RELEASE OF VENTURIA-INAEQUALIS ASCOSPORES DURING UNSTEADY RAIN - RELATIONSHIP TO SPORE TRANSPORT AND DEPOSITION

Aerial concentrations of ascospores of Venturia inaequalis were measured by using Burkard spore traps located in several orchards in North Carolina during 1977-1980. Rainfall amounts were recorded continuously with rain gages located either next to the spore traps or at nearby locations. During the course of 20 separate events, for which significant ascospore release occurred, rainfall rates, R, ranged from 0 to 19.6 mm h-1, and 1-h average aerial ascospore concentrations ranged from 2 to 488 spores per cubic meter of air. A significant portion of the total ascospores (approx. 44%) were collected during hours when R was less than 0.25 mm h-1. A model of spore transport and deposition was developed that accounts for deposition of ascospores as a function of R. This model was used to calculate spore deposition for the temporal values of R and aerial ascospore concentration, C, observed for each spore release event. Deposition was also calculated by using the average rainfall rate, R(avg), for each event. These two methods of calculation were compared by using the ratio of spore deposition calculated with R(avg) to that calculated with the time history of R. This ratio ranged from 0.6 to 5.1 (mean = 1.5) at 0.5 km and from 1.0 to 10.6 (mean = 3.2) at 10 km downwind from a source. Our model presents a method for estimating deposition of spores released during rain and for estimating the range of error likely to occur in predicted spore deposition rates if average rainfall rates are used in lieu of actual time courses of rainfall in the calculation.