downy mildew of watermelon
Figure 1. Chlorotic lesions of downy mildew on a watermelon leaf. Note that a few of the lesions are starting to become necrotic. 
Figure 2. Chlorotic and necrotic lesions on watermelon leaf. 
Figure 3. Underside of watermelon leaf with downy mildew. Note sporulation of downy mildew-see red arrows. 
Figure 4. Severe outbreak of downy mildew on watermelon. Note that stem and fruit of watermelon are not directly affected.