Age-related susceptibility of strawberry leaves and berries to infection by Podosphaera aphanis

Powdery mildew, caused by Podosphaera aphanis, is a major disease of strawberry worldwide. The importance of the disease varies with production system (June bearing vs day neutral) which could be explained in part by the concurrent presence of susceptible leaves or berries and abundant airborne inoculum. Age-related susceptibility was studied by inoculating strawberry leaves and berries at different growth stages of the June bearing cultivar Jewel and the day neutral cultivar Seascape. On eight occasions in 2007, five plants of each cultivar were inoculated with dry conidia using a settling tower. There was a significant effect of leaf and berry growth stage on the susceptibility, which decreased exponentially as leaves or berries aged to reach almost zero when the leaves were completely expanded or the berries at the pink stage at the time of inoculation. The proportion of maximum mildew severity as a function of leaf or berry growth stage was predicted using non-parametric regression (R(2) = 0.96-0.97). The predicted values were validated with data collected in fields or tunnels naturally infected by P. aphanis. There was a linear relation between predicted and observed proportion of maximum mildew severity (R(2) = 0.95-0.99). The results of this study showed that timing fungicide sprays based on periods of high leaf and berry susceptibility should greatly improve management of strawberry powdery mildew. (C) 2010 Elsevier Ltd. All rights reserved.