Looks like Powdery Mildew.
Powdery Mildew is caused by the fungi Erysiphe cichoracearum and Sphaerotheca fuliginea occurs when temperatures are between 50 and 90 °F, during dry weather with high relative humidity.
You can try mixing one tablespoon baking soda, one-half teaspoon of liquid, non-detergent soap (like Seven Generation) with a gallon of water, and spray plants very well.
YES! Mulching helps ANY kind of soil. It gives the roots places to really expand. There are other plants to grow or Let grow that will break up your soil, but our tropical vegetables have a have time with it. By mulching you create an environment like when a property is abandoned and tree leaves fall and decompose and that helps most plants to sprout to grow well.Ok I dug up a plant no grubs ! I’m really happy about that . A mix of water baking soda and a bit of dish soap took care of the powdery mildew . Tomorrow I’m going to add some compost around the stems that are now very exposed and mulch . I don’t know if it will help? ducks4you our soil is sandy not clay does the mulching still apply to my kind of soil ?