New Fungicides to Manage Phytophthora Root and Crown Rots of Sweet Cherry in California

A limited number of effective fungicides available warranted the evaluation of new compounds to manage Phytophthora diseases of sweet cherry. In multiyear greenhouse and field studies with Phytophthora cambivora-, P. cactorum-, and P. citricola-inoculated soils, soil applications with the Oomycota fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin significantly reduced incidence and severity of Phytophthora root and crown rots with comparable or higher efficacy than the registered mefenoxam or potassium phosphite. Mid-day stem water potential measurements in the field to determine water stress resulting from root and crown infections agreed with visual tree health ratings. Fluopicolide caused leaf phytotoxicity at a field site with a Monserate sandy loam soil but not at a site with a Yolo silty clay loam soil. Injury was significantly reduced when fluopicolide was rotated with oxathiapiprolin. Soil-applied ethaboxam, fluopicolide, and oxathiapiprolin moved into roots and then acropetally into rootstocks and scions at different amounts where they reduced canker development after wound inoculation with P. citricola. Fluopicolide showed higher mobility than ethaboxam or oxathiapiprolin. Fungicide mobility was also empirically predicted by the adaptive Bromilow model, which uses the partition coefficient logP (indicating lipophilicity or hydrophilicity) and the acid dissociation constant pKa of a compound. Results of our studies support registration of these fungicides and will help to optimize their use on cherry and other tree crops for disease prevention and management.