A model simulating deposition of Venturia inaequalis ascospores on apple trees *

Based on existing physical theories and models, a dynamic model estimating the concentration of Venturia inaequalis ascospores in the orchard air and their deposition on apple leaves was elaborated. The model produces two main outputs: number of ascospores deposited per leaf and proportion of ascospores discharged from pseudothecia deposited onto the leaves. The model has a relatively simple structure, and computations are based on few algorithms, which are implemented on an electronic data sheet of common use. Nevertheless, it preserves the accuracy of more complex physical models reasonably well. The model includes the effect of meteorological conditions and horticultural characteristics, and thus provides information for each type of orchard. Few input variables are required: wind speed and rainfall rate can be measured in standard meteorological stations; horticultural characteristics of the orchard can be determined for each type of orchard. The model produces conservative estimates of ascospore deposition, because it assumes a complete retention of the spores deposited by rainfall and does not consider either deposition on stems and flowers or the spatial distribution of plant surfaces. After further validation under orchard conditions, the model will be used to obtain better estimates of scab infection risk in current scab control strategies.