Downy Mildew in Cucurbits
Dark purplish gray spores of the downy mildew fungus only develop on
lower surfaces of leaves and are easily distinguished from white spores
of the powdery mildew fungus. These spores are diagnostic but unfortunately
are not always present (see Fig 4). Note that downy mildew lesions usually
do not enlarge beyond major veins, giving spots an angular appearance.