We examined the influence of an arbuscular-mycorrhizal fungus, Glomus intraradices (INVAM CA 501), on black foot disease caused by the fungus Cylindrocarpon macrodidymum on Vitis rupestris cv. St. George under controlled conditions. Mycorrhizal or nonmycorrhizal grape rootings were inoculated with the pathogen. Eight months following inoculation with the pathogen, we evaluated disease severity, vine growth, and mycorrhizal colonization. Mycorrhizal plants developed significantly less leaf and root symptoms than nonmycorrhizal plants (P = 0.04 and P < 0.0001, respectively). Only nonmycorrhizal grape rootings inoculated with the pathogen had significantly less dry root and leaf weights compared with the noninoculated control (P = 0.0021 and P = 0.0017, respectively). Mycorrhizal colonization was high (48.3% for the noninfected control and 54.5% for plants infected with C. macrodidymum) and not significantly affected by inoculation with C. macrodidymum (P = 0.2256). Thus, V. rupestris preinoculated with G. intraradices were less susceptible to black foot disease than nonmycorrhizal plants. Results from this study suggest that preplant applications of G. intraradices may help prevent black foot disease in the nursery and in the vineyard.