RELATIVE RESISTANCE OF THE POTATO CULTIVAR KRANTZ TO COMMON SCAB CAUSED BY STREPTOMYCES-SCABIES AS DETERMINED BY A CLUSTER-ANALYSIS

Twenty potato breeding selections and five potato cultivars (25 clones ) were evaluated in replicated field plots for their resistance to com mon scab caused by Streptomyces scabies, at Presque Isle, ME and Crane sville, WV in 1993. At harvest, all tubers in each plot were individua lly scored for the surface area covered (0=0% to 5>75%) and for lesion type (0=no lesions to 5=pitted lesions). The individual tuber scores for either surface area covered or lesion type for each plot were tota led and divided by five times the number of tubers to create an index of surface area covered (SAT) or lesion type (LI), respectively. Clona l mean SAT ranged from 0.05 to 0.96 in Maine and 0.21 to 0.89 in West Virginia. Clonal mean LI ranged from 0.06 to 1.0 in Maine and 0.48 to 1.0 in West Virginia. There were significant differences among clones for SAI and LI. Clones were clustered on mean SAT and LI in ME and WV. The clones clustered into four groups. The resistance of Krantz and t wo breeding selections, B0348-2 and B0339-1, was similar to Ontario.