i have something similar on my watermelons but it is a lot darker. some kind of fungus maybe?

Look at the photo link below. Note the drying edge that is curling upward.

One of the fastest ways to help is probably to use fish emulsion. You can dilute it and spray/pour it on the leaves, and/or pour some around the roots.

Fish Emulsion also has nitrogen, potash, and many micronutrients, all good for the plants and the soil.

Sue

Here is a link that might be useful: Potassium deficiency photo

I guess it could be a potassium shortage.

But in most home gardens, it's an environmental disturbance which temporaily disrupted water transport to the plant -- among them, erratic soil moisture level, low humidity, high heat.

Thanks so much for your thoughts. It probably is time for another feeding of fish emulsion, so I'm sure it won't hurt it. I've tried to water fairly regularly, and we have high humidity along with our high heat here.

Jean, they tend to produce the same symptoms because they're both causing the same problem. Potassium is a cell-regulating element; it regulates osmotic pressure in cellular tissue and fluids. When you don't have enough potassium, you see the same problem: getting water to the plant.

Sue