Cause The fungus Peronospora potentillae or similar species. The disease has been reported in Washington and found a few times by the OSU Plant Clinic. The cultivar Fire Storm is susceptible. Overhead watering favors disease development.
Symptoms Lower leaves develop vein-bounded, angular chlorotic lesions that became necrotic as the disease progressed. Sporulation may occur on the underside of the leaf and look like fuzzy or downy gray patches. May be confused with nutrient deficiency.
Cultural control Scout for symptoms of the disease.
- Scout plants for any sign of the disease. Check newly planted stock originating out of state. If it is suspected, get diseased plant samples to a public diagnostic laboratory as soon as possible to confirm the diagnosis.
- Remove and destroy infected plant and plant parts.
- Space plants for better air circulation. Water in the morning or in such a way that leaves do not remain wet for extended periods of time. Use drip irrigation.
Chemical control There are no fungicides specifically registered for this crop. These fungicides are legal to use if not phytotoxic and may have activity against this disease:
- Aliette at 2.5 lb/100 gal water. Do not use within 14 days of copper spray. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- Heritage at 2 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Segovis at 0.6 to 3.2 fl oz/100 gal water plus another fungicide. Group 49 fungicide. 4-hr reentry.
- Stature SC at 6.12 to 12.25 fl oz/100 gal water. Group 40 fungicide. 12-hr reentry.
Reference Wallace, E.C., Daughtrey, M.L., Rane, K.A., Salgado-Salazar, C., and Crouch, J.A. 2018. First report of Peronospora sp. causing downy mildew disease on Geum sp. in Maryland and New York. Plant Disease 102:1463-1464.