Pests, Diseases and Other Problems with Conifers

by Howard Drury

Brown patches within conifers are becoming an increasingly common and can be caused by a number of reasons. Pests or diseases are responsible for some cases, but others may be caused by growing conditions, weather conditions or routine operations such as trimming. Symptoms  Brown patches can develop anywhere within the hedge, varying in size from 1-2cm (½-1in) to 3-4m (10-14ft). Patches develop most commonly in the summer, when the trees are in active growth and prone to stresses such as water shortage. To give a correct diagnosis it is important to observe if the browning is confined to whole branches as in the case of soil borne diseases such as honey fungus or spread across several stems and branches which normally indicates an environmental issue, be it pest, disease, climatic or cultural issues.

Brown patches can appear on any conifers, but most problems are with × Cuprocyparis (formerly Cupressocyparis) leylandii (leyland cypress), Chamaecyparis lawsoniana (lawson cypress) and Thuja plicata (western red cedar).

Possible Causes

Pests

Cypress aphid

Cypress aphid is one of the causes of dieback on conifer hedges. It has become an increasingly damaging pest on conifer hedges since the 1980s, particularly on Leyland cypress hedges.
Cypress aphid is a blackish brown sap-feeding insect that infests the stems of some hedging conifers in early summer. This causes a gradual drying up of the foliage.

Cypress aphid (Cinara cupressivora) is a relatively common cause of brown patches. A recent collaborative research project between the RHS and East Malling Research found cypress aphid to be associated with half of the cases of brown patches investigated.

Damage caused by cypress aphid develops in late spring and summer. It is found most often at the base of the hedge, but can develop at any height. Large greyish greenfly are sometimes found, but the browning often develops long after the aphids have left the foliage. Clues are left behind, including cast aphid skins and a black fungal growth (sooty mould) that grows on the sugary honeydew excreted by the pest.

Cypress aphid is a blackish brown sap-feeding insect that infests the stems of some hedging conifers in early summer. This causes a gradual drying up of the foliage.

Symptoms

Yellowing shoots in summer; by late summer, many of these will be brown and dead
On clipped hedges the dieback can be quite pronounced, with the lower parts more severely affected than the top
A black powdery coating of sooty mould often develops on the stems and foliage  Damaged hedges can recover, but it is likely to be a slow process

Biology

Damage is caused by greyish brown aphids which are active from May to November, reaching peak numbers in early to mid-summer.
• They suck sap from the stems and even quite light infestations can have a significant effect
• Although the aphids are fairly large, 2-4mm long (about 1/8in), they are difficult to see
• A useful confirmation of aphid damage is the presence of a black sooty mould, as there are other causes of dieback on conifers

Control

Chemical control

Spray in early summer to prevent damage occurring.

Suitable products contain thiacloprid (Bayer Provado Ultimate Bug Killer Concentrate), acetamiprid (Scotts Bug Clear Ultra) or thiamethoxam (Westland Plant Rescue Bug Killer Ornamental Plants

It is often difficult to spray large dense hedges thoroughly, and in years when the aphid is abundant, some damage will occur

Green Spruce Aphid

Unlike most aphids, the green spruce aphid is active and causes damage to spruce trees (Picea species) over the winter months.

Green spruce aphid is a sap-sucking insect that infests spruce trees (Picea species).

Symptoms

Green spruce aphid is up to 2mm long and is dull green with dark red eyes.
It is most likely to be seen on spruce trees during late autumn to spring.

Old foliage develops a pale mottled discolouration during the winter, and many of these needles will fall off in the spring. A black sooty mould may be noticeable on the stem joints. 

New growth produced in spring is unaffected, and its bright green appearance contrasts strongly with the discoloured and sparsely foliated older stems.

Control

Non-chemical control

Natural enemies such asladybirds, lacewings, hoverfly larvae and parasitic wasps may help to limit infestations during the summer.

Damaged trees can be helped to recover by keeping them watered during dry spells and feeding with a general fertiliser in spring. It will take several years for a badly damaged tree to regain an attractive appearance.

Chemical control

Small trees can be sprayed with thiacloprid (Bayer Provado Ultimate Bug Killer Concentrate), acetamiprid (Scotts Bug Clear Ultra Concentrate) or deltamethrin

Scale insects

Scale insects are found quite commonly in affected areas. These, like the cypress aphid, feed by sucking the sap from the foliage. Heavy infestations are thought to cause some dieback, but scale insect is very common and is often incidental to the problem. If there are similar numbers of scale insects on parts of the hedge unaffected by the dieback then this pest is unlikely to be the primary cause of the problem.

