Stewart et al. 2014 reported four Washington P. rubi isolates from 4 fields were sensitive to mefenoxam (0.062 µg/ml).
2015: Assessed 112 Washington P. rubi isolates, including Stewart’s four isolates, from a total of 22 fields.
The majority of isolates are sensitive (0.118 µg/ml).
One isolate was highly resistant (199 µg/ml). One isolate was moderately resistant (52 µg/ml). A few isolates might have low resistance (3 µg/ml), but need retesting.
2016 in progress: Approximately 60 new Washington P. rubi isolates from 6 new fields. Also resampled the field where the highly resistant isolate was collected.
Total: 28 fields from 5 counties surveyed: Whatcom (18 fields), Skagit (5 fields), Clark (3 fields), Cowlitz (1 field), and Puyallup (1 field).
Resistance to mefenoxam does not appear to be a problem at this time. The majority of WashingtonP. rubi isolates are sensitive to mefenoxam, the active ingredient of Ridomil.
Recommendation: Continue switching between fungicide chemistries (e.g., mefenoxam and phosphorous acid) to prevent the development and spread of fungicide resistance.
Stewart, J. E., Kroese, D., Tabima, J. F., Larsen, M. M., Fieland, V. J., Press, C. M., Zasada, I. A., and Grünwald, N. J. 2014. Pathogenicity, fungicide resistance, and genetic variability of Phytophthora rubi isolates collected from raspberry (Rubus idaeus) in the Western United States. Plant Dis. 98:1702-1708.