Plant Problem Diagnostic
White Mold: Diagnosing Sclerotinia on Vegetables
The short answer
White mold, caused by the fungus Sclerotinia sclerotiorum, attacks a wide range of vegetables, beans, lettuce, cabbage, and many others, usually in cool, moist weather under a dense canopy. It begins as water-soaked lesions on stems and pods near the soil or where growth is crowded, then develops a fluffy, white, cotton-like mold, and finally hard black sclerotia (about the size of rat droppings) inside stems and on the surface, after which plants wilt and die.
The sclerotia survive in soil for years and, when the canopy shades moist soil, germinate to form tiny mushroom-like cups (apothecia) that forcibly release airborne spores. Those spores typically infect flowers and senescing tissue first before spreading into stems, and the disease is especially common with overhead irrigation and dense plantings.
Management is cultural first: use wider spacing to increase airflow and keep foliage dry, avoid overhead irrigation or water so plants dry before nightfall, rotate away from susceptible hosts toward non-hosts like small grains and corn, control weeds, and remove and destroy infected plants while burying sclerotia by deep tillage. Any fungicides are timed around bloom per label, but home control leans on the cultural steps.
Identifying white mold
The progression is distinctive. UC IPM and Michigan State Extension describe infected stems, leaves, and pods first appearing water-soaked, then developing a fluffy, white, cotton-like fungal growth in moist conditions. As lesions girdle the stem, the affected tissue dries and bleaches to a pale tan or almost white, papery look, and when the main stem is attacked near the soil line the entire plant can be killed.
The clincher is the sclerotia. Michigan State notes hard resting structures form in and on the decaying tissue, starting white to cream-colored and gradually turning black with age, large enough (roughly a quarter to half inch) to resemble rat droppings; they often form inside hollow stems. Splitting a wilted bean or cabbage stem and finding black sclerotia embedded in white moldy pith confirms white mold over other stem rots. Because the fungus attacks so many crops, the same signs apply across beans, lettuce, cole crops, and more.
The cool, wet, crowded conditions it needs
White mold is favored by cool, very moist conditions and dense canopies that trap humidity and free moisture for extended periods, and UC IPM notes mycelium develops most rapidly around 68 to 77F. A thick planting that never quite dries out, especially under overhead irrigation, is the ideal setup, which is why the disease often shows up first in the densest, shadiest part of a bed.
The disease cycle explains why flowers and old leaves get hit first. Sclerotia survive in the soil for years and, when the canopy shades the ground and soil stays moist, germinate to form cup-shaped apothecia that forcibly eject airborne ascospores. Those spores are carried by wind and typically colonize flowers and senescing (aging or dying) tissue before spreading into healthy stems, so bloom time in a wet, crowded planting is the high-risk window.
Managing white mold
Open up the planting and keep it dry. Use wider row and plant spacing to increase air movement and let foliage dry, and avoid overhead irrigation or time watering so plants dry well before nightfall, UC IPM specifically ties severity to heavy canopies that raise humidity and to overhead watering. Michigan State adds avoiding dense-canopy cultivars and eradicating weed hosts, since many weeds also carry the fungus.
Then manage the soil-borne sclerotia over time. Rotate away from susceptible hosts (beans, lettuce, cole crops, potato) toward non-hosts such as small grains and corn to starve the fungus, remove and destroy infected plants promptly, and bury sclerotia by deep tillage so fewer reach the surface to sprout. Fungicides (classes such as boscalid or thiophanate-methyl) are applied around bloom per label where pressure is high, but for home gardens the cultural program does most of the work. If you are unsure whether a stem rot is white mold versus another disease, splitting the stem for black sclerotia is the fast check, and an Extension lab can confirm.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- UC IPM - White Mold, Dry Beans — White mold (Sclerotinia sclerotiorum) begins as a watery rot on stems, leaves, and pods with white mycelium in moist conditions and black, irregularly shaped sclerotia on infected parts; affected tissue bleaches to pale tan or white and whole plants can be killed when the main stem is attacked near soil level; mycelium develops most rapidly at 68 to 77F and is more severe under heavy canopies that raise humidity; sclerotia germinate directly or produce mushroom-like apothecia that release airborne spores infecting stems, leaves, and pods; management includes wide row spacing to increase air movement and dry foliage, and rotation with non-hosts such as small grains and corn to reduce soil inoculum.
- Michigan State University Extension - White Mold (E2989) — White mold (Sclerotinia sclerotiorum) produces water-soaked lesions and a white cottony growth that girdles stems and causes wilting, with lesions later turning beige, tan, or bleached and papery; sclerotia form in decaying tissue, are about a quarter to half inch in diameter, and start white or cream then turn black with age; it thrives in very moist conditions and is especially common with overhead irrigation and dense canopies; the fungus forms apothecia that forcibly eject ascospores dispersed by wind; management includes eradicating weed hosts, crop rotation with non-susceptible hosts, irrigation management to reduce humidity, avoiding dense-canopy cultivars, and timing irrigation so plants dry before nightfall.