The leaves on my rhododendrons, one especially, look as if they have a bit of rust sprinkled on them, what causes this? I have 3 broad-leaf, two are English Roseum and the other is a nova zembla. The nova zembla's leaves look good but the leaves on the english roseum do not. Is there anything I can do to make them healthier?

I live in Massachusetts, Zone 5, my soil is acidic clay and there is a high water table in my area.

Any advice is greatly appreciated.

Linda