The Q&A Archives: Rhododendron Rust

Question: Over the past few weeks I have noticed a problem with my rhododendrons which are located on a southweasterly exposure but most within an area shaded by taller trees.

Particularly near the bottom of the plants, the leaves are dying back from the end of the branch. The brown dead part at least appears to start at the end and then "backwards" over several leaves which then also turn brown and die. I am suspecting a fungus but need to know what it is and proper treatment.

Also, some of the other leaves have brown discolored spots as well. Hmmm - I have just brought in a few of these leaves to examine and describe and it seems the leaf is first turning brown from the stem end. These brown spots follow the veins of the leaf as well as showing general discoloration on the edges. On the underside of the leaf this discoloration is rust colored.

Can you assist with diagnosis and treatment?

Many thanks.

Answer: One thing to note is that rhodendrons have very shallow roots, so if you've done anything to disturb the top few inches of soil, that could be a factor in weakening the plants.

It's possible that your plants are suffering from an infection of rust, a disease caused by the fungus Pucciniastrum vacinii. Symptoms include rust-colored pustules on the undersides of the leaves, which eventually release golden or brownish spores. This fungus requires an alternate host--hemlock--to complete its life cycle, so if there are hemlocks nearby, suspect rust.

Rust can be difficult to control once an infection has taken hold. You can try spraying with a sulfur-based fungicide every 10 days or so during July and August. Or you could try spraying a neem-based product. Neem is a botanical extract that has fungicidal properties. Also, prune and destroy all affected foliage.

I can't say for sure if you are dealing with rust, but I hope this offers some help.

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