Mefenoxam Sensitivity in Phytophthora cinnamomi Isolates.

Phytophthora cinnamomi is a destructive root pathogen of numerous woody plant species in the ornamental plant nursery. Sixty-five isolates of P. cinnamomi were evaluated for mefenoxam sensitivity on 20% clarified V8 agar amended with mefenoxam at 0 or 100 μg/ml. In the presence of mefenoxam at 100 μg/ml, eight isolates were intermediately sensitive, with mycelium growth ranging between 11 and 18% of the nonamended control, and 57 isolates were highly sensitive, with little or no mycelium growth. Five intermediately sensitive and five sensitive isolates were chosen to characterize their responses to mefenoxam at 0, 0.1, 1, 10, and 100 μg/ml. For intermediately sensitive isolates, the mefenoxam concentration causing 50% inhibition of mycelium growth (EC values) ranged between 0.03 and 0.08 μg/ml; EC values for sensitive isolates varied from 0.01 to 0.02 μg/ml. Five intermediately sensitive and seven sensitive isolates were selected further to assess in vivo sensitivity to mefenoxam using Lupinus angustifolius 'Russell Hybrids'. Lupine seedlings were treated with distilled water or mefenoxam at label rate (Subdue MAXX, 1 fl. oz. of product per 100 gal.) and then, 2 days later, inoculated with a 5-mm-diameter mycelial plug of P. cinnamomi on each cotyledon. Mefenoxam-treated plants averaged more than 96% less disease than water-treated plants. Mefenoxam provided adequate protection of lupines from infection by all 12 isolates regardless of their in vitro levels of sensitivity to mefenoxam. The ability to develop mefenoxam resistance was assessed in P. cinnamomi isolates with different mefenoxam sensitivity by UV mutagenesis and adapting mycelium to increasing concentrations of mefenoxam. Both UV mutagenesis and mycelium adaptation generated isolates with reduced sensitivity to mefenoxam. These isolates, however, did not grow as quickly as their corresponding parent. This study suggests that P. cinnamomi populations from ornamental nurseries in Virginia are sensitive to mefenoxam.