Relationships between Venturia inaequalis potential ascospore dose, aerial ascospore concentration, climatic risks and scab development; effects on experimental designs

Abstract: During 2008-2009, in an experimental apple orchard located at Gotheron (Drôme,France), Venturia inaequalis potential ascospore doses, aerial ascospore concentrations and totalascospores trapped were compared in different plots of a pure stand and a cultivar mixture withor without sanitation practices. The climatic risks were evaluated and scab development wasobserved on leaves and fruits. A very weak correlation between potential ascospore dose, aerialascospore concentration and total ascospore trapped was observed in 2008 in spite of theexperimental design with an edge and a distance of 15 meters separating the plots. In 2009 thiscorrelation was significantly increased, after putting up a net between the plots. Methodologicaleffects for the experimental assessment of efficacy of the sanitation practices are discussed.Moreover, our study permitted in 2009 a field validation of one of the conditions defined byStensvand et al. (1997) for infection by ascospores at low temperatures.