Sclerotinia

Sclerotinia is a fungal disease that can affect a wide variety of vegetables, but is seen most often by home gardeners in stored carrots.

Symptoms:

• White cottony mould or mycelium on the leaves near the crown is seen following extended periods of cool wet weather in late summer/fall in actively growing plants.
• Once harvested, the infection spreads from the crowns to engulf the entire carrot root in a white fuzzy fungal mass.  As the fungus matures it begins to form sclerotia (1-2cm long oval, hard, black resting bodies).  
• A secondary bacterial soft rot may follow the initial sclerotinia infection of stored carrots.

Control:

• Practice a minimum three-year rotation of carrots with non-host crops in the garden (ex. Onion, beet, spinach, and corn).   A good rotation would be: Year 1 = carrots, year 2 = beets, year 3 = corn, year 4 = carrots)
• Keep weeds to a minimum and provide adequate spacing between plants to ensure there is good air movement in the garden; this will help reduce relative humidity.
• Avoid over-fertilizing the carrots which encourages excessive top growth which interferes with air flow near the crown.  
• Avoid watering late in the evening.
• Plant carrots on raised beds.  Control soil moisture as well as flooding. 
• Harvest when the carrots are cool (less than 5˚C).   Sort the carrots and discard any obviously infected roots and don't store carrots that are split or broken, or otherwise damaged.
• Avoid washing carrots after harvest - cure them in a cool, dark, dry place for a few hours to help  toughen the outer cells (skin) of the root, which protects them from invading bacteria or mold. After curing, gently brush off any excess soil.
• Don't trim your carrots before storing them - simply take off the tops and leave the root at the bottom intact. Any cuts (from trimming) and bruising from washing is an opportunity for the fungus and/or bacteria to take hold.
• Store carrots in the crisper with a loose wrapping of plastic or in perforated plastic bags.
• Provide good air flow and carefully maintain storage temperatures.
• Inspect the stored carrots often and immediately remove any carrots showing any sign of decay.

Sources:

Howard, R. J., Seaman, W. L., & Garland, J. A. (Eds.). (2002). Diseases and pests of vegetable crops in Canada. Vancouver, British Columbia: The Canadian Phytopathological Society and the Entomological Society of Canada.