Diseases

Pestalotiopsis dieback

What is Pestalotiopsis?  Species of the fungus Pestalotiopsis are often found associated with leaf spots and diebacks, not just of conifers but of a very wide range of woody plants. They are usually regarded as being weakly pathogenic, that is they require a plant to be weakened or damaged in some way in order to colonise it. However, once within the plant tissues they can sometimes cause quite extensive damage. Pestalotiopsis is often found colonising the shoots of conifer hedges that have suffered initial damage from aphid infestation. Wet weather favours dispersal of the spores and their ability to infect the plant.

Symptoms

You may see the following symptoms:

Leaves turn yellow and then brown, often progressing back from the tip of a shoot
A constricted, girdling area of browning sometimes develops on a twig, with the entire twig dying above this point
Numerous black, ‘pinhead’-sized fruiting bodies, just visible to the naked eye, develop within the affected plant tissues
Under wet conditions, thin black tendrils of spores ooze from the fruiting bodies

N.B. The development of brown patches and shoot dieback in conifer hedges can have a number of different causes (e.g. aphid damage, Pestalotiopsis, pruning at the wrong time of year, or various interactions between these factors). Laboratory examination of samples of affected shoots may be required to determine the precise cause(s).

Control

Non-chemical control

Try to avoid or prevent any of the factors that can lead to plant damage or stress. For example:

Prevent or control pests such as cypress aphid
Ensure that newly-planted conifers establish well
Water plants during periods of extended drought
Avoid unnecessary physical damage
Prune hedges at the correct time of year hedge pruning times
Avoid pruning during prolonged periods of wet weather (which are suitable for spore germination and infection), or during drought conditions (which can itself result in dieback)

If dieback due to Pestalotiopsis is diagnosed, pruning out the affected shoots will reduce the number of spores available to set up new infections. However, remember that many conifers will not produce new growth if they are cut back hard into old growth.

Chemical control

There are no fungicides available to amateur gardeners with specific recommendations for use against Pestalotiopsis. However, the fungicides difenoconazole (Plant Rescue Fungus Control), myclobutanil (e.g. Systhane Fungus Fighter, Fungus Clear 2 GUN!) and triticonazole (Fungus Clear Ultra) are labelled for the control of some other diseases on ornamental plants, and could therefore be used legally on conifers (at the owner’s risk) to try and control Pestalotiopsis.

These are the words of the Royal Horticultural Society and not mine, but I do agree that pesticides and fungicides recommended to use on ornamentals may be used on other ornamentals not listed provided there is no risk whatsoever of humans eating any parts of sprayed plants.
There is no specific information available as to the efficacy of these products against the fungus, however. It is possible that repeated sprays would be required where the disease is present, particularly during unsettled weather.

Thuja Blight

Thuja blight is a fungal disease caused by Didymascella thujina (syn. Keithia thujina) that attacks the leaves and shoots of Thuja species, particularly Western red cedar (Thuja plicata). Young trees can be badly damaged, and the disease can also be unsightly on larger specimens. Lower branches tend to be worst affected, particularly in dense plantings with poor air circulation.

Symptoms

You may see the following symptoms:

Individual scale leaves turn yellowish and then brown in late spring and early summer
Large, oval, brown-black fungal fruiting bodies, visible to the naked eye, develop in the leaf. There is usually one fruiting body per scale leaf, but sometimes two or three
The fruiting bodies fall out once they have released their spores, leaving a cavity in the leaf (which could be mistaken for pest damage)
Dead leaves can persist on the tree throughout the winter
Heavy infections can lead to widespread browning of leaves, and sometimes twig dieback. Seedlings and young plants (up to four or five years old) can be killed

Control

Non-chemical control

Avoid planting Thuja plicata (particularly seedlings and young plants) in areas where air circulation is poor

Remove and dispose of any twigs that are shed as a result of infection by Thuja blight
Cut out heavily-infected shoots, if this can be done without spoiling the overall shape of the tree

Chemical control

There are no fungicides available to amateur gardeners with specific recommendations for use against Thuja blight. However, the fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are labelled for the control of some other diseases on ornamental plants, and could therefore be used legally on Thuja (at the owner’s risk) to try and control Thuja blight. The formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests.
There is no specific information available as to the efficacy of these products against Thuja blight. It would be impractical to treat large, mature specimens, but treatment could be attempted on young trees or trees grown as hedging. Research carried out on commercial plantations of Thuja (using a product unavailable to gardeners) has shown that two treatments, in late March and late April, can be effective, with possibly a third spray in late June in wet seasons.

