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 the comments box at the bottom of this page or 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.
Powdery mildew (Erysiphe spp)
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!
Bud blast (Seifertia azalea)
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.
Petal blight (Ovulinia azalea)
Spots appear on petals in wet conditions. These spread rapidly and cause petals to collapse in a slimy mess.
Azalea leaf gall (Exobasidium japonicum)
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.
Leaf spots (mainly Colletotrichum gloeosporioides but some other species also)
Purple or brown roundish spots on upper leaf surfaces usually of evergreen species.
Rust (Chrysomyxa rhododendri)
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:
Phytophthera Root Rot (various Phytophthera spp.)
Sudden wilting and collapse of whole plant or slower and more gradual decline.
Honey fungus (Armillaria spp.)
Bodies of fungus sometimes seen developing on dead or dying parts.
Silver leaf (Chondrostereum purpureum)
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
Marion Kinns, 7th September 2024
diseases’ into the search box)
Pests affecting Rhododendrons :
Scale insects
These small oval shaped insects attach to the stems and leaves of rhododendron plants. They feed on the sap and can cause yellowing, wilting and stunted growth. Excretions of honey dew from these insects attracts a black fungal growth of ‘sooty mould’ which is very unsightly. It is difficult to eradicate without chemical control of the insect. If the air circulation around the plant is improved, there may be less sooty mould.
Lace bugs (Stephanitis takeyai and Stephanitis rhododendri)
Lace bugs (Stephanitis takeyai and Stephanitis rhododendri) were first recorded in the UK in 1998. They are tiny sap sucking insects native to Japan. They were introduced to the USA and Europe via the horticultural trade. The south east of England was affected initially but incidences have been recorded from Leicestershire, Rutland, Nottinghamshire and parts of Wales . Rhododendron growers in the USA complain bitterly about the problems caused by these pests and find that some rhododendron subsections are more affected by lace bugs than others. They are named for the lacey appearance of their wings.
Vine weevils
Adults cause characteristic semicircular notching of the leaves of rhododendrons. In late summer and over the winter, larvae eat the bark, usually circling the stem just above the ground and causing the plant to die.
(A lot of very useful information is to be found on the Glendoick website under the heading ‘What is
wrong with my rhododendron – ‘Rhododendron problems, pests and diseases)
Other pests & diseases to be aware of :
Sirococcus tsugae. Host plants: Cedrus and Tsuga; cedars and hemlocks.
This was originally seen in the United States but first reported in the UK, including Scotland, in 2014 and 2015. It affects the genera Cedrus and Tsuga; cedars and hemlocks. Symptoms are severe shoot blight and defoliation. Dead needles have a characteristically pink colour and later go brown. There can be cankers and bleeding of resin from the bark. Spread is via rain splash and high winds. There is no effective control other than biosecurity and plant hygiene. More information
Marion Kinns, 2022
Stink bug, Halyomorpha halys. Host plants: Main threat to orchards
Of less concern to us is a new Stink Bug originating from China and Japan. This has been highlighted by the RHS. The brown marmorated stink bug, Halyomorpha halys spread to the USA in the 1990s and is now found in various European countries. So far only adults have been discovered in the UK and no breeding populations.
They differ from native stink bugs with pale bands on the antennae, a row of pale dots on the body and a rectangular head. They appear to overwinter in houses.
The main threat is to orchards where a small number can cause great damage. Gardeners are asked to watch for them. If you find them, capture them and report suspects to bmsb@niab.com or via the RHS gardening advice service rhs.org.uk/myadvice . More Information
Marion Kinns, 04/02/22
Phytophthera pluvialis. Host plants: Affects a variety of trees including western hemlock, tanoak, Douglas fir and various pines.
Presently known to affect a variety of trees including western hemlock, tanoak, Douglas fir and various pines. It was first reported from Oregon, USA in2013 on tanoak ( Lithocarpus densiflorus) and Douglas fir and has been shown to be the causative pathogen in ‘red needle cast’ in radiata pine in NZ. It also causes shoot die back and lesions on stems, branches and roots.
It wasn’t seen in the U.K. until September 2021 in a Cornish woodland where it was found to be affecting mature western hemlock and Douglas fir. Further outbreaks have been found in Cornwall, Devon, Cumbria and near Loch Carron in Scotland. Wales reported their first outbreak in Dec2021.
Many members in our society will grow conifers so this fairly recent pathogen must be of concern. We have also seen how other Phytophtheras ( ramorum and kernoviae) affect a wide range of hosts including the genus Rhododendron.
