Interactions among fungicides applied at different timings for the control of Botrytis bunch rot in grapevine

Botrytis bunch rot (BBR), caused by Botrytis cinerea, is one of the main diseases affecting grapevines. Due to the complexity of the B. cinerea life cycle and the existence of different infection pathways affecting grapevine tissues at both early and late growth stages, fungicides are usually applied sequentially at the end of flowering (A), prebunch closure (B), veraison (C), and before harvest (D). Interactions among fungicides (from different groups) applied at these growth stages were evaluated in this work, with focus on the strategies in which early- and late-season applications are combined (i.e., strategies AB, CD, ABC, ABD, ACD, BCD and ABCD). The evaluation was performed in a set of 116 studies carried out in different years and locations, by comparing the observed (b(obs)) and expected (b(pred)) efficacies in controlling BBR; b(obs) was calculated as the reduction of BBR severity in treated plots compared to untreated ones, while b(pred) was calculated by using a mathematical function. Early-season sprays (i.e., A and B) showed non-additive interactions (i.e., the observed efficacy was significantly lower than expected in case of additive effect) while late-season sprays (i.e., C and D) did. No significant synergistic effects were observed among fungicide sprays. In the early-season, spraying in A was more effective than in B, and both sprays (A and B) were useful under high disease pressure only, when the full ABCD strategy was needed for effective BBR control. Otherwise, the most effective combination was ACD, able to exploit the additive control of the early-season infection pathways and the multiple infection events during berry ripening.