UC Davis

Background and goals In California’s southern San Joaquin Valley, applications of pruning-wound protectants may lower the incidence of grapevine trunk diseases. Rain is important for spore dispersal of the causal fungi, which infect pruning wounds. Despite low annual rainfall in this region, pruning wounds are at risk of infection, based on reports of symptoms and pathogen detection in vineyards, and from controlled inoculations demonstrating susceptibility of Vitis vinifera table grape cultivars. Therefore, our goal was to evaluate protectants for pruning wounds, testing two commercial fungicides, thiophanate-methyl (Topsin; UPL) and pyraclostrobin + boscalid (Pristine; BASF), for three years in an 8-yr-old Autumn King vineyard. Methods and key findings For three years, liquid formulations of fungicides were applied alone or with Pentra-bark (Quest Products Corp.), an adjuvant to enhance pesticide penetration of the bark of woody crops. After spray application, pruning wounds were inoculated with spores of the fungi that cause the trunk diseases Botryosphaeria dieback (Neofusicoccum parvum), Esca (Phaeoacremonium minimum), and Eutypa dieback (Eutypa lata). Both thiophanate-methyl and pyraclostrobin + boscalid were effective against N. parvum, with greater efficacy using Pentra-bark in two study years. Both fungicides were moderately effective against E. lata, with greater efficacy using Pentra-bark in one study year. Neither fungicide, alone or with Pentrabark, was effective against P. minimum. Conclusions and significance One application of thiophanate-methyl or pyraclostrobin + boscalid to pruning wounds (after pruning and before rain) may prevent Botryosphaeria and Eutypa dieback, but not Esca. Greater fungicide efficacy with Pentra-bark against E. lata and N. parvum in some years warrants further testing, for example, at higher concentrations.