Box Blight and Box Caterpillar

Buxus species and vulnerability

Box, whose botanical name is Buxus sempervirens is most commonly recognised as one of the most pervasive small evergreen bushes shaped into geometric patterns and shapes in ornamental gardens.  Over the years it has been pestered by a fungal disease known as Box Blight, that has caused damage to its foliage and naturalistically vandalised the displays that make use of the plant.  Gardeners have struggled to fight and manage this infection but Box continued to be used widely in gardens.  In more recent years an additional pest has become prevalent that consumes and decimates the foliage of the plant.  This is Box Caterpillar and its management is much harder.  Whilst it is possible to control, it requires committed and thorough management techniques that conflict with modern day garden ethics of sustainability and biodiversity.  Consequently, the zeitgeist surrounding what to do with Box has pretty much become replace it.  I have produced an article detailing candidates for alternatives to Box that can be found here.

In some rare cases, it might be desirable to attempt to keep an existing Box planting healthy, and so this article shares some information on the pests and how to combat them.

Content from this article is mostly adapted from articles published by the RHS, and versions of those articles from 2023.  The links here are the most recent versions of those pages and guidance from the RHS may have changed in the intervening years.  Most likely is that the RHS will reduce their guidance that includes pesticdes and fungicides which they increasingly discourage.

https://www.rhs.org.uk/disease/box-blight

https://www.rhs.org.uk/biodiversity/box-tree-caterpillar

Box Blight

Box blight is a fungal disease of the Buxus genus that causes bare patches, brown leaves and dieback, it is particularly active during wet periods.  There are two species active in the UK and their botanical names are Calonectria pseudonaviculata and Calonectria henricotiae. 

Symptoms of box blight include leaves turn brown and fall, leading to bare patches.  Black streaks and dieback occurs on young stems.  In wet conditions, the white/pink spore masses of the fungus may be seen on the undersurfaces of infected leaves.  

Lifecycle

Calonectria pseudonaviculata and C. henricotiae can survive as resting structures or mycelium on fallen leaves for up to 6 years and produce spores (inoculum) when conditions are suitable. The spores are dispersed in water and probably by animals and birds. They may be spread in wind-blown rain, but are unlikely to travel long distances on the wind.

As well as natural means of dispersal, they have been spread widely by human activity, especially on infected plants from nurseries. Both species can infect unwounded plants and cause serious damage.

Cultural control and hygiene

Hold any commercially sourced plants in isolation for at least four weeks to confirm they are free of infection before planting out. Commercial nurseries may use fungicides which suppress but do not kill the fungus, and this isolation technique will allow time for suppressed disease to become visible. 

Inspect plants for early symptoms as box blight spreads very rapidly in warm and humid conditions and is difficult to manage.

Reduce the frequency of clipping to create more ventilation throughout the plants (regular pruning creates dense foliage and less air movement). Prune a hedge with a convex top rather than flat and prune only in dry conditions. Clean pruning tools with a garden disinfectant or mild bleach solution between different areas of the garden and between gardens to minimise unwitting spread of the disease.

Avoid overhead watering as box blight thrives in humid conditions. Use mulch under plants to reduce rain splash. Feed plants moderately.

Treatment

Box blight doesn't kill the roots of box plants so in theory they can recover if cut back. Fortunately, box responds well to clipping (which is why it is such a good hedge and topiary plant) and will also respond to box blight by producing new shoots. The risk is that the new ones will also become infected.

Management focuses on removing sources of infection, protecting uninfected and new growth and managing for healthy recovery. Doing nothing seldom works in the long term.

Step 1: Review garden hygiene

These measures are targeted to manage a box blight outbreak, but are worth considering for good general garden hygiene;

• Bag up and dispose of infected material to avoid dropping debris around the garden. Do not compost

• Clean pruning tools (for instance, with a garden disinfectant such as Jeyes Fluid or a dilute bleach solution) and clean clothing after working in infected areas

• Check soles of shoes for leaves and soil; brush any off and clean them as for tools to remove adhering spores

• Applying a fungicide (see section below) prior to cutting back infected plants may reduce the risk of spore development and spread during and immediately after cutting

• Remember to quarantine new plants if replacing with more box in the garden

Step 2: Control the infection

The box blight fungi (Calonectria pseudonaviculata and C. henricotiae) survive and reproduce in infected leaves and stems, including fallen leaves and dead stems. Fungal spores within these infected tissues act as pockets of the disease (i.e. the inoculum) that can invade previously uninfected leaves and stems.

