Improving the toolbox to manage Phytophthora diseases of strawberry: searching for chemical alternatives
Florida strawberry production is affected by two economically important Phytophthora diseases, Phytophthora crown rot (PhCR) and leather rot (LR), caused primarily by P. cactorum and P. nicotianae. Although products are available, chemical control is limited to mefenoxam, phosphites, and azoxystrobin and resistance to mefenoxam and azoxystrobin has been reported. Both in vitro and in vivo assays were conducted to evaluate the effectiveness of eight different chemical classes on controlling PhCR and LR of strawberry. The fungicides mandipropamid, fluopicolide, oxathiapiprolin, and cymoxanil completely inhibited mycelial growth of both species at 1µg/ml. The same fungicides controlled LR and PhCR caused by P. cactorum, in detached fruit and greenhouse trials, respectively, including mefenoxam-resistant isolates. However, for PhCR caused by P. nicotianae only oxathiapiprolin and cymoxanil were effective in controlling the disease. Cyazofamid, fluazinam, propamocarb, and tebuconazole did not control PhCR caused by either Phytophthora spp. Except for tebuconazole, all fungicides tested reduced incidence of LR caused by P. cactorum in the detached fruit assay; in contrast, fluazinam, cyazofamid, and propamocarb had no effect on P. nicotianae. This study identifies four fungicides that could potentially be registered to manage both PhCR and LR of strawberry. The registration of additional fungicides with different modes of action would allow fruit and nursery growers to alternate products to reduce fungicide resistance risk.