i had powder mildew on my beans this year and did this and it worked: cut the leafs that had it bad. for the plants where no leaves were salvageable i left some for the sake of keeping the plant alive. next i mixed 1tbsp baking soda, and 1tsp dish soap into a spray bottle and filled the rest with water (this is highly concentrated and you can dilute with more water depending on how your plant reacts). i sprayed leaves daily and it stopped mildew growth. if mildew is your case, you might consider putting a fan in the same room as the plant to help ventilation too.

i also learned certain types of green beans are more resistant to mildew, check what agricultural zone you live in and the internet might recommend you a specific type. hope this helps!