Answer: It is really difficult to diagnose a plant problem without being able to see it!
Spider mites can be a serious pest of watermelons, especially during hot, dry weather. Infested plants appear yellow and become visible from a distance. Mites are primarily found on the undersides of leaves making the leaves appear tan or yellow and have a "crusty appearance". Mites feed on the plant sap and can defoliate vines in a few weeks in hot, dry weather. Defoliated plants tend to yield small, poor quality fruit.
Powdery mildew, a fungal disease, can also cause the same symptoms.
I can't really recommend a control without knowing whether either of these are the cause of the problem. Wish I could offer more help!
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