A simple way to answer this is to match each pathogen to the disease it is best known for. Uncinula necator is the classic powdery mildew fungus of grapevine, which forms a white powdery coating, different from downy mildew. Phytophthora infestans is remembered for the Irish potato famine caused by late blight, and Pythium aphanidermatum is a common soil pathogen causing damping off of young seedlings. None of these three match grapevine downy mildew. The oomycete Plasmopara viticola specifically infects grapevine leaves and produces the yellowish oily spots with white downy growth underneath that define downy mildew. \[\boxed{Plasmopara\ viticola}\]