Climate, asynchronous phenology, ontogenic resistance, and the risk of disease in deciduous fruit crops

Abstract: Minimum chilling requirements of perennial fruit crops have been extensively studied, but little is known of how the degree and depth of winter chilling affects synchronization of host regrowth upon emergence from dormancy. The European grapevine species (Vitis vinifera) is a useful model system for studying the interactions between chilling, asynchronous phenology, development of ontogenic resistance, and the consequent risk of disease. Mean temperature of the three winter months ranged from -4.1 to 11.8 °C among 15 sites on 3 continents, and was associated with duration of bloom at each site: 2 d at the coldest sites, and > 2 wks at the warmest sites. This 7-fold increase in the duration of bloom directly translated to protracted susceptibility due to delayed development of ontogenic resistance to major fungal pathogens, including Erysiphe necator and Plasmopara viticola. Downstream effects of asynchronous bloom such as asynchronous ripening and sugar accumulation were also recorded. Asynchronous regrowth following unusually warm winters has been noted in grapevine, apple, and stone fruits. This asynchrony will likely increase the risk of disease in many pathosystems typified by phenology-defined windows of susceptibility. Bloom is an easily monitored phenological event that can be used to quantify the impact of winter chilling on the degree of asynchrony in perennial fruit crops. Once the relationship between winter temperatures and the duration of bloom is determined for a perennial fruit crop, the impact of climate change on the foregoing can be projected by examining these interactions across existing climatic gradients.