Growers are asked to be alert to this disease and report any concerns via The TreeAlert online portal. Phytophthera pluvialis currently meets the criteria to be classified as a GB Quarantine pest for regulatory purposes. More Information
Marion Kinns, 2022
Update: “Early investigations at various gardens have been encouraging in that there has been no sign of this particular fungal pathogen causing a problem in rhododendrons.“ Marion Kinns 2024
Phytophthera austrocedri. Host Plants: native juniper
This soil and water born fungus affects our native juniper which is already rare so there is some real cause for concern. It affects and damages roots leading to death. Discolouration of the foliage is seen which is either a uniform bronze following root damage or patchy from lesions on stems or branches. Tongue or flame shaped lesions up to 50cm can be seen under the bark and sometimes resin pockets. There is no natural resistance but plants on drier sites may survive. More Information
Marion Kinns 04/02/22
Plant Health Questions & Answers :
“Your President, Willie Campbell asked me, in my capacity as Plant Health watchdog for the SRS, to investigate problems he had observed in his locality of Doune and in Central Scotland generally. Willie’s observations were that ‘Horse Chestnuts were dying and cypress hedges, especially the golden ones were dying off’. The following was the result of my investigations.”
Horse Chestnut Bleeding Canker, Pseudomonas syringae, pathovar aesculi..
This is caused by a bacterium, Pseudomonas syringae, pathovar aesculi. From a tree survey conducted across the UK in 2007, it was discovered that half of all trees surveyed showed some symptoms. This disease was first seen in the UK in 1970 and is thought to have arrived from India. Trees of all ages can be affected. Some infections can last for years with little impact on the crown but in other cases the spread can be rapid causing crown thinning, die back and sometimes death of part or of the whole tree. Symptoms noted are cracks in the bark and oozing of a reddish brown sticky liquid. These dry in the winter to leave a rusty brown or black deposit. Under the bark the wood is found to be discoloured with brown or purple discolouration as opposed to the usual white/pinkish colour. In older plants dead bark may fall away to expose the wood beneath.
All plants now sold in the UK must be produced in the UK. If this bleeding canker is observed it should be reported to plant health authorities via Tree Alert at forestresearch.go.uk
(this information was found on the Woodland Trust website www.woodlandtust.org.uk)
Marion Kinns,
Horse Chestnut leaf blotch fungus, Phyllostica paviae.
This is caused by the fungus Phyllostica paviae. Irregular brown blotches of dead tissue are seen on the leaves. Sometimes these have a yellow edge and sometimes the leaf shrivels totally. This is more prevalent in the south and west of the U.K. and was introduced to this country from N. America in the twentieth century.
Horse chestnut leaves are also sometimes seen to gradually turn brown and shrivel all round the edges, looking like severe water stress. This is not leaf blotch and the cause of this condition is not yet known. (this information was found on the RHS website www.rhs.org.uk)
Marion Kinns,
Horse Chestnut leaf mining moth, Cameraria ohridella.
This was first found in the UK in 2002 and the moth concerned is Cameraria ohridella. White or brown batches are seen on the foliage from June to September. Caterpillars or circular pupal cocoons can be seen in the mined area of the leaf if it is held to the light. By September the tree may look as if it is dying but it will survive. Leaves may be dropped early and conkers may be smaller. (this information was found on the RHS website www.rhs.org.uk)
Marion Kinns,
“There are three potential problems which could affect Leyland Cypress trees – a blight, a canker and root rot. There is also a pest, an aphid.”
Cypress needle blight, Passalora sequoia.
This is caused by another fungus, Passalora sequoia. Symptoms are needles browning and dropping, usually beginning on the lower branches. Spores are spread by rain, winds and tools. To avoid this disease, trees for hedging should be placed far enough apart to allow sun and air to pass through the branches. Infected branches should be pruned out and the loppers used should be sterilised. Do not chip or compost infected branches.
Marion Kinns,
Canker in Cyprus trees, Seiridium unicorne or Botryosphaeria
This is another fungal disease and is caused by either Seiridium unicorne or by Botryosphaeria . In the former, the cankers rarely girdle the stem and in the latter they can girdle the stem. The cankers are dry lesions and are often sunken. Surrounding bark can show dark brown or purple discolouration and water flow can be restricted by these cankers. Branches can be seen to die back and there can be oozing of sap. It is found that canker tends to occur only on stressed plants. Fungicides are not effective. Recommendations are to plant trees a minimum of 12-15ft apart and avoid over fertilising. Infected branches should be pruned out, sterilising the pruners. A programme of regular irrigations should be followed and the trees should be mulched to the drip line.
Marion Kinns,
Root rot, Phytophthera
Species of Phytophthera fungus are responsible for this condition. Roots die off and foliage goes yellowish, purple or tan. The whole of the tree is affected. There is no chemical treatment. It is often caused by planting where the drainage is poor and young plants are most often affected.
(information for the above three diseases was found at www.gardeningknowhow.com and the RHS website www.rhs.org.uk
Marion Kinns,
Cypress aphid, Cinara cupressiora
Damage develops in late spring and summer and is most often found initially at the base of a hedge. The pest is a large greyish greenfly. Brown patches often develop after the aphids have gone. It is, however, possible to see cast aphid skins and sooty mould which grows on the honeydew excreted by the aphids. Damaged hedges can recover but this can be a slow process. The RHS suggests the use of biological controls to avoid killing beneficial insects. Information from RHS website www.rhs.org.uk
Marion Kinns,