Infection has probably progressed further than it shows, so it pays to cut out more than you think you need to. It is important to cut out infection when the disease strikes, even if it is not the normal time of year to trim box. However, if the plants are wet such as after rain, wait until conditions are dry before beginning cutting back, otherwise you risk spreading the disease further.

Cutting back or cutting out

Removing some or all of the affected areas will reduce the amount of inoculum and improve the chances of healthy recovery. Prioritise your plantings; removing infected, less important, plantings may safeguard prized plantings.

How can I tell what is diseased? Typically you are looking for patches on your box plants where the leaves have gone brown or have fallen, leaving bare stems. Infected stems will have distinctive black streaks and dieback (i.e. are no longer green under the bark).

Choose from one of  four options, depending on the severity of the infection and your priority;

Option 1: remove minor or isolated infections

(Good where infections have been detected early, for topiary specimens, or where fungicides will be applied.)

Cut out a little more than you think is necessary as some infections are likely to be latent (not yet visible)

Do not touch healthy box plants immediately after working on an infected area

Consider applying a fungicide (see below) before cutting and again after two weeks

Option 2: reduce height by about a half

(Good for hedges and parterres. Also where infection is spread over large areas. This approach creates a more open structure in the plants that discourages box blight.)

Try to cut stems so no black streaks remain. This may require reducing width as well as height

Consider applying a fungicide (see below) before cutting and again after two weeks. Reduction can be less severe if you intend to use fungicides

Clean pruning tools regularly during process

Option 3: cut back to stumps

(Good where infection is severe but is in a position where you want to try to save the box feature (e.g. part of a parterre) or where you do not wish to apply fungicides. This more drastic approach aims is to remove all infected tissues to maximise the chances of removing all inoculum.)

Cut back all plants showing symptoms to ground level, leaving just the stumps

Plants can be cut back at any time, but beware of the risks of transporting the pathogen around the garden in wet conditions

Option 4: remove affected plants

(Good where infection is very severe, the box is not of high value, or it threatens other box plantings.)

Remove plants at any time, but beware of the risks of transporting the pathogen around the garden in wet conditions

Avoid planting box in the same place for several years (spores can remain viable on fallen leaf litter for at least 6 years)

Remove fallen leaves and infested soil

• Lay down a tarpaulin or sheeting before clipping, to make clearing up clippings and debris easier.

• Remove dislodged leaves and other debris from the canopy after cutting back

• Clear away as many fallen leaves from under and around the plants as possible (even a few remaining leaves can lead to new infections)

• Remove a layer of soil under the plants to get rid of infected leaf fragments. Replace with fresh soil (do not overfill)

• Apply a mulch

Disposing of diseased material

Home composting

Although compost bins rarely reach temperatures sufficient for killing pathogens, they are suitable for disposing of material infected by some less-persistent diseases. Common foliar fungal diseases such as powdery mildews and rusts can not usually survive for long once the plant material has fully degraded. This is especially true if the infection has been caught early, before the pathogen has had time to produce resilient spores or structures

Green waste

Council composting facilities should reach temperatures capable of killing most diseases. Although transport and collection makes this option less sustainable than home-composting, many gardeners may be more comfortable sending infected material out of the garden. Some councils vary in what they are prepared to accept – thick branches and large volumes of rotten fruit may not be, for example. If in doubt, check with your local authority

Burial

This is a simple, sustainable way of handling some diseased material on site; for example, leaves infected with foliar diseases such as black spot and fruit suffering from rots such as brown rot. Find a site in bare ground between shrubs or in a vegetable garden and bury material at least 30cm (1ft) deep so that it will not be unearthed by cultivation or mammals. Do not bury material infected by soil-borne diseases such as club root and sclerotinia or any woody material that will be slow to break down

Bonfires

Dry, woody material can be burnt in gardens that are large enough to have a safe bonfire or incinerator. This is ideal for branches affected by canker and fireblight and for woody roots infected with honey fungus. Traditionally, annual autumn bonfires would have been the way of disposing of uncompostable material, but do bear in mind that leaving diseased material stacked until bonfire night can be unhygienic. Bonfires create pollution and may also annoy neighbours. They should be conducted when wind and weather mean that smoke will not enter houses or inhibit others from enjoying their garden. Here are some government guidelines on bonfires in smoke control areas.

