Rhododendron leaf problem

Asked April 23, 2016, 11:48 AM EDT

We recently moved into a creekside home that has at least 30 old (50-100 years), large Rhododendrons. Most, if not all, of them have a fine spotting, coupled with larger, more scabby looking spots on their leaves. I am also finding the fine spotting on some of the Azaleas. What is it and what should I do about it? I'd hate to lose these Rhodies!

Clackamas County Oregon

2 Responses

Thank you for sending images with your inquiry. They clearly indicate what has occurred.

The leaves with large brown, scabby spots suffered sunburn sometime in the past year. Perhaps they were on the dry side during some of our bright sunny weather. Even though they’re large, old specimens, monthly irrigations will help them through our dry summers.

The fine-textured spots in the remaining pictures reveal damage by azalea lace bug, an invasive pest which arrived relatively recently. Azalea lace bugs feed primarily on azaleas but also attack rhodies. The small pale areas on the top indicated that the bugs have fed from the underside of the leaf. The dark spots on the reverse are fecal spots from the previous season.

The affected azaleas and rhodies will survive but will tend to look rather tough from summer through winter until fresh new leaves cover the old. Even so, the shrubs will flower profusely every year, which is precisely why we grow them.

Management includes the following:

- Reduce stress by supplying sufficient water through the year.

- Insecticide sprays can be useful; most are contact materials; in other words, the spray must thoroughly cover the undersides of the leaves. (Choices include Azadirachtin; Insecticidal soap; and Neem oil.)

- The most effective time to spray is right after the hatch, when the nymphs are in a rather tight cluster on the underside of the leaves. In this warmer than usual year, that may be anytime now. Monitor to determine the appropriate timing: Every several days check the undersides of the leaves as you look for the small dark colored nymphs (youngsters).

See http://oregonstate.edu/dept/nurspest/azalea_lace_bug.htm and http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/40424/em9066.pdf

Thank you so much for your thorough and detailed answer. I am so glad to know that there are things I can do to help and that the symptoms are not indicative of imminent plant death.