The biochemical basis of resistance to the leaf spot/blight pathogen A. helianthi was compared in six wild Helianthus species, possessing three ploidy levels (diploid, tetraploid and hexaploid) and different degrees of resistance to A. helianthi, and cultivated sunflower (H. annuus cv. CO-4, susceptible check) in terms of sugar, phenols and isozymes of peroxidase, polyphenol oxidase and chitinase. The resistant species of wild sunflowers (H. tuberosus, H. occidentalis) possessed higher levels of constitutive as well as induced total phenols and total sugars as compare with cultivated sunflower and susceptible wild sunflowers. Polyacrylamide gel electrophoretic (PAGE) isozyme analysis of defence-related enzymes showed a positive correlation of resistance with chitinase and polyphenol oxidase and a negative correlation with peroxidase.