Cause A fungus, Sclerotinia trifoliorum, that can affect alfalfa, birdsfoot trefoil, crown vetch, and clover. The fungus produces small, black sclerotia within infected tissue and these will fall to the soil. During the fall or winter, the sclerotia produce small mushroom-like structures called apothecia. The apothecia produce spores (ascospores) that are blow by wind to susceptible plants. Plants of all ages are susceptible and colonized by the fungus. Sclerotia are produced on or in stem as well as crown tissues.
Symptoms In late autumn, brown leaf spots appear. Diseased leaves fall and are covered with white fungus growth. The disease spreads to the crown and roots. In spring, a crown and root rot is evident, and the new growth wilts, dies, and may be covered with fungus growth. Hard, black, fungus bodies (sclerotia) about the size of wheat grains are produced in diseased tissue. Late spring snows favor disease development. Diseased plants initially may be in patches within the field.
- Plant pathogen-free seed.
- Clean crop rotation. Plant grass or grain for 4 years, and control volunteer legumes.
- Plow deeply to bury sclerotia.
- Avoid over-fertilization, especially with nitrogen.
- Some red clover cultivars are resistant.
- Endura (Group 7) at 5 to 8 oz/A. Do apply more than once. For only crimson clover grown for seed in Oregon (SLN OR-07001). 12-hr reentry.
- Iprodione formulations (Group 2) are labeled.
- Nevado 4F at 1.5 to 2 pints/A. Do not apply more than twice. For clover for seed in Oregon only. 24-hr reentry.
- Rovral 4 Flowable at 1.5 to 2 pints/A in 12 to 40 gal water /A. One (1) to two (2) applications can be made but no more than two (2) applications per season. If a single application is made, the maximum rate allowed should be used. For crimson, red, or white clover grown for seed in Oregon only (SLN OR-050002). 24-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of any 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Flint at 2 oz/A. For crimson clover grown for seed in Oregon only (SLN OR-050030). 12-hr reentry.
- Quadris Flowable at 10 fl oz/A. Do not apply within 14 days of harvest for forage or hay. 4-hr reentry