UC Davis

Five Phytophthora species comprising a total of 243 isolates (77 P. cinnamomi, 23 P. citrophthora, 18 P. multivora, 18 P. pini, and 107 P. plurivora) were screened for sensitivity to mefenoxam, fosetyl-Al, dimethomorph, dimethomorph + ametoctradin, and fluoxastrobin using amended agar assays. Mefenoxam-insensitive isolates were detected within P. cinnamomi (4%), P. multivora (11%), and P. plurivora (12%) even atapproximately 2.5×the recommendedlabelrate. These isolates were also insensitive to higher (off-label) concentrations of fluoxastrobin. Concentrations of dimethomorph (400 lg/ml) and dimethomorph + ametoctradin (100 lg/ml) were mostly effective in mycelial growth inhibition, but two P. plurivora isolates were insensitive, suggesting that resistance management is required. All mefenoxam-insensitive isolates were sensitive to fosetyl-Al at the label rate. Surprisingly, populations of P. cinnamomi from mid-Atlantic oak forests included insensitive isolates. For mostspecies, isolates recovered from asymptomatic hosts (e.g., soil/potting media collected from randomly selected asymptomatic hosts) had a significantly greaterrelative growth ratewhencompared with isolates recovered from symptomatic hosts (e.g., isolates recovered from lesions or wilted plants). These findings suggest that mefenoxam and fluoxastrobin should be used sparingly to manage oomycetes in Maryland ornamental nurseries.