Cause A fungus-like microorganism, Peronospora trifoliorum, which overwinters on the plant and on diseased crop residue. Cool, moist conditions promote the development of downy mildew. It seldom seriously reduces seed production, and is rarely a problem in forage fields. The disease has been reported on crimson, red, and white clover in other regions of the US and the OSU Plant Clinic in Corvallis received a crimson clover sample with downy mildew in 2024.
Symptoms Affected leaves develop yellowish patches and the underside of the affected leaf is covered with a purplish-gray growth. Severely infected leaves yellow and die.
Cultural control
Plant varieties with resistance to downy mildew. Red clover varieties with improved resistance to downy mildew include Duration, RC3, Red Carpet XL, and Renegade.
Chemical control
- Formulations of phosphorous acid (Group P7; formerly Group 33) are registered for use, including:
- Fungi-Phite at 0.5 to 2 quarts/A on 7- to 14-day intervals. 4-hr reentry.
- K-Phite 7LP at 1 to 3 quarts/A on 7- to 28-day intervals. 4-hr reentry.
- Rampart at 1 to 3 quarts/A on 14- to 21-day intervals. 4-hr reentry.