P-054 - Preliminary field trial results on biopesticides against grape downy mildew in Virginia, 2024

Grape downy mildew, caused by an oomycete Plasmopara viticola, damages leaves and fruit of grapevines (Vitis spp.), potentially reducing crop yields by up to 75%. We are facing limited chemical options due to fungicide resistance and possible legal restrictions. In 2024, two field trials were conducted at experimental vineyards in Winchester, VA, to assess the effectiveness of nine biopesticides and four conventional products on Vitis vinifera 'Chardonnay' vines. The experimental design was a randomized complete block design with four blocks. We applied treatments to three consecutive vines every 14 days for six to eight weeks and visually estimated the incidence and severity of downy mildew from 60 leaves per treatment per block. A generalized regression procedure (JMP Pro, ver. 17, SAS Institute) was used to evaluate the effect of treatment on disease incidence and severity. Among treatments, the mean disease incidence and severity ranged from 39 to 100% and 1.1 to 32%, respectively. As expected, Captan, Cueva (copper octanoate), and Ranman (cyazofamid) resulted in significantly lower mildew severity (P ≤ 0.05) than the no-spray control treatment. Among the biopesticide treatments, Zonix (rhamnolipid) resulted in significantly lower incidence and severity in one trial. We did not find statistical significance with Lifegard (Bacillus mycoides isolate j) and Actigard (Acibenzolar-S-methyl), but disease incidence and severity were consistently lower than the control. Our results will aid growers in their product choices for effective grape downy mildew management.