Box: problems

Among the thousands of plants RHS Gardening Advice receives enquiries about every year, box sits up there in our top 20 – evidence of just how popular a plant it is with gardeners. However, it is troubled by a number of pests and diseases, not least the dreaded box blight which continues to cause problems for those growing box in the UK. Box tree caterpillar is now also causing serious problems in parts of southern England.

Box discolouration Credit: RHS/Tim Sandall

Quick facts

Common name Box problems
Scientific name Various
Plants affected Box, Buxus balearica, B. sempervirens, B. microphylla
Main symptoms Various
Most active All year

Image gallery

Here are some images of the diseases and pests common to box.

Diseases of box

Box blight (Cylindrocladium buxicola)

Watch for leaf spots, stem dieback and bare patches which are tell-tale signs of box blight.

See RHS research into box blight.

Volutella blight

Volutella blight is a disease of box caused by the fungus Pseudonectria buxi. Volutella blight has similar symptoms (twig and leaf death) to the more serious box blight, although the characteristic black streaks of box blight do not form and defoliation is not common. Spore masses, developing on undersides of leaves in wet conditions are pink for Volutella blight and white for box blight.

P. buxi requires wounds for infection and is associated with environmental stress or clipping in wet weather. It is not a serious disease and improving cultural conditions will usually lead to plant recovery. Diseased branches should be pruned out when the foliage is dry, and old fallen leaves removed from the interior of affected plants.

Box rust

Box rust is caused by a fungus called Puccinia buxi. This is not particularly troublesome and seldom causes serious problems.

It can be seen as thickened rusty blister-like pustules on both sides of the leaves. The fungus forms only one type of spore which develops in autumn and winter and breaks through the epidermis of the leaves in spring. New leaves are infected in the spring and early summer. The fungus continues to grow during the summer and autumn and as a result the infected spots become thicker. Box rust does not have an alternate host.

To control it either clip off the affected shoots or spray with one of the fungicides labelled for rust diseases.The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are approved for the control of rust diseases on ornamental plants.

The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Westland Resolva Rose 3 in 1, Doff Rose Shield Bug & Fungus Killer, Scotts Roseclear Ultra Gun 2, Vitax Rosegarde) and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun).When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Macrophoma leaf spot

Macrophoma leaf spot is caused by the fungus Macrophoma candollei. It is a weakly pathogenic fungus, resulting in numerous tiny black raised fruiting bodies found on dying or dead straw-coloured leaves. Like Volutella blight, it is associated with plants under stress and is easily managed by improving cultural conditions.

Phytophthora root rot

Both new and well established plants may fail completely for no obvious reason. This may be due to a soil-borne disease called Phytophthora root rot.

Pests of box

Box sucker

This insect causes the leaves to become cup-shaped and in spring there are splashes of a waxy white material on the foliage which can also be blown around in the wind. In most cases the damage can be tolerated.

Box tree caterpillar

This moth has become established in south east England, and its caterpillars can completely defoliate plants. For more information and to submit reports of this insect visit the box tree caterpillar profile

Box leaf-mining gall midge

The box leaf-mining gall midge, Monarthropalpus flavus, is uncommon in Britian but occasionally heavy infestations occur. This tiny fly deposits its eggs in the new leaves during late April - May. The larvae feed inside the foliage, causing a yellowish discolouration on the upper leaf surface. The lower leaf surface develops a slight swelling in the area affected by the larva's feeding. The yellow larvae are up to 3mm long and they feed inside the leaves during summer – winter before pupating within the mines in spring. Several mines can develop in a leaf and heavily damaged ones often drop off.

Whilst the damage caused can be unsightly it rarely affects the vigour of plants and light infestations can be tolerated. 

Mussel scale on box

Mussel scale, Lepidosaphes ulmi, is a sap-sucking insect that is found on a range of woody plants, including box, apple, hawthorn, ceanothus, cornus and cotoneaster. The blackish-brown shells or scales are shaped like mussels, up to 3mm in length, and are attached mainly to the bark but sometimes they spread to the foliage. Heavy infestations can result in plants dying back.

Box red spider mite

A fine whitish mottling on the foliage of box plants can be caused by the box red spider mite, Eurytetranychus buxi, which is a mite that is specific to box. This tiny creature feeds by sucking sap from the undersides of the leaves, particularly during spring and early summer. By late summer infestations die out and the mite overwinters as eggs, which are laid on the stems and underside of leaves.

Box tree red spider mite is difficult to control. Fortunately, although the mottling may be considered unsightly, this mite does not cause serious damage to the plants and so it can be tolerated.  If treatment is considered necessary mite numbers can be reduced by spraying the undersides of the foliage thoroughly with sprays based on fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, Bug Clear for Fruit and Veg). Applications on three or four occasions at about five-day intervals when signs of renewed feeding damage are seen on new foliage may be necessary to control newly hatched mites and break the life cycle.

Do not spray on or near plants in flower due to the danger to pollinating insects.

Inclusion of a pesticide product does not indicate a recommendation or endorsement by RHS Gardening Advice. It is a list of products currently available to the home gardener.

Environmental stresses

When the leaves take on distinctive yellow tips or an orange or bronze colouration, it suggests the plants are under environmental stress.

Many shrubs can suffer brown leaves. Trying to diagnose the problem as soon as possible may help save the plant.

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