Is it possible to predict the aerial concentrations of Venturia inaequalis ascospores in apple orchards?

Abstract: Daily aerial concentrations of ascospores of Venturia inaequalis, the infectious agent responsible for apple scab, were observed over four years in apple orchards in the Drôme and Maine-et-Loire departments in France. These concentrations were recorded throughout the entire primary ejection period with Burkard 7-day volumetric spore traps, placed directly on the ground at the inter-row level of the orchard. During days with particularly high ejections, i.e., greater than 5% of the total quantity of ascospores trapped for the year, concentrations of more than 400 ascospores/m3 of sampled air could be observed in the two regions. Using meteorological data recorded by the weather stations located near the orchards studied, it was possible to model daily ascospore ejections with two types of decision support software used on a regular basis in France for agricultural warning systems. However, these models did not correctly estimate a significant number of large ejections for some of the years. It would therefore be unrealistic to recommend the use of these modelled values of daily ascospore ejections for pest control strategies requiring precise details about these quantities, without taking excessive risks. On the other hand, it seems possible to use these two models to determine the period (from 1 to 2 months, depending on the year) during which the aerial concentrations of ascospores are the highest.