From Alex Harris, an Undergraduate Student Experiential Learning student with OMAFRA and the University of Guelph, and the OMAFRA Simcoe Specialty Crop Team:

Sclerotinia white mould is a fungal disease affecting numerous herbaceous crops including beans, sunflowers, cole crops, carrots and cucurbits. Affected herbs include basil, fennel, mint, parsley, and tarragon, although it has been reported most often on basil in Ontario.

Sclerotinia develops under moist conditions when temperatures are above 10°C. Under these conditions, a characteristic white, fluffy mould will develop, usually along the stem, causing individual or clusters of plants to wilt and collapse. Affected areas initially look brown or dark-green and appear water-soaked. Eventually, hard black clumps called sclerotia will develop within the mould, which are the long-term survival structures of the fungus.

Sclerotinia white mould on a basil stem

In addition to rotation out of crops that are susceptible to the disease, avoid over-watering and close plant spacing. This will help reduce the wet, humid conditions under which Sclerotinia thrives.