The Q&A Archives: Black Leaves on Rhododendrons

Question: I moved into a new house that has several rhodies. Many of the leaves are black and dead. Thet also have holes in them that may be insects(none seen yet).The plants have many many buds and look healthy other than some of the leaves dieing. All my neighbors have the same problem with their plants. Soil is clay. I'm trying to loosen it with gypsum and I have fertilized them with Miricle Grow. Thanks....Paul

Answer: Fungal diseases are quite common on rhododendrons grown in the Pacific Northwest. What you describe sounds like Phytophthora blight. It is a fungal disease whose symptoms are often confused with chemical or mechanical injury or leaf scorch. Irregular dead spots on the tips or margins of leaves, shiny black cankers on shoots, and leaf loss and tip dieback on affected branches are among the common symptoms. The disease is spread by wind and splashing water. To manage, avoid overhead watering, avoid wounding plants, as wounds provide ideal infection sites. Be sure to rake and destroy fallen leaves to prevent the spread of infection.
Remove infected leaves from your plant. Prune infected branches back into healthy wood. Destroy, do not compost, diseased plant material.

Rhodies do appreciate an acidic soil. You can topdress with peatmoss which will help acidify the soil, and you can feed with an acidified fertilizer (such as Miracle Gro).

If the holes in the leaves are notches on the margins, root weevils have been feasting on them. The adult insects hide in the soil during the day and feed at night. You can go outside with a flashlight after dark and probably catch them feeding. If this is what's nibbling on the leaves of your plant, you can trap the pests with a sticky product called Tanglefoot. Apply according to label directions.

Best wishes with your rhodies!

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