The cultural characteristics and histochemical variations following infection with Alternaria helianthi was studied in six wild Helianthus species of three ploidy levels (diploid, tetraploid and hexaploid) possessing different degrees of resistance to the pathogen and in cultivated sunflower (H. annuus cv. CO-4, susceptible check). Of these, the species H. occidentalis and H. tuberosus were found to be highly resistant while H. hirsutus was moderately resistant. Media supplemented with leaf extracts of wild species, with the exception of H. grosseserratus, supported less growth and sporulation of A. helianthi than media supplemented with leaf extract of cultivated sunflower. Reduced infectivity of the pathogen was recorded when grown on leaf extract media of H. occidentalis, H. hirsutus and H. tuberosus. Furthermore, abnormalities in the shape of conidia were noticed on supplemented media with leaf extracts of H. occidentalis and H. tuberosus. RAPD analysis of the fungal DNA isolated from the pathogen grown on leaf extract media of cultivated and wild Helianthus species revealed no polymorphism. Histochemical studies showed restriction of the pathogen to epidermal cells in resistant wild sunflowers as well as an increased accumulation of phenols.