powdery mildew of watermelon
Figure 1. Leaf lesions of powdery mildew of watermelon may be white due to the sporulation of the fungus or colonies on the reverse of the leaf may show up chlorotic. 
Figure 2. Leaf with sporulation of powdery mildew. 
Figure 3. Powdery mildew of watermelon fruit. (Photo by Wenjing Guan.) 
Figure 4. Powdery mildew on stem of watermelon.