Household waste

Plant material such as rotten bulbs and tubers, collapsed seedlings in contaminated compost and small woody prunings can be thrown away in the household waste. From here they will be sent to landfill sites to be buried deeply in a lined and sealed environment. Consider this only when other methods are not appropriate, as landfill space is in short supply and biodegradable material in landfill releases methane (a greenhouse gas contributing to climate change)

Council refuse site

Where burning is not possible, woody material and rootballs may need to be taken to the tip/recycling centre. It is a requirement to deposit these with general waste as they are usually too large for shredding for compost.

Using fungicides

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Bayer Fungus Fighter Plus and Bayer Fungus Fighter Concentrate have label recommendations for box blight and can be applied up to six times per year. If treatment is needed in spring, consider saving some applications in case it recurs in the autumn. Triticonazole (Fungus Clear Ultra) is labelled for the control of other diseases on ornamentals and could therefore be used legally on box (at the owner's risk) to try and control box blight. Follow label instructions for use (avoid spraying when rain is expected, but also in bright sunshine or when plants are dry at the roots).

Box Tree Caterpillar

Box tree caterpillars are the larvae of a moth that feeds on box (Buxus) plants. It is native to East Asia and became established in Europe in 2007. Although the first adult moth arrived in Britain in 2007, followed by several moths to light traps in 2008, caterpillars were not found in private gardens until 2011 in the home counties. By the end of 2014 the moth had become established in parts of London and surrounding areas. For growers in the south-east of England, it is now a problem that is likely to reoccur repeatedly throughout the growing season and in successive years.

Symptoms

Gardeners are likely to become aware of box tree caterpillar when they notice defoliation or find webbing and caterpillars on box plants.

• The caterpillars eat box leaves and produce webbing over their feeding area. Plants may also show patches of dieback, which can be especially apparent on trimmed plants. 

• Newly hatched caterpillars are greenish-yellow, with black heads. Older caterpillars reach up to 4cm (1¼in) in length and have a greenish/yellow body with thick black and thin white stripes along the length of the body

• Caterpillars are also capable of stripping bark, which can result in girdling of affected sections

• The pupae are concealed in a cocoon of white webbing spun among leaves and twigs

• The adult moth usually has white wings with a faintly iridescent brown border, although the wings can be completely brown or clear. The moth has a wingspan of around 4cm (1¼in)

• The pale yellow, flattish eggs are laid sheet-like, overlapping each other, on box leaves. These are very difficult to find.

Lifecylcle

There are at least two generations of the box tree moth a year, with the majority of feeding damage caused between March and October.

Box tree moth overwinters as small caterpillars, hidden between box leaves that have been webbed together in late autumn. These caterpillars begin to become active as the weather warms in the spring – they have been observed feeding as early as February on particularly warm days. By June, caterpillars will have finished feeding, pupated and emerged as adult moths.

There is then a second emergence of adults in late July/early August as the next generation of box tree moths emerge, looking for mates. During summer overlapping of generations has been observed, with multiple life stages of the moth present at the same time. However, by late September/October the majority of the population are early instar caterpillars.

The adult moths are good flyers and are attracted to light.

Management

Check box plants frequently from early spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

Non-pesticide

• Where practical, caterpillars should be removed by hand

• Pheromone traps, can indicate whether there are moths in the area and their level of activity. 

• The mixed nematode biological control sold as Fruit and Vegetable Protection has some effect on the caterpillars.

• The caterpillars have a range of natural enemies, including parasitoid flies, parasitoid wasps, social wasps, spiders and ants.

• There have been reports of garden birds including blackbirds, starlings, magpies and tits feeding on the caterpillars in some locations. It is not yet clear if this predation will result in a reduction of box tree moth numbers

• Consider choosing alternatives to box plants.

Pesticides

The RHS recommends that you don't use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects.

Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control and can lead to resurgence of the target animal.

The shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife.

The pesticides listed are legally available in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally.

Always follow the instructions on the products. 

Homemade products are not recommended as they are unregulated and usually untested.

Products such as Xentari are not registered for use in the UK and the RHS do not advise on their use.

Plants in flower should not be sprayed due to the danger to pollinating insects.

Extensive populations can be treated with an insecticide. Thorough spray coverage is required if control is to be achieved

Forceful spraying is needed to penetrate into the interior of box plants through the webbed-together leaves

Organic contact insecticides containing natural pyrethrins include Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer. Several applications of these short persistence products may be necessary to give good control.