Thirty wild Helianthus species, several interspecific hybrids and 380 recombinant interspecific inbred lines (RIILs) derived from interspecific crosses between the cultivated sunflower and four diploid annuals (wild H. annuus, H. argophyllus, H. petiolaris and H. debilis) and three diploid perennials (H. occidentalis, H. maximiliani and H. divaricatus) were studied for reaction to rust caused by Puccinia helianthi under severe natural epiphytotic condition. The disease incidence ranged from 0 to 100% in the stable interspecific derivatives while none of the wild sunflowers and their F1 hybrids revealed susceptibility to rust. The disease severity in the RIILs varied between 0 and 40%. The mean disease severity was low in lines derived from diploid annuals (6.0-10.4%) as compared with those derived from diploid perennials (23.1 31.1%). One line (PS 1089) derived from H. argophyllus ×cultivated sunflower and two lines (PS 2011 and PS 2032) derived from H. petiolaris × cultivar crosses were found to be immune. Disease severity and disease incidence were correlated with various physiological traits.