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Sick Rhododendron (Suspect Pythopthora Root Rot).


By alanb


We have had a beautiful red Rhododendron in our garden for 20 years. It has been a lovely feature each May and early June.

It flowered again nicely mid May this year, but now when it is normally full of new shoots it looks decidely sick. Last year we ‘lost’ the Rowan with a disease, this was planted in close proximity and can be seen in the photo above. I believe nearby Ness gardens, lost some Rowan’s last year with disease.

Photos of the Rhodendron taken today are posted below.

I wondered if the disease which attacked the Rowan last year has spread to the Rhodendron, prior to me cutting the Rowan down last year.

Would anyone be able to confirm that my suspicions are correct please.

Thank you for any comments or advice you can offer.

Footnote: I have fed and watered the Rhodendron this year, as in previous years.

Following Seaburngirl’s comments, I have included a photo
of three cut stems below. The cut areas were white and moist … not brown and dry as I expected.

Roots taken out today … Photo below.


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if you cut one of the wood stems what colour is the wood inside?

I don't know the answer, but you could send this picture and query to the RHS. if you are a member their advice service is free. Not sure if they charge for non members.

12 Jun, 2020


Due you know what the Rowan's had? Did they have splits in the bark that oozed so the bark looked as if it had dark liquid running down it?

the whitish growth on the branches look like lichen growth and that wont cause any problem.

12 Jun, 2020


Hi Seaburngirl, thank you for your comments and interest.

The Rowan died over a period of 12 months killing large branches on one side initially. I removed these in an attempt to save it, but gradually the rest of the tree died.
I understood it to be 'Ash Dieback' ... but have read that Rowan is not considered to be a member of the Ash family?

I have left the tree trunk in the ground and use it to hang bird feeders on. Perhaps I should have removed the trunk and roots completely. Perhaps I haven't done myself any favours here.

I don't recall seeing any splits in the bark 'oozing dark liquid'.

I have cut three branches off the Rhododendron just now and included a photo in the blog. I was surprised to see it white and quite moist. I was expecting dry, brown and dead.

12 Jun, 2020


well that rules out some of the fungal attacks as you usually find rings of discolouration.
Do you get toadstools around the base of the Rowan in late summer? [thinking honey fungus] Rowans are in the genus Sorbus and are actually in the rose family. So not ash die back.

cant be any more help I am afraid.

12 Jun, 2020


Thank you again Seaburngirl for your time. Helpful to have eliminated Ash Dieback.

In answer to your question, never any toadstools round base of Rowan.

12 Jun, 2020


Seaburngirl, following your advice re contacting RHS, I took a look at the RHS and Glendoik websites. These gave lists of diseases that affect Rhododendrons.

I believe I have found the answer to my problem.


(a) Phythopthora Ramorum (sudden oak death) or
(b) Phythopthora Cinnamoni

SYMPTOMS: 'Sudden wilting, yellow or sparse foliage occurs, with eventual collapse of the whole plant. Above ground symptoms do not appear until root decay is well advanced. Caused by a micro organism.

Phythopthora Cinnamoni is most active in high soil temperatures (22-28 degrees C). Temperatures were very warm here in April and May.

Infected soils can spread on foot wear and garden tools (spades/secateurs etc.,). This can be prevented by good garden hygiene washing tools in hot soapy water and footwear in Jeyes Fluid, according to the instructions. Infected soil should be discarded, spores can survive for several years.

12 Jun, 2020


SADLY today, I have cut all the branches off the Rhododendron. I will burn them so as not to 'recycle' the disease. I will include a photo of the root system later when I dig them out.

13 Jun, 2020


That is such a shame - it was a gorgeous shrub. Perhaps you'll find space to replace it in another part of the garden...

22 Sep, 2021


Sue, it was our showpiece in the garden in late May, but unfortunately you sometimes lose a plant or shrub. Thankfully not very often.

22 Sep, 2021

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