Probably Downey mildew. It starts out with black spots with a yellow center and then the black spots come together to form a mass. Other possibilities are herbicide injury and anthracnose. It’s also possible that the white spots are aphids on top of Downey mildew. Can you scrape them off? Aphids are easily washed off with a strong spray of water. The Plant disease clinic could give a definite diagnosis. See the following websites for comparison. 1. https://aggie-horticulture.tamu.edu/vegetable/watermelon/foliar-diseases/ 2. https://plantpath.ifas.ufl.edu/u-scout/cucurbit/downy-mildew.html 3. http://apps.extension.umn.edu/garden/diagnose/plant/vegetable/melons/ 4. https://pdc.umn.edu/