I live on the gulf coast near Tampa FL. I have a 2-year old garden in a box about 2 feet high filled with a sandy topsoil i had brought in. I keep it waterd via dedicated sprayers in the garden hooked up to the regular watering system. I cannot get rid of powdery mildew. I am sprayig "Triple Action Neem" oil about twice a week (twice as often as prescribed) and barely keep it in check. Plants are looking bare near the bottom because leaves have died and fallen off and not producing many tomotoes. Any guidance i appreciated!
Most nights my plants are getting wet enough from fog and dew that the water is dripping off. Check before sun up. You may need to stop watering. I'm only watering about once a week. I had a real problem with the mildew in the spring and read somewhere to spray with a mix of 1/4 milk to 3/4 water and it worked. It took 3 sprayings several days apart to get rid of it all but it worked. be sure to get under leaves and stems.
I have a tiny rambler rose that my Grandmother moved from Miss. in 1908. It is real subject to powdery mildew. I tried everything on the market commercially but nothing stopped it. Then a lady on Dave's recommended baking soda and dish washing liquid in water. It stopped it in its tracks. Might take a couple of sprayings but it works. Now I start spraying when it first begins to leaf and about once a week until it is through blooming. Very cheap fix. Give it a try and try to get under the leaves as much as possible.