Grey leaves on basil and watermelon
The basil probably has downy mildew mildew, which is a relatively new disease in Maryland. Most basils are becoming affected by it at this time of year and there is no cure for it. https://extension.umd.edu/growit/downy-mildew-basil The melons are susceptible to several types of leaf spot diseases. The reddish spotting on the one melon leaf looks like a symptom of ozone pollution. There really isn't any thing you can do to alleviate these symptoms. Harvest the melons and pull up/discard the affected plants.
Regarding the basil, you might want to plant varieties that are known to be more resistant to downy mildew. Cornell University lists several more resistant types on their research web page. http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html