VIEW ARTICLE

Ecology and Epidemiology

Incidence and Patterns of Association of Pathogens in a Leaf Spot Disease Complex on White Clover in the Piedmont Region of North Carolina. Scot C. Nelson, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh; C. Lee Campbell, Professor, Department of Plant Pathology, North Carolina State University, Raleigh Phytopathology 82:1013-1021. Accepted for publication 26 May 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1013.

Incidence of pathogen species and their patterns of association in a leaf spot disease complex on white clover (Trifolium repens) were observed in plots and adjacent areas in a pasture during 1990–1991 and in a monthly (July–October) survey of commercial pastures during 1991. Leaf samples from seven pastures in five counties (Chatham, Franklin, Nash, Orange, and Wake) in the Piedmont region of North Carolina and one county in western North Carolina (Yancey) were examined. The presence or absence of pathogens was confirmed by diagnostic, visual symptoms and by observation of pathogen reproductive structures on leaves. The most prevalent diseases were black spot (Pseudomonas andropogonis), Cercospora leaf spot (Cercospora zebrina), and summer blight (Rhizoctonia solani). Certain diseases were conspicuous in certain sites or months, e.g., Curvularia leaf spot (Curvularia trifolii) in Wake County, Cercospora leaf spot in Nash and Orange counties, and pepper spot (Leptosphaerulina trifolii) in October. Other diseases observed were Stagonospora leaf spot (Stagonospora meliloti), sooty blotch (Polythrincium trifolii), anthracnose (Colletotrichum trifolii), rust (Uromyces sp.), and Pseudopeziza leaf spot (Pseudopeziza sp.). The leaf spot disease complex comprised 10 pathogens at the regional level (Piedmont), five to eight pathogens within single pastures, two to five pathogens at the plant level, and generally one to two pathogens on a particular leaf. The results of a test for interspecific associations showed there was a significant net negative association among pathogen species at the leaf level. A test for pairwise pathogen associations indicated many negative associations and few positive associations at the leaf level.