Weeds were sampled in grapevine rootstock mother fields, open-root field nurseries, and commercial vineyards of Albacete, Alicante, Castellón, Murcia, and Valencia provinces in Spain between June 2009 and June 2010 and evaluated as potential hosts of black-foot and Petri disease pathogens. Isolations were conducted in the root system and internal xylem tissues for black-foot and Petri disease pathogens, respectively. Cylindrocarpon macrodidymum was successfully isolated from the roots of 15 of 19 weed families evaluated and 26 of 52 weed species. Regarding Petri disease pathogens, one isolate of Phaeomoniella chlamydospora was obtained from Convolvulus arvensis, and three isolates of Cadophora luteo-olivacea were obtained from Bidens subalternans, Plantago coronopus, and Sonchus oleraceus. Pathogenicity tests showed that Cylindrocarpon macrodidymum isolates obtained from weeds were able to induce typical black-foot disease symptoms. When inoculated in grapevines, isolates of Cadophora luteo-olivacea and Phaeomoniella chlamydospora were also shown to be pathogenic on grapevine cuttings. Our ability to recover grapevine pathogens from vineyard weeds and to demonstrate pathogenicity of recovered strains on grape suggests that these weeds may serve as a source of inoculum for infection of grapevine.