I'm growing watermelons for the first time, and things were looking great. But this evening I saw that some leaves were curling up. A closer inspection show black spots all over the leaves, top and bottom. Spots is actually the wrong word. It looked as if a million tomato hornworms have pooped on the leaves. The black stuff is not flat, like a spot. I assume a fungus of some sort.

I live in a hot dry area, with drip irrigation, so the foliage stays dry.

Does this ring a bell with anyone?