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 2.5x the recommended label rate. These isolates were also insensitive to higher (off-label) concentrations of fluoxastrobin. Concentrations of dimethomorph (400 g/mL) and dimethomorph + ametoctradin (100 g/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, the populations of P. cinnamomi from mid-Atlantic oak forests included insensitive isolates. With almost all species, isolates recovered from asymptomatic hosts (e.g., soil/potting media collected of randomly selected asymptomatic hosts) had a significantly greater relative growth rate when compared to isolates recovered from symptomatic hosts (e.g., isolates recovered from lesions or wilted plants). These findings suggest that mefenoxam should no longer be used to manage oomycetes in Maryland ornamental nurseries and that the use of fluoxastrobin should be limited.

Exploring Phytophthora agathidicida Oospore Viability and Germination

“…These examples of resistance, among others in the literature relating to almost all oomycides (Beaulieu et al, 2021;Cai et al, 2016;Dobrowolski et al, 2008;Hunter et al, 2018;Pang et al, 2013) -and especially the evidence of cross-resistance (Diriwächter et al, 1987;Ziogas et al, 2006) illustrate the actively evolving nature of Phytophthora pathogens, and the arms race that is occurring in order to keep these devastating plant pathogens under control. This has sparked research to develop new oomycides and management strategies.…”

“…Other targeted oomycides have since been developed, including oxathiapiprolin, metalaxyl, dimethomorph, fluoxastrobin and mefenoxam (Beaulieu et al, 2021;Cohen et al, 1979).…”

“…Phosphite resistance has been reported in several Phytophthora species, although most studies report large variances between isolates (Adaskaveg et al, 2017;Beaulieu et al, 2021;Hao et al, 2018;Hunter, 2018;Hunter et al, 2022;Hunter et al, 2018;Veena et al, 2010;Wilkinson et al, 2001). The mechanisms for this resistance vary, e.g., exclusion of phosphite from entering the pathogen's cells (Griffith et al, 1993), cellular resistance to phosphite (Griffith et al, 1993;Niere et al, 1994) and efflux of phosphite from the cell (Dunstan et al, 1990).…”

“…The mechanisms for this resistance vary, e.g., exclusion of phosphite from entering the pathogen's cells (Griffith et al, 1993), cellular resistance to phosphite (Griffith et al, 1993;Niere et al, 1994) and efflux of phosphite from the cell (Dunstan et al, 1990). Resistance to phosphite has been seen in Phytophthora species in both agriculture and forest settings, and resistance has been linked to the historic use of phosphite, which implies the use of phosphite is selecting for resistant isolates (Beaulieu et al, 2021;Dobrowolski et al, 2008;Hao et al, 2021).…”