the velocity (V), momentum (M), and kinetic energy (E) of the drop at impact with the fruit surface. The best multivariate regression model included W, M, and V as predictors of responses 1, 2, and 4. M alone was the best predictor of response 3; in contrast, V was significant alone for responses 5 and 6. Except for droplets produced by the impact of 1-mm-diameter drops, the average diameter of droplets carrying sporangia was always significantly greater than the diameter of droplets not carrying sporangia (P = 0.05). The distribution of the number of sporangia per droplet was generally well described by the logarithmic distribution with zeros for each combination of incident drop diameter and height of release. The ? parameter, representing the proportion of droplets with sporangia, declined in relation to momentum at impact. The ? parameter, a measure of the mean number of sporangia per droplet (of droplets bearing sporangia), declined with increasing mass of the incident drop. There was no relation between droplet size and the number of sporangia per droplet. For all splash droplets and those with sporangia, the distribution over distance was accurately described by the Weibull distribution whose scale parameter (representing the distance within which 63% of the droplets landed) varied linearly and positively with incident drop velocity. The Weibull shape parameter (representing curve skewness) was best described as a function of both drop velocity and kinetic energy or momentum. Droplets with sporangia did not travel as far, on the average, as droplets without sporangia. |