Abstract: Potato cultivars (Solanum tuberosum) differed in their susceptibility to stem canker and black scurf disease caused by Rhizoctonia solani (teleomorph Thanatephorus cucumeris [Frank] Donk). Disease reactions were determined for nine commercial potato cultivars under greenhouse conditions. The nine tested cultivars were divided to three groups. Lady Rosetta was the most susceptible cultivar followed by Monalisa and Mondial cvs. Spunta, Nicola and Hermes cultivars showed moderate susceptibility, While Draga followed by Cara and Diamont were the most resistant. The two potato cultivars Draga, the resistant cv. and lady Rosetta, the highly susceptible were selected to study the nature of disease resistance among potato cultivars. Fraction of phenolic compounds by HPLC, oxidative enzymes and thio-amino acid contents were determined in leaves and roots of these potato cultivars. The resistant Draga cultivar showed the highest amounts of free, conjugated and total phenols, fraction of phenolic compound (Coumarin, Caffic acid, p-Coumaric acid, Benzoic acid, Resorcinol and Apigenin), oxidative enzymes (peroxidase and polyphenoloxidase) and thio amino acids (Cysteine, Cystine and Methionine) compared with the highly susceptible lady Rosetta.