downy mildew of pumpkin
Figure 1. Downy mildew of pumpkin. Lesions tend to be angular and are initially chlorotic. Note older lesions have turned necrotic. 
Figure 2. Downy mildew of pumpkin. 
Figure 3. Under moist conditions, the causal fungus for downy mildew of pumpkin can be observed to sporulate on the underside of the leaf.
Figure 4. Sporulation on the underside of a pumpkin with downy mildew. Note pumpkin leaf is wet. 
Figure 5. Downy mildew of pumpkin. Sporulation is visible on the underside of the leaf near the vein where moisture has accumulated.