Thank you for contacting Ask Extension. Before I can recommend something, we need to determine what insect, if any, is causing this damage. Do you see anything on the plant? You would need to check early morning/late evening, when insects do appear. Is the plant in an open area, or does it have protection above, like other trees?

Once I receive these answers, I can better answer your issue.



Thank you for contacting Ask Extension. The last photo show what look like some of the pretty common foliar fungal diseases that show up from time to time on these plants (e.g. Cercospora leaf spot). They are more or less a sign that conditions were good for fungal diseases last year and it might be a good idea to implement some simple cultural management practices to prevent the issue from persisting. It does not appear to be on a large amount of the plant, so I would only keep an eye out. If it shows up in over 30-40% of the plant, then pruning out affected leaves/branches and cleaning up fallen leaves will do a lot to reduce disease incidence next season. A protective fungicide spray could be used in the spring, but that is probably not warranted unless the infection rate is really high. You can read some general information about these kinds of diseases in this article from University of Maryland Extension: https://extension.umd.edu/resource/fungal-leaf-spots-shrubs/n.

Since you do not see any signs of insects on the buds, then it is probably due to a few different factors. One potential cause could be late frosts, which we had, that can damage the developing buds. Another possibility is under-watering, which can also affect the development of the buds and the opening of the blossoms. It's important to ensure that the rhododendron is receiving adequate water and that it is not being exposed to late frosts to prevent bud damage. https://extension.missouri.edu/publications/g6825

If we can be of further assistance, please let us know.