Cause This disease, caused by the fungus-like microorganism, Peronospora trifoliorum, is favored by cool, wet springs. Losses usually are restricted to the first cutting, and severe damage is seldom noted. The fungus overwinters in crown buds or as dormant spores in old, dead leaves. Only young tissue is susceptible to this fungus.
Symptoms New growth becomes pale green to yellowish green; leaflets may roll or twist down. A delicate, violet-gray, moldy growth may be abundant on leaf undersides in cool, wet weather.
Cultural control
- Plant resistant varieties.
- When disease is severe, cut early to reduce foliage loss. Cutting also removes disease tissue and increases air circulation.
Chemical control
- Seed treatment will suppress early season downy mildew the first crop year. Treat seed with:
- Apron XL at 0.64 fl oz/100 lb seed plus a dye. See label for reentry restrictions.
- For foliar application
- MilStop SP (85% potassium bicarbonate) at 2 to 5 lb/A. Oregon and Washington only. 1-hr reentry. O
- Phosphonates formulations (Group P7) are effective. Fosphite at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. 4-hr reentry.
- Priaxor Xemium (Group 7 + 11) at 4 to 6.9 fl oz/A on 14- to 21-day intervals. Do not make more than 2 applications per cutting or 3 applications per year. Preharvest interval is 14 days. 12-hr reentry.
- Quadris Flowable (Group 11) at 6 to 15.5 fl oz/A. Use of an additive such as crop oil concentrate or nonionic surfactant is recommended. Do not make more than three (3) foliar applications any Group 11 fungicides per acre per season. Do not apply more than 15.5 fl oz/A per cutting. Do not apply within 14 days of grazing or harvest for forage and hay. 4-hr reentry.
Biological control
- Regalia (Group P5) at 1 to 2 quarts/A on 7- to 14-day intervals starting prior to infection. Does not benefit from the addition of an adjuvant. Under high-disease pressure, use in combination with another fungicide. 4-hr reentry. O