Sclerotinia sclerotiorum
Sclerotinia sclerotiorum can attack a wide range of vegetables and other plants. It causes white mold on beans, cottony soft rot of carrot, watery soft rot of cole crops, root rot of pea, and timber rot of tomato, among others. Symptoms vary with host plant.
On cabbage, initial symptoms include water-soaked areas on the stem and leaves. The leaves wilt and the plant collapses. Cabbage heads become covered with a cottony mass of fungi with irregularly shaped, hard, black bodies. The disease organism overwinters in the soil and can also be brought in on infected transplants. It is infectious over a wide temperature range (50 to 76 degrees F), but requires moisture to germinate and infect. Rotation to resistant crops (beets, onion, spinach, peanuts, corn, and grasses), and flooding of the field for 23 to 45 days lower disease incidence.
Growing vegetables with an upright rather than a sprawling growth habit, wide plant spacing, and low plant density also reduces disease development. Sclerotia, the overwintering structures which are the precursors of infectious propagules, survive for 3 or 4 years in the soil. Thus the disease is not easily controlled by incorporating residues because tillage practices to bury sclerotia also bring additional sclerotia to the surface. In Quebec, feeding in muck soils by native populations of dark-winged fungus gnats (Bradysia winnertz) significantly reduced germination of sclerotia of S. sclerotiorum.
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