|I have a medium sized rhody that has developed tiny black spots mostly on the underside of the leaves, though on some it appears to be coming through to the top. It almost looks like dirt splashed up on the leaves, but it's not (I checked to see if I could rub it off - can't). There is no bug infestation that I can see, just the occasional, normal nibble out of a leaf here an there.
|What you're describing sounds like powdery mildew, especially if the undersides of the leaves have a brown, feathery looking discoloration. Eventually the upper sides of the leaves will develop yellowish discoloration, followed by brown spots. Black spots on the undersides of leaves are usually the fruiting bodies of a fungus called Phyllosticta. Rhododendrons are susceptible to both of these fungal problems. You can prune to remove affected plant parts and stop the spread. If this is not practical, use a fungicide such as Funginex or Safer's Garden Fungicide. Be sure to follow label directions.|