Were you treating the powdery mildew with fungicides? If so which were you using and how often applied? If not, may I ask why? If left untreated it will slowly kill the plant. I also notice a great deal of weeds in the area of the plant which only harbors the spores and contributes to the problem. I assume no over-head watering or has excess rain been an issue?

Dave

I was spraying the leaves lightly with a milk/water combination. I had been over-head watering them with a sprayer hose, but I switched to just letting the water pour out of the hose onto the soil last week.

If organic use only is the goal then there are a number of ORMI approved fungicides available that would have been far more effective than some milk and water combination. As a fungicide its effect is minimal at best. Given the decline of the plants in the pics it may well be too late to save them now. Not that I would pull them but I'd sure start spraying the new growth with a real fungicide.

Dave