Grapevine downy mildew is a serious disease caused by the oomycete Plasmopara viticola. This thesis evaluated the prevalence of fungicide resistance in Georgia populations of P. viticola using bioassays and molecular methods, and the field efficacy of fungicides commonly used to manage the disease. Resistance to quinone outside inhibitor (QoI) fungicides (G143A mutation) was widespread throughout the state in this pathogen (present in 82.1% of vineyard-year combinations), whereas no carboxylic acid amide or phenyl amide fungicide resistance was detected. Moreover, QoI fungicides provided the least amount of disease control in on-farm field trials and performed no different from untreated controls when tested in a research vineyard. In contrast, other groups of fungicides tested provided adequate or exceptional control of downy mildew, documenting practical disease control failures associated with QoI use.