White mold—Sclerotinia sclerotiorum
Plants affected by white mold generally show white, cottony mycelium on the surface of the infected area. On or inside the white mycelium appear black, round to irregular-shaped structures called sclerotia.
On flowering plants such as tomatoes or cucurbits, symptoms include water-soaked areas on flowers and at stem joints where senescent flower petals have fallen. The infection quickly kills stems, which eventually dry and take on a bleached appearance. Water-soaked stem lesions may also appear at the soil line if senescent plant debris is present around the plant. Infected fruits turn gray and rot.
On carrots, white mold, sometimes known as cottony soft rot, may occur at any stage of growth. Extensive root decay may occur before symptoms of wilt appear on the upper part of the plant.
Sclerotia survive in the soil. When they are within the top 2 to 3 cm of soil they can germinate to form a saucer-shaped fruiting body called an apothecium. Each apothecium produces millions of spores, which are disseminated by wind. Both infection and subsequent spread of white mold are determined by temperature and moisture. White mold is favored by cool, moist conditions. Spores germinate on and colonize dead or senescent tissue; senescent flowers are frequently the source of new infections.
White mold is favored by a wet soil surface and high temperatures. Avoid planting in infested soil. Use of raised beds and careful furrow irrigation that does not overflow onto bed surfaces can help limit damage. Avoid overhead sprinklers. Space plants well enough to allow good air circulation. Remove and destroy entire infected plants and crop residues as soon as you see the mold. Bury old plant debris to help destroy sclerotia.
Bleached appearance of tomato stems
Black sclerotia on bleached stems