We want your grape downy mildew!

For the 2026 growing season, OMAFA has funding to screen Ontario grape downy mildew for sensitivity to phosphorous acid. Contact us if you are interested in participating, we can come pick up your samples!

Wendy – wendy.mcfadden-smith@ontario.ca; 905-932-8965

Katie – katie.goldenhar@ontario.ca; 519-835-5972

Phosphorous acid products (ex. Phostrol, Confine Extra, Rampart) have become an important part of our downy mildew management program.  While they have plant host defense activation activity, they also reduce work directly on the pathogen to reduce sporulation on lesions. This direct pathogen activity is the efficacy we are aiming to test for based on the methods from the study described briefly below (Rotella et al., 2026).

In a study conducted in Maryland and Pennsylvania, 352 samples of grape downy mildew (Plasmopara viticola) were screened for resistance to Group 11 (azoxystrobin), 4 (Ridomil – metalaxyl-M) and P07 (phosphorous acid).  Isolates were inoculated on leaves treated with label rates of each fungicide.  Isolates were considered susceptible if no symptoms developed.  If oil spots formed, whether sporangia were produced or not, the isolate was considered resistant. From 6 to 64% of the P. viticola isolates were resistant to phosphorous acid.

What does this mean for Ontario grape growers? Group 11 resistance is known to be widespread in Ontario (Sharma et al., 2025) and group 40 resistance has been documented, so more intensive use of other fungicides is occurring. We are gathering information on phosphorous acid sensitivity to inform your management decisions so be sure to participate!

Rotella, S., Clippinger, J., Hed, B., Campbell, M., & Hu, M. (2026). Characterization of Fungicide Resistance in Plasmopara viticola Isolates from Maryland and Pennsylvania Vineyards. Plant Disease, 110(5), 1903. https://doi.org/10.1094/PDIS-11-24-2499-RE

Sharma, N., Heger, L., Combs, D. B., McFadden Smith, W., Holland, L. A., Brannen, P. M., Gold, K. M., & Miles, T. (2025). Prevalence of Mutations Associated with QoI, QiI, QioSI and CAA Fungicide Resistance Within Plasmopara viticola in North America and a Tool to Detect CAA Resistant Isolates. Phytopathology. https://doi.org/10.1094/PHYTO-08-24-0257-R