Pests & Diseases investigated by SRS Research Director
If you have noticed any particular problems with trees and shrubs in your own area and would like further information, please free to contact Marion Kinns. I make no claims to be an expert but I am happy to investigate to find what information I can. This can then be shared with the membership. Members with more information than I have discovered will be welcome to make comments/ additions; potentially leading to useful discussions.
Membership of the SRS declares us as having an interest in, or in some cases obsession with, the genus Rhododendron. However, quite a few of our members have big gardens with a range of other genera so these questions can be quite wide ranging.
Contact via email: enquiries@scottishrhododendronsociety.org.uk
Contents:
- Diseases affecting rhododendrons.
- Pests affecting rhododendrons.
- Other pests and diseases to be aware of.
- Plant Health Questions and Answers.
Diseases affecting Rhododendrons :
Diseases that have been and are the cause of most concern are the two Phytophtheras – ramorum and kernoviae. Leaf die back and shoot/branch die back are the obvious signs. These fungal diseases are still notifiable and have been the cause of loss of large numbers of rhododendrons from historic collections through death of plants and enforced removal.
Other common specific fungal diseases that affect rhododendrons follow. The most common are Bud blast and Powdery mildew.
Affects some species more than others. Subsection Cinnabarina is particularly susceptible as are the Fortunea. Infection causes pale green, reddish purple or yellowish spots on the upper surface of evergreen rhododendrons. Deciduous species may show white powdery fungus on the leaves. This isn’t usually fatal for the plant but unsightly and some years are worse than others. It is not recommended to remove affected leaves or plant could be defoliated!
Causes buds to go brown and remain attached. Later buds may turn silvery grey before becoming covered in small black bristles. This should not be confused with frosted buds which remain brown and can be knocked off easily. Remove and destroy affected buds of bud blast promptly.
Spots appear on petals in wet conditions. These spread rapidly and cause petals to collapse in a slimy mess.
This is usually found on ‘azaleas’ and leaves become pale, swollen and distorted then covered with a bloom of white fungal spores. Remove galls promptly when seen before the spores appear and spread the disease.
Purple or brown roundish spots on upper leaf surfaces usually of evergreen species.
Rust (Chrysomyxa rhododendri) seen as discoloured spots on the upper leaf surface which correspond to pustules on lower surface containing dusty orange spores.
Rhododendrons can also be attacked by more generalist diseases:
Sudden wilting and collapse of whole plant or slower and more gradual decline.
Bodies of fungus sometimes seen developing on dead or dying parts.
This can cause dieback of the branches with fruiting bodies sometimes developing on dead or dying parts. It affects the wood of the plant and is often spread by infected pruning tools. The silver appearance of the leaves is due to the top layer of leaf cells separating from the one below as a reaction to the fungus in the bark. Light is then reflected differently.
(much of this information came from the RHS website rhs.org.uk , then enter ‘Rhododendron
diseases’ into the search box)
Marion Kinns, 7th September 2024
Page under development. Please check later!
Pests affecting Rhododendrons :
- Plant Health Questions & Answers by SRS research director, Marion Kinns
- A response to comments in the press on Rhododendron ponticum by John Hammond and Ian Douglas
- Some thoughts on Cinnabarina, its Hybrids and Powdery Mildew by John Hammond. SRS Yearbook No 23