Pumpkin (below): Yellow to orange spots develop on the upper surface of leaves. Affected tissue can quickly turn brown and die. On the underside of leaves opposite these spots the pathogen produces dark spores. As the downy mildew pathogen grows inside a leaf it often cannot progress through vein tissue, thus symptoms typically are angular in shape. When not suppressed by fungicides, downy mildew can kill leaves and cause plants to die prematurely resulting in rotten pumpkin handles after the vine dies.
Imitator (below): Saprophytic fungi attacking old powdery mildew growth produce dark spores that can be confused for the downy mildew pathogen. In contrast with downy mildew, the saprophytic growth occurs more generally on the leaf rather than in defined, typically vein-delimited spots and there are no leaf spot symptoms on the upper leaf surface.
Watermelon (below): Dark brown leaf spots caused by downy mildew are irregular in shape. Leaves die prematurely when fungicides are not applied. The pathogen produces few spores on the underside of leaves, even following 1-day incubation in high humidity (inside plastic bag on wet paper towel); therefore, presence of fuzzy pathogen growth cannot be used to diagnose downy mildew in watermelon as it can in other cucurbit crop types (in particular cucumber and butternut squash). Watermelon tends to be less commonly affected than other cucurbit crop types. Images below were taken in 2013. Symptoms appeared on watermelon very early in the 2015 growing season in the northeastern US.