milk sounds promising along with some other natural treatments

http://www.growingformarket.com/articles/powdery-mildew-solutions

You can't avoid exposing the plants to the spores. The best thing to do is make the plants less susceptible to infection. A plant that is growing strongly, has all the nutrients, water and sun it needs, will not get infected. This is a bigger problem container gardening like you are.

To replant your planter, you should first dump the soil into a wheelbarrow, and mix in some organic matter (compost, worm castings, something like that) and then a complete organic slow-release fertilizer (I recommend the brand Down to Earth). Add lime or sulfur depending on whether your next plant prefers acidic or alkaline soil. Don't overdose these additions, just enough to fluff out your soil and provide enough nutrients for the growing season, which will depend on the size of your planter.

Then replant. You will be fine for awhile, but once the plant gets really large (and what happened to your strawberries after a couple years), is that the roots get root bound in the pot. The plant, having a lot of leaf matter to provide moisture for, quickly draws up all the water in the soil. When you go to water it, the root-bound soil can't absorb very much water at all, and most of it drains out the bottom of the planter. To you, this may look like you saturated the soil, because it's coming out the bottom and you assume you don't need to water more. The reality is exactly the opposite. You need to soak the soil long enough to let it start to soak up moisture through the root-bound system.

You can do this a couple ways... with drip systems that release water really slowly, by putting your hose in the pot on a trickle for a couple hours, or by filling a large container with water and putting your planter inside that container. The water will soak up from the bottom through the drainage holes.

But what happens otherwise is your dehydrated (and possibly nutrient-starved plant if you haven't fertilized with complete, mineralized fertilizer) succumbs to the powdery mildew. Dehydration is the #1 cause of powdery mildew infection. For the reasons above it's even a bigger problem with plants in planters.

Hope this helps.

What probably happened was dehydration. I got sick, old age, better than the alternative, in April, and am just getting to doing stuff again. Watering went by the wayside. The planter is enormous, 4(?) feet by 2 feet x 2 feet, so I don't think anything is root bound. I did fertilize until I keeled over