Biocontrol and epidemiology of lettuce drop caused by Sclerotinia minor at Bacchus Marsh, Victoria

Abstract

The mycoparasite Coniothyrium minitans, but not a commercial preparation of Trichoderma or a local isolate of Trichoderma virens, significantly increased the percentage of sclerotia infected and reduced the percentage viability of sclerotia when applied to lettuce plants showing the earliest symptoms of lettuce drop caused by Sclerotinia minor in commercial crops. C. minitans always infected untreated controls in these experiments and was evidently a natural resident in the soils; it is likely that it effects a degree of natural biocontrol of sclerotia of S. minor in these soils. In spring and summer, the viability of untreated sclerotia among plant debris on the surface of the soil declined greatly, providing evidence that prolonging the exposure of sclerotia on the soil surface before cultivation for the next crop can greatly reduce sclerotial inoculum. Far fewer sclerotia were formed on infected, untreated plants in late spring to summer than in autumn and early spring. Epidemics of lettuce drop caused by S. minor on salad lettuces developed rapidly in the last 1 or 2 weeks before harvest. This was as a result of infection mainly via the lower leaves drooping onto the surface of the soil or touching adjacent infected plants. Clumping of diseased plants was evident only late in the epidemic. Cutting of infected mature plants at an early stage of symptom development and leaving them in the field did not reduce the number of sclerotia formed on them compared to plants left in the ground, indicating that cut plants should be removed from the field. Possibilities for disease control are discussed.