Worldwide distribution. On watermelon (and squash), and other members of the cucumber family, including weeds. A minor disease, worse under shade.
Small, about 5 mm, round spots with grey centres, brown margins, and sometimes yellow halos. Centres may fall out, spots merge, resulting in black areas, and leaf fall.
Spores on the underside of the leaves are spread in wind and rain.
Cultural control: do not plant new crops next to old; use 3-4-year rotation; weed; do not grow watermelon under shade; collect and burn trash after harvest. Chemical control: copper, chlorothalonil, or mancozeb.