Detection of Oomycide-Insensitive Phytophthora Isolates in Maryland Ornamental Nurseries and Mid-Atlantic Landscapes Provide Data for Reconsidering Management Strategies

Abstract

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 at approximately 2.5× the recommended label rate. These isolates were also insensitive to higher (off-label) concentrations of fluoxastrobin. Concentrations of dimethomorph (400 μg/ml) and dimethomorph + ametoctradin (100 μg/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 most species, isolates recovered from asymptomatic hosts (e.g., soil/potting media collected from randomly selected asymptomatic hosts) had a significantly greater relative growth rate when compared 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.