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Analysis of Progress of Alfalfa Leaf Spot Epidemics. Wayne M. Thal, Former graduate assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Present address: Oxford Research Station, USDA, ARS, Box 1555, Oxford, NC 27565; C. Lee Campbell, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 78:389-395. Accepted for publication 2 September 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-389.

The progress of alfalfa leaf spot diseases, caused primarily by Leptosphaerulina briosiana, was monitored to compare characteristics of disease development among five cultivars in successive growth cycles within a growing season. The rate of disease progress and the shape of the disease progress curve were generally similar among cultivars but varied among growth cycles. Disease severity often decreased toward the end of an epidemic within a growth cycle. Defoliation and accumulation of debris toward the end of epidemics generally corresponded to decreased levels of visible disease near the time of harvest. A simple theoretical simulator, constructed to examine some of the unique factors that appear to influence disease development in the alfalfa leaf spot pathosystem, indicated that defoliation was not the sole cause of the decreased disease severity at the end of an epidemic.