Phakopsora muscadiniae on Vitis rotundifolia parasitizes grape cultivars derived from V. labrusca and V. vinifera

Two grapevine leaf rust fungi, Phakopsora uva and P. muscadiniae, are distributed in the geographic range from southern North America through northern South America.Phakopsora uva is assumed to parasitize only grapevine species in the subgenus Euvitis and P. muscadiniae only those in the subgenus Muscadinia of the genus Vitis.Experimental urediniospore inoculations onto two grape cultivars, Delaware (derived from V. labrusca) and Kyoho (derived from a hybrid between V. labrusca and V. vinifera), V. coignetiae, V. flexuosa (subgen.Euvitis) and V. rotundifolia (subgen.Muscadinia) were carried out to confirm the parasitic ability of the two rust species on different Vitis species and cultivars in the two subgenera.Phakopsora uva parasitized only Delaware and Kyoho, but not V. coignetiae, V. flexuosa and V. rotundifolia.By contrast, P. muscadiniae parasitized the two cultivars in addition to V. rotundifolia.Although unique taxonomic status of V. rotundifolia in the subgenus Muscadinia suggested the inherent resistance against grapevine leaf rust fungi pathogenic to grapevine species classified in the subgenus Euvitis, it supported infection and sporulation of P. meliosmae-myrianthae pathogenic to Kyoho.