Cottony Soft Rot (White Mold)

Causal Agent

Cottony soft rot is a white mold caused by the fungus Sclerotinia sclerotiorum. The wide host range includes succulent plants, ornamentals, and several vegetable crops. For root crops, carrots are most susceptible.

Symptoms

Sclerotinia causes infection during any growth stage of the carrot. Extensive root decay will lead to wilt and collapse the aboveground portion of the plant. The “cottony” white mycelium appears on infected tissue, usually at the base of the plant. Inside this mycelium are black, irregular-shaped structures called sclerotia (hard balls of mycelium), which range in size from 0.10.4 inches wide. 

Disease Cycle

During late summer/early fall, S. sclerotiorum produces sclerotia that germinates and produces hyphae directly on the surface or inside of infected plant tissue. The following spring, these structures sometimes produce fruiting bodies called apothecia, which have thin stalks with small, cup-like structures.  Eventually, the ascospores are released and carried by the wind to susceptible hosts where they cause new infections. The optimal conditions for infection are cool temperatures (60 °F to 70 °F). 

Management

• Till the soil. Turning over the soil can bury the sclerotia, which reduces the chance of disease infection the following season.
• Rotate crops. Rotating crops from carrots to nonsusceptible hosts such as grains and corn will prevent the disease from building up.
• Supply adequate ventilation. Spacing crops apart can reduce humidity and prevent spread.
• Flood the site. Emerging the site underwater for several weeks can "drown" the sclerotia.