How to kill Downey mildew? Last spring I found leaves becoming brown and dropping off on. 3 Rhododendrons planted close together. It was finally diagnosed as Downey mildew and I sprayed off and on all summer. We had a very cold, wet winter and the spring is not much better. The mildew is still present on the plants and I am wondering what is the remedy to kill it? They are very mature plants and I would hate to lose them but fear that is going to happen. I live in Portland, Oregon and communicated with master Gardners last year but their suggestions have not worked. Any suggestions? Harriett Heisey Hheisey1000@gmail.com
Certified GKH Gardening Expert
I suggest spraying the plants with a product called Green Cure at 4 to 5 day intervals for at least 5 sprayings. Be sure to mix the product with water at the Cure Rate listed on the product label. Downy Mildew is a tough customer to get rid of, so you must keep after it. If things do not appear to be improving after the 5 spraying regimen. Then do yet another regimen of spraying. Another option would be spraying with a fungicide known as Mancozeb. It does leave a yellowish powdery residue on the plants leaves but that is part of how it works. Use the Mancozeb at 7 day intervals.