Sclerotinia crown and stem rot on clovers, alfalfa, vetches, and related forage legumes. A cool, wet-season disease that can kill seedlings or wipe out patches after fall establishment, wet winters, or prolonged snow cover.
Triggers: Cool moist weather, fall seeding, wet soils, dense canopy, poor air circulation, and prolonged snow cover all favor disease.
What it looks like: Stems wilt first, then in damp weather a white, cottony growth shows on stems, crowns and sometimes the soil beside them. Hard black lumps the size of peppercorns, white inside, form at the stem base, on the soil or inside dead stems, which go hollow and flatten easily.
Disease or overwatering? White cotton on the stems plus black peppercorn lumps confirm this rot. Pinch a dead stem flat between your fingers and you can often feel the lumps inside. Plants wilting in soggy soil without the white growth or black lumps point to too much water instead.
Will it recover? Infected seedlings die, but in established plants every stem can die while the crown lives and sends up new growth in spring. Cut or mow the foliage back before winter, and keep weeds such as chickweed out so air reaches the base of the plants. Late winter sowing usually escapes the disease.
Risk fades when: Drier conditions reduce active infection pressure, though stand damage may already be visible by spring.
Source for the signs above: UC IPM Pest Management Guidelines: Alfalfa, Sclerotinia Stem and Crown Rot (White Mold)
