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Phytophthora (fy-toff-thor-uh)

Symptom ID Guide: Rhododendron species

Phytophthora species are microscopic organisms that can cause disease in Rhododendron plants.

You can learn more about Phytophthora and Sudden Oak Death at the links below.

Rhododendron species

Diversity

There are many beautiful Rhododendron species. Many are exotic to the Pacific Northwest, but thrive in managed gardens.

rhododendron with small leaves and one red bloom

rhododendron with larger, wide leaves and multiple pink blooms

More information about the diversity and characteristics of Rhododendron species are available at the links below.

Susceptibility to Phytophthora ramorum

Our program has conducted a little research on the differences in susceptibility between a few varieties. Our research projects page has some information about Rhododendron susceptibility.

Some Rhododendron species and cultivars that have tested positive for P. ramorum in nursery inspections are listed below. This is not a complete list.

Cultivar	Parents
‘Unique’	Campylocarpum hybrid
‘Baden Baden”	‘Essex Scarlet’ (parents unknown) x forrestii ssp. forrestii Repens Group
‘Anna Krushke’	ponticum seedling
‘Roseum Elegans’	catawbiense hybrid
‘Bruce Bretchbill’	bud sport of ‘Unique’
‘Edith Bosley’	Dexter Bosley #1035 x Lee’s Dark Purple (catawbiense x unknown )
‘English Roseum’	catawbiense x unknown
‘Lee’s Dark Purple’	catawbiense x unknown
‘Purple Splendour’	ponticum x unknown
‘Rocket’	‘Cunningham’s White’ x red catawbiense seedling
‘The Hon. Jean Marie de Montague’	griffithianum x unknown
‘Chionoides’	ponticum hybrid
‘Daphnoides’	R. virgatum ssp virgatum x unknown
‘Holden’	red catawbiense seedling x ‘Cunningham’s White’
‘Nova Zembla”	Parsons Grandiflorum (parents unknown) x hardy red hybrid
‘Catawbiense Album’	catawbiense hybrid or selection
‘Christmas Cheer’	caucasicum hybrid
‘Colonel Coen’	unknown, but contains ponticum
‘Cunningham’s White’	caucasicum x ponticum, white flowered
‘Grace Seabrook’	Jean Marie de Montague’ x strigillosum
‘Royal Purple’	unknown

Phytophthora ramorum Symptoms on Rhododendron

Leaf blight

Phytophthora ramorum symptoms on Rhododendron consist of leaf blight and shoot dieback. These symptoms are indistinguishable from symptoms caused by other Phytophthora species.

Leaf lesions will have dark discoloration that often follows the midrib or petiole. These lesions are often most severe at places where water accumulates, such as the leaf tips.

a long, narrow leaf with a large lesion along one side of the midrib and a smaller one on the opposite side towards the edge. — Foliar infection by *P. ramorum* on Rhododendron

Shoot blight

Phytopthora ramorum can also cause shoot blight on Rhododendrons. The wood will be stained brown under the bark and the bark will be blackened.

Shoot blights are also caused by Botryosphaeria and Phomopsis, which can look similar to those caused by P. ramorum. However, tiny pinhead-like fruiting bodies can sometimes be seen, whereas with a Phytophthora infection, visible fruiting bodies are not present.

the shoot dieback shows as a dark brown lesion below the top of the branch. — Rhododendron shoot blight caused by *P. ramorum*

Dead rhododendron shoot, showing gray leaves hanging down from the branch and the pistils remaining from the flower pointing vaguely upward

Rhododendron plants in pots. Light green leaves pointing upwards are new growth. Many of the darker green leaves further down show evidence of lesions caused by Phytophthora.

Sun scorch

Sun scorch and other leaf spots caused by fungi can resemble symptoms of P. ramorum.

These symptoms differ from those of P. ramorum by having defined margins, fruiting bodies (in the case of fungi), and often do not follow the midrib or petiole. They can occur anywhere on the leaf, not necessarily where water collects.

rhododendron leaves, two of which show sunscald lesions — Sun scorch injury on Rhododendron leaves (Photo: Rich Regan, Oregon State University)

Wilting

Wilting can be a symptom of infection, but there are many causes of wilting. If adding more water does not help, it could indicate the plant is infected with a pathogen such as Phytophthora or or another fungus such as Botryosphaeria, but it does not necessarily indicate that Phytophthora ramorum is involved.

There are many species of Phytophthora that can cause a plant to wilt and die. For example, Phytophthora cactorum, P. cinnamomi, and P. citricola cause root rot and stem cankers.

rhododendron leaves turning yellow with clusters of pink and white blooms. both the leaves and the blooms show signs of wilting.

Rhododendron plants in pots with their leaves drooping downward and their flowers crispy and brown.

Dead rhododendron in a pot, surrounded by healthy plants.

More Resources

Learn more about Sudden Oak Death

More information, including links to youtube videos, about Sudden Oak Death.
You can also find information for specific audiences (e.g. nursery professionals, garden managers, master gardeners, etc.) on our Sudden Oak Death Education Program webpage.

View more online symptom guides

We have prepared a list of links to other symptom guides. Viewing multiple photos of symptoms on Rhododendrons may help you determine if Phytophthora ramorum is involved on yours.

Management

Visit our webpage for Nursery Productions if you are looking for Best Practices for managing Phytophthora ramorum. These practices are also useful for managing other Phytophthora diseases, such as those that cause root disease, which can be more damaging than P. ramorum on their hosts.

Relevant WSU Extension Document

Cover photo of WSU Extension Publication: "identifying, treating, and avoiding azalea and rhododendron problems"

Other Rhododendron Diseases

rhododendron with sparse foliage turning yellow and showing signs of wilting — Wilting rhododendron

Diseases of Rhododendrons are not limited to those caused by Phytophthora species. The American Phytopathological Society website lists a number of diseases and pathogens impacting Rhododendron species.

If you are uncertain and concerned about the possible pathogens affecting your Rhododendron plants, we encourage you to contact us or submit a sample to the WSU Plant and Insect Diagnostic Laboratory (see below).

Plant Disease Diagnostics

Guidelines for submitting a sample

Visit our Sample Submissions Guidelines webpage for information to help determine whether it will be helpful to submit a plant sample for professional diagnostics.