Sclerotinia minor on lettuce: Effect of plant growth on susceptibility to infection

Lettuce varieties such as 'Buttercrunch', 'Great Lakes', and 'Triumph' which produced lower leaves that were in continual contact with the soil as they senesced were far more susceptible to infection by Sclerotinia minor than a variety, 'Cos', with an upright growth habit. The primary inoculum in this disease is probably mycelium originating from sclerotia lying on the soil surface underneath the lower leaves. The pattern of disease development discounts the possibility that ascospores are an important source of inoculum.