White Mold
Sclerotinia sclerotiorum, Sclerotinia minor
Overview
White mold, caused by Sclerotinia sclerotiorum, is the classic cottony soft rot of beans, canola, lettuce, tomatoes, and many flowers. Cool moist weather during bloom lets ascospores from soil-borne sclerotia infect petals and then stems. Hard black sclerotia form inside stems and on residue, surviving for years. Organic control combines wide rows, varieties with open architecture, irrigation timing, biological sclerotia parasites, and rotation away from highly susceptible hosts.
Symptoms
Pale water-soaked lesions on stems, leaves, or pods expand into soft rot blanketed with white mycelium. Black sclerotia resembling mouse droppings form on and inside tissue. Plants wilt above girdling stem lesions. Dense flowering canopies show nested patches of collapse after cool rainy blooms.
Environmental Conditions
Cool
Prevention
Rotate with corn or grasses for several years. Plant into well-drained beds. Avoid excessive nitrogen that creates lush closed canopies. Purchase clean seed. Time plantings so peak bloom misses the coolest wettest stretch when possible. Do not move soil from white mold hotspots into clean ground.
Management
Remove diseased plants with sclerotia when numbers are small. Widen spacing and choose upright cultivars that dry after rain. Apply Coniothyrium minitans products to soil to reduce sclerotia where available. Avoid irrigation during bloom if soil moisture allows. After harvest, remove residue rather than leaving a sclerotia bank.