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Evaluation and Characterization of Advanced Potato Breeding Clones for Resistance to Scab by Cluster Analysis. R. W. Goth, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Vegetable Laboratory, Plant Sciences Institute, Beltsville, Maryland 20705. K. G. Haynes, and D. R. Wilson. Research Plant Geneticist, and Horticulturist, U.S. Department of Agriculture, Agricultural Research Service, Vegetable Laboratory, Plant Sciences Institute, Beltsville, Maryland 20705. Plant Dis. 77:911-914. Accepted for publication 2 May 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/PD-77-0911.

Seventeen potato breeding lines and four cultivars were evaluated in replicated field trials in Presque Isle, Maine, from 1988 to 1991 for their reaction to scab (Streptomyces scabies). All tubers were individually scored for type of lesion (0 = none to 5 = pitted scab) and surface area covered (0 = 0% to 12 = 100%). Computations were made for lesion index (LI), surface area index (SAI), and overall scab index (OSI). Analyses of variance of LI, SAI, and OSI revealed significant differences among potato lines in 19891991. Cluster analyses on the combination of LI and SAI were superior to clustering on OSI alone. Five clusters were defined in 19891991. Cluster analysis provides a means to quantitatively compare the scab reactions of new germ plasm with that of cultivars used as disease standards.