Link below is to several discussions of the same question that the search pulls up and will answer your question. Looks like you have both powdery and downey mildew problems and both are discussed in the link. Dave Here is a link that might be useful: Squash leaves turn yellow discussions

You never want to let cukes dry out between waterings. At best you'll end up with very bitter cucumbers. Cukes require consistently moist, not wet but moist, soil. Did you harden them off well before moving them out to the garden? If not that would explain some of the damage as nothing can go straight from inside out to the garden without a gradual period of adjustment to full sun and wind first. So are these symptoms on all the leaves, even the new ones, or just the older leaves? Does it appear to be spreading up the plant? Does it rub off with your fingers? Is it more yellow than white or brown? If so then it is Downey Mildew as opposed to Powdery Mildew.

Why the epsom salts? Potting mix isn't normally low in magnesium and that is the only benefit one gets from E. salts. You don't mention which Dr Earth product you used and they make many but if I recall correctly they are all organic blends. Great for a in ground garden with active soil bacteria but won't work at all well in containers as there is no soil bacteria in potting mix to digest it so the plants can use it. You need to use a water soluble fertilizer of some type, something the plants can actually benefit from quickly. There are many brands available both synthetic and organic. They very likely need a good feeding and if they were like this before you transplanted them then they likely have needed some nutrients or some time. Dave

I am in zone 7, just about when spring time starts. Greenhouse is maintained above 60F and up to 80F. Humidity varies depending on the weather since there is on and off rain and sun, but in general, the dehumidifier keeps it below 67% humidity. This does not seem to be due to the environment, since the other cucumber plants in top soil in pots are perfectly healthy, if not slightly smaller. The grow bags are experimental, and are like the hydroponic grow bags that commercial greenhouses use. The bags don't get very hot as the environment isn't very hot. The bags are for sealing moisture, to test if I can grow cucumbers with reduced watering and if that can help water loss during hot summer days when they wilt everyday.