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Assessment of Resistance to Leaf Spot Diseases Among Alfalfa Cultivars in North Carolina Fields. Wayne M. Thal, Graduate assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Present address: Oxford Research Laboratory, USDA-ARS, Box 1555, Oxford, NC 27565; C. Lee Campbell, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 77:964-968. Accepted for publication 22 December 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-964.

Experimental plots were sampled destructively and leaves on individual stems were evaluated visually for leaf spot severity to assess differences in disease level among 16 alfalfa cultivars. Samples were collected from sites in Rowan and Washington counties during 1983 and in Rowan, Sampson, Wake, and Washington counties during 1984. Disease severity was estimated for each leaf on a stem. Total length, length of defoliation up the stem, number of intact and defoliated nodes, and number of abscised main stem leaves were measured for each stem. Significant differences were detected among cultivars in disease severity, maximum disease on a stem, and percentage of nodes with abscised main stem leaves. A cultivar by location interaction was not detected. Florida 77 appeared to be more susceptible than most other cultivars, whereas Raidor had a higher level of resistance to leaf spot diseases than other cultivars. Stability analysis indicated that Raidor has the most stable disease resistance. High correlations (r ? 0.70) were detected among several of the variables measured. Cluster analysis was used to group cultivars on the basis of disease severity and percentage of defoliated nodes.