This past Saturday, the mildew had spread across the patch of summer squash. While *we* had been careful to avoid watering late evening, to avoid water sitting on leaves not drying off, Mother Nature had no such qualms. For a week she rained on and off as she pleased, day and night, throwing in a few thundershowers for good measure. Plus with the high humidity, and therefore a high dewpoint, we surely had leaves soaked in dew every night.
Nevertheless, we benefited significantly from using potassium bicarbonate (couple teaspoons mixed in a few quarts of water -- we didn't add soybean oil because that gummed up the sprayer) to prevent mildew. Indeed, the cucumber leaves were still free of mildew, and we've had a much more abundant crop of cucumbers this year than last. The winter squash along the fence showed some mildew encroachment, which by today according to our Thursday waterer had spread entirely over that patch. Interestingly, the winter squash under the beans and corn was still free of mildew. Perhaps yet another advantage of growing the Three Sisters together!