VIEW ARTICLE

Ecology and Epidemiology

Effects of Plant Debris on Intensity of Leaf Spot Diseases, Incidence of Pathogens, and Growth of Alfalfa. James A. Duthie, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695, Current address: Department of Plant Pathology, University of California, Davis 95616; C. Lee Campbell, associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Phytopathology 81:511-517. Accepted for publication 20 November 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-511.

When leaf spot diseases are severe, early harvest of alfalfa (Medicago sativa) has been recommended to preempt defoliation, preserve yield and quality, and prevent accumulation of pathogen-infested leaf debris. Effects of debris on incidence of all leaf spots taken together (percentage of leaves with lesions caused by species of Leptosphaerulina, Stemphylium, Phoma, and/or Cercospora), frequency of leaf spot pathogens (percentage of diseased leaflets infected by each pathogen), and yield were evaluated during five periods of alfalfa growth in each of 2 yr among untreated plots (10.0 × 4.1 m) and plots in which naturally infested debris was either removed or added immediately after each harvest. Effects of treatments on disease incidence and on frequencies of leaf spot pathogens were small or absent during most periods of alfalfa growth. Mean disease incidences in debris-removed, untreated, and debris-added plots were 17, 22, and 23%, respectively, in 1987, and 30, 32, and 33%, respectively, in 1988. Frequency of Leptosphaerulina sp. on diseased leaflets, but not other leaf spot pathogens, was increased significantly by adding debris and was reduced significantly by removing debris in each year. Environment had a much greater effect on disease incidence and pathogen frequency than levels of debris. Under moist conditions, disease incidence increased rapidly. There was no evidence of a decrease in yield due to increased levels of debris and disease in any period of alfalfa growth. During hot and dry periods in both years, dry-matter yield was reduced by removing debris, apparently because soil moisture was reduced. Thus, under these or similar growing conditions, early harvest may not be a useful method of reducing yield losses caused by leaf spot diseases.