Rhododrendon disease

Asked May 17, 2014, 11:18 AM EDT

One rhododrendon in my yard has a disease that is exhibiting itself on the leaves. It has been here for over a year and has resulted is a slow dieoff of branches. Can you identify it and provide a treatment? Any reason why it is on this bush only and has not effected others?

Multnomah County Oregon

1 Response

Your rhododendron displays unthrifty growth, with leaves only at the tips of the branches. I suspect it is receiving insufficient light and is crowded by surrounding plants. The brown lesions are one way powdery mildew shows up on rhododendrons. (The other, more familiar, appearance is powdery white to grayish areas on the leaf.) Your other rhododendrons may be more resistant to the mildew or they have better growing conditions unsuited to the disease.

Cultural suggestions include these:
- Reduce relative humidity, reduce shading and increase air circulation by thinning out growth on surrounding plants. - Apply a deep watering (8 or 10 inches deep) once a month during summer.
- Apply water at the shrub’s base; avoid overhead irrigation via sprinklers
- Consider replacing it with a resistant kind.

If you would like to try a spray, begin applications when you first notice the disease on current-year leaves. Products available for home use include Bayer Advanced Disease Control, Monterey Bi-Carb Old Fashioned Fungicide, and Safer Garden Fungicide. Follow label directions for dilution rate and re-application times. The brown spots will remain.