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Microorganisms Associated with Bottom Rot of Lettuce Grown on Organic Soil in New York State. D. J. Pieczarka, Graduate Research Assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853; J. W. Lorbeer, Professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, New York 14853. Phytopathology 65:16-21. Accepted for publication 28 June 1974. DOI: 10.1094/Phyto-65-16.

Rhizoctonia solani, Fusarium oxysporum, Mucor sp., Trichoderma sp. and Alternaria sp. frequently were isolated from leaves and heads of lettuce grown on organic soils in New York and exhibiting initial symptoms of bottom rot. R. solani consistently was isolated from infected tissue at all stages of symptom development. It was the only fungus of the group pathogenic to lettuce when nonwounded detached leaves were inoculated. Under field conditions Erwinia carotovora, Pseudomonas marginalis, and P. fluorescens frequently were present in addition to R. solani on plants with extensive maceration and vein-browning of heads. In pathogenicity tests using detached lettuce leaves, all bacterial isolates required a wound before infection occurred. When excised leaves with lesions formed by R. solani were inoculated with soft rot bacteria, the resulting rate of decay was greater than on leaves infected with only R. solani. This suggests that R. solani frequently plays a role in providing a mode of entry for the soft rot bacteria under field conditions.