downy mildew of watermelon

Downy mildew-This disease does not occur every year in Indiana and watermelon may be out of production by the time the fungus-like-organisms arrives in Indiana. Initial symptoms on leaves are often light chlorotic lesions with diffuse margins. Lesions on the underside of leaves may have a black or purple sporulation under moist conditions.  The center of lesions may turn necrotic with time. Stems and fruit are not directly affected. 

Figure 1. Chlorotic lesions of downy mildew on a watermelon leaf. Note that a few of the lesions are starting to become necrotic. 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 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 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. Figure 4. Severe outbreak of downy mildew on watermelon. Note that stem and fruit of watermelon are not directly affected.