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Seasonal Occurrence of Leaf Spot Pathogens of Alfalfa in North Carolina. Kilmer Von Chong, Graduate Student, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. C. Lee Campbell, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 72:667-672. Accepted for publication 3 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0667.

In order to monitor leaf spot pathogens, 25 alfalfa stems were collected at 2-wk intervals from March to October in 1985 and 1986 at two sites in Wake County and 50 stems at monthly intervals from May to October in 1985 in Rowan and Washington counties of North Carolina. Disease severity estimates were made on each leaf. Fungi were identified after incubation of leaflets in petri dish moist chambers or on V-8 juice agar. Correlations between pathogen occurrence and weather data (maximum and minimum temperature, amount and frequency of rain) from the NOAA weather stations nearest to each alfalfa plot were examined. Leptosphaerulina briosiana was the most frequently occurring pathogen during both 1985 and 1986, and occurrence was not limited to cool and wet weather. Stemphylium botryosum was present early in the 1985 season but occurred primarily in the spring and fall in 1986; occurrence decreased as mean temperature and rainfall increased. Phoma medicaginis var. medicaginis was more frequent in the spring than in the fall, and occurrence seemed to be related to frequency of rain rather than to mean rainfall. Cercospora medicaginis was present from mid-June until October. The seasonal distribution of each of the four leaf spot pathogens is not mutually exclusive.