Simulating the susceptibility of clusters to grape black rot infections depending on their phenological development

Abstract In the years 2006–2008, clusters of Vitis vinifera (cv. Riesling) were inoculated with a conidia suspension of Guignardia bidwellii , the causal agent of grape black rot, at weekly intervals from one week before until eight weeks after bloom. Berries’ susceptibility turned out to be strongly linked to the speed of the phenological development in the respective season. In view of this, a degree day-based model simulating the relative susceptibility as a function of the phenological development was established. Infections on inflorescences were already possible prior to bloom. The period of highest susceptibility for direct infection on berries lasted from fruit set to the beginning of bunch closure. Successful infections took place until the end of bunch closure. Ontogenic resistance occurred clearly before veraison. Consequently, control measures should focus on the period between directly prior to bloom and bunch closure. Thus, the time span of black rot control coincides with the period in which vineyards need to be protected against the two major grape pathogens, Erysiphe necator and Plasmopara viticola . Applications of fungicides aimed at powdery and downy mildew control, also effective against G. bidwellii (e.g. strobilurins, triazoles and dithiocarbamates), offer a simultaneous control strategy for these three fungal diseases.