My zucchini plant has mature leaves that are turning white along the center and spreading out toward the edges but the edges of the leaves are still green. Can someone tell me what this is and what I should do about it?

Hi! njmccune and Welcome to AMHD :)

Some varieties of zucchini have white lines naturally, but it sounds like Powdery mildew:

Mix 2-3 Tablespoons of baking soda in one gallon of water and spray this all over your plants and garden soil. Make sure you spray the undersides of the leaves, too. Spray everything, not just the zucchini.

This solution is organic and will help control a multitude of garden problems. Repeat the application every 7-10 days or more frequently if you get rain right after an application.

Add a layer of mulch around your plants to prevent water from splashing on the leaves. Mildews and other fungi are spread this way.

I have the same issue with my own zucchini, however, they naturally turn a shade of "dusty" white upon maturity - totally normal. Powdery mildew looks about the same shade of white as the natural leaf coloring, but looks much more like mildew, or mold, growing as patches or spots on the leaves. Think of mold growing in the grout of bathroom tile... that's how powdery mildew would look in shape, possibly a bit larger in size, though (dime to nickel sized, until fully engulfed prior to the leaf wilting and eventually dying).

As a precaution, you can always spray with the 10/1 water and milk mixture and prevent the mildew from forming, especially if you live in a cooler, more northern climate. Doing so won't harm the plant and may give you a little peace of mind. I live in central Florida, and because of the warm and mild climate, there is less of a chance of the fungus forming here, but, I too occasionally spray my plants (zucchini, squash and watermelon) just to be on the safe side. With our "come and go" flash rain storms in late spring through summer, the chances get a bit higher because of the rapid warming of the foliage after a shower which can lead to a fungal infection.

I hope this helps and I wish you good luck and a bountiful harvest!

Adam