I am a new member and have recently bought one Nova Zembla and one Catawbiense plant from Costco. The plants were quite large, approx. 3' high and looked fairly healthy. When I examined the underside of the leaves I noticed small dark spots all over and some leaves are yellow. Costco sold them for $22 which is amazingly cheap but I am sure these plants were overfertilized. I have planted them in a good spot for Rhododendrons in acidic soil and semi shade. Is there anything I can do against the spots, Iron sequestrants etc?