Amer J of Potato Res (2002) 79:451-457 451 Foliar Resistance to Late Blight in Potato Clones Evaluated in National Trials in 1997 K.G. Haynes 1., B.J. Christ ~, D.P. Weingartner~, D.S. Douches 4, C.A. Thill5, G. Secor ~, W.E. Fry 7, and D.H. Lambert 8 IUSDA/ARS, Vegetable Laboratory, 10300 Baltimore Ave., Beltsville, MD 20705. 2Department of Plant Pathology, Pennsylvania State University, University Park, PA 16802. ~Ag~culture Research Center, University of Florida, Hastings, FL 32145 4Depagment of Crop and Soil Sciences, Michigan State University, East Lansing, M148824. ~Department of Horticultural Science, University of Minnesota, St. Paul, MN 55108. 6Department of Plant Pathology, North Dakota State University, Fargo, ND 58105. 7Department of Plant Pathology, Cornell University, Ithaca, NY 14853. 8Department of Plant, Soil, and Environmental Sciences, University of Maine, Orono, ME 04469. ABSTRACT Changes in the oomycete Phytophthora infestans in the United States and other parts of the world pose a sig- nificant threat to potato production. A continual evalua- tion of potato clones for resistance to late blight is necessary to identify clones with resistance and to moni- tor the stability of resistance in light of the emergence of new and more aggressive strains of this pathogen. Twenty- two potato clones (10 cultivars and 12 selections) were evaluated in 1997 for late blight resistance at seven U.S. locations. Seven late blight differentials (RIR2R3R 4, RIR2R4, R1R3R4, R3, Rs, R10, and Rmult i) were also included in the test at five of these locations. The US-8 strain of/'. infestans was present at all locations. Percent infected foliage was recorded at approximately weekly intervals following the onset of disease. Area under the disease progress curve (AUDPC) was calculated. The nonpara- metric stability statistics mean absolute rank differences (S(1)i) and variances of the ranks (S(2)i) were used to ana- lyze phenotypic stability. Although neither of these sta- tistics was significant for individual clones, both of these statistics were significant when summed over clones, indi- cating the importance of genotype x environment inter- actions on the rmakings of these clones across locations. The most late blight-resistant and susceptible clones were the most stable; clones in the intermediate ranges were most subject to rank changes due to genotype x environ- ment interactions. The most late blight-resistant clones were AWN86514-2, B0692-4, B0718-3, and B0767-2. The most susceptible clones were B0811-13, B1004-8 , Nor- Donna, and Krantz. AUDPC was very low for the late blight differentials R s and Rmulti, moderately low for R l0 and very high for the remaining differentials. This study is important in characterizing the reaction of potato clones to new strains ofP. infestans. Accepted for publication October 4, 2002. ADDITIONAL KEY WORDS: Phytophthora infestans, genotype x envi~ ronment, phenotypic stability. Abbreviations: AUDPC-area under the disease progress curve; FL- Florida; MD-Maryland; ME-Maine; MI-Michigan; MN-Minnesota; ND- North Dakota; NY-NewYork; PA-Pennsylvania