Volume 5 Issue 10
Jerry Weiland, USDA-ARS Plant Pathologist
Email: Jerry.Weiland@ars.usda.gov
PRELIMINARY RESULTS FOR RIDOMIL (MEFENOXAM)
- 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).
- Conclusions:
- 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.
Reference:
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.