Biology

Infection is favoured by leaf wetness so on larger trees the lower branches, where air circulation is poorer, tend to be the worst affected. On smaller, younger trees the entire plant can be affected, particularly in dense plantings.
The black fruiting bodies develop just below the leaf surface, and when they are mature the epidermis of the leaf splits open and spores are released. The spores are produced from May to October. They are resistant to desiccation, and spores shed in the autumn can overwinter on the leaf surface to germinate the following spring.

Coryneum canker

This disease is caused by the fungus Seiridium cardinale, and can cause branch die-back of leyland cypress and western red cedar. It is found most commonly on large trees, and is relatively rare in smaller hedges.

Honey fungus or Phytophthora root rot

If the browning affects most or all of the foliage of an individual tree in the hedge, or a tree dies completely, a root disease such as honey fungus or Phytophthora root rot could be responsible. Revised fact sheets on both honey fungus and Phytophthora will be available shortly on my website.

Hedge trimming

All conifers have little or no capacity to regrow from old wood. Over-enthusiastic hedge trimming can result in bare patches. The time of year when trimming is also important, even if it is just light trimming. A recent research project found that die-back appeared to be slightly more common after autumn trimming (mainly October). Trimming in the summer during times of plant stress, such as prolonged drought or hot, dry spells may also be a factor. Some years I have been advocating the trimming of conifers in April and early May and although theoretically late summer pruning can be carried out from my experience dieback can occur especially during severely inclement weather.

For many years much has been written about lightly trimming conifers during the summer and I have been against this front number of reasons. Originally I was concerned that exposing otherwise shaded foliage could lead to stress and dieback. More recently I have come to believe that some of trimming allows a stronger scent to be given from the cut surfaces which can attract pests such as the conifer aphid which feed on lesser and in turn caused dieback. I am not convinced that it is simply feeding that causes this dieback and suspect aphids may be carrying the disease but this will require further research before any conclusions can be reached.

Growing conditions

The RHS Advisory Service believes that many brown patches are likely to result from adverse growing conditions such as drought, frost, waterlogging or cold, drying winds, all of which could inhibit regeneration from the trimmed foliage. I also believe that conifers in the main conference cool damp regions and our recent periods of hot and dry weather stress plants, which in turn leads to other problems such as leaving them prone to insect attack. Observation over the years also shows that conifers by the action of their roots alter the soil structure leaving it devoid of most nutrients, extremely acid in nature and dry in the vicinity making it difficult to establish other plants in close proximity to larger specimens.

If the browning affects most of the foliage of individual trees, check for root diseases, waterlogging, dryness of the root and even has one recent case in South Staffordshire demonstrated a gas leak from the mains supply which turned the roots of the conifer blue.

Prevention and control

Prevention

Avoid trimming during hot or dry weather or in the autumn, and never cut into old wood.
Trimming two or three times, in April, June and early August, is usually relatively safe. Removal of competing vegetation within 30 cm (1ft) each side of the hedge, feeding the trees with a general fertilizer in late winter and mulching the base of the hedge with a 8 cm (3in) layer of well-rotted organic matter should also help. I am very wary of trimming in August if weather conditions are hot and dry, and I now recommend only trimming when cool, damp conditions prevail during midsummer.

Non-chemical control

The Royal Horticultural Society suggest that bare patches may (depending on the cause) gradually fill in, but in serious cases this can take several years. It may be possible to tie in nearby healthy shoots to fill bare areas if these are not too extensive. I would argue that in most cases the majority of species with the exception of some Thujas and Taxus do not recover from areas that have previously died back and while it may be possible to carefully retrain any existing green foliage that is still alive I find the resulting visual appearance is less than satisfactory and in many cases specimens and hedges are best removed which gives an opportunity to re-plan the area without using conifers.

© Howard Drury and the Royal Horticultural Society, 14/02/2022