Answer: Wow, it's hit your garden hard! Once it has reached this stage, there's not much you can do except keep your plants well fed and watered, and treat any new growth with preventive measures (mentioned below). In your region, you can still plant another round of beans and cucumbers (choose resistant varieties), so that may be the best strategy at this point. You can eat the fruit from infected plants, but discard any fruit that has mildew growing on it.
Powdery mildew is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather, as well as in high humidity.
The first line of defense is to grow resistant varieties, and secondly, keep a close eye out for early infection, and use preventive measures to keep it from spreading.
The general advise to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, you can actually inhibit infection with periodic strong sprays of water to dislodge mildew spores (not so strong as to damage the plant.) Some people have had good luck with pesticides containing neem, a plant extract. You can also try using a homemade baking soda spray--mix 1 teaspoon baking soda per quart of water; add 1/4 tsp mild soap to help it stick to the leaves. Apply this once a week, and hose down plants between sprayings. Be sure than any fungicide you use is labeled for powdery mildew, since some are ineffective against this disease.
If you believe the plants are beyond hope, remove all of the diseased plants and, bury or throw everything in the garbage. Unless your compost heap really heats up (at least 160F), the fungal spores can overwinter there and reinfect your garden. Plan to grow varieties resistant to the disease next year. Following the ideas above, your mildew problem should be better under control. Best of luck!
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