Suppression of Lettuce Drop Caused by Sclerotinia minor with Composted Sewage Sludge. R. D. Lumsden, Research Plant Pathologist, Soilborne Diseases Laboratory, Plant Protection Institute, USDA, Agricultural Research Service, Beltsville, MD 20705. P. D. Millner, Research Microbiologist, Soil Microbial Systems Laboratory, Agricultural Environmental Quality Institute; and J. A. Lewis, Research Soil Scientist, Soilborne Diseases Laboratory, Plant Protection Institute, USDA, Agricultural Research Service, Beltsville, MD 20705. Plant Dis. 70:197-201. Accepted for publication 20 August 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/PD-70-197.

Amendment of soil with composted sewage sludge significantly reduced the incidence of lettuce drop caused by Sclerotinia minor in the field during a 4-yr period in both fall and spring plantings. Compost was added to the soil in the first 2 yr and residual effects occurred in the final 2 yr of the study. Suppression of disease was correlated with increased soil microbial activity, assessed by a dehydrogenase method, and with total nitrogen, phosphorus, magnesium, calcium, and organic matter content of the soil. Size and weight of lettuce heads were similar from compost-amended and nonamended soils. Survival of sclerotia of S. minor was unaffected by the addition of compost to soil, and there were no differences in types or numbers of fungi associated with sclerotia recovered from the soils. Growth of mycelia and formation and germination of sclerotia in vitro were adversely affected by sterilized compost-amended soil and crude soil extracts but not by filter-sterilized extracts of compost-amended soil, compost, or sludge. The suppressive effect of compost on disease of lettuce caused by S. minor is complex and may be related to improved physical structure or modified nutrient content of the soil, resulting in increased soil microbial activity.