Box blight is a disease of box leaves and stems caused by two closely related fungi, Calonectria pseudonaviculata and Calonectria henricotiae (syn. Cylindrocladium buxicola). The two species of Calonectria differ in their sensitivity to some fungicides (triazoles). They largely affect Buxus spp. (box) in the UK, but other plants in the Buxaceae family are also susceptible.

Box blight is just one of a number of problems box suffers from. A second blight, called Volutella blight, also affects the plant, but is less serious. Box in the UK is also increasingly affected by box caterpillar.

Correct diagnosis is important because of the effort involved in managing box blight. Try to catch it early, because the further advanced the epidemic, the harder box blight is to manage. You may see the following symptoms:

• Leaves turn brown and fall, leading to bare patches
• Black streaks and dieback on young stems 
• In wet conditions, the white spore masses of the fungus may be seen on the undersurfaces of infected leaves (place leaves in a plastic bag with moist tissue for a few days to check). Note that spore masses of Volutella blight will look pink
• While box caterpillar also causes defoliation, this is distinguished by cobweb-like webbing, the presence of green and black caterpillars, and curved cut-outs in leaves from chewing

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods,

Gardeners often use the word variety when referring to a specific plant, but the correct botanical term is 'cultivar'. Whichever word you use, it means a distinctive plant or plants, given a specific cultivar name and usually bred to enhance certain characteristics, such as flower or fruit size, colour, flavour or fragrance, plant size, hardiness, disease resistance, etc. Additionally, it is worth knowing that, botanically, variety has another meaning - it refers to a naturally-occurring distinct plant that only has slight differences in its looks. For example, Malva alcea var. fastigiata differs from typical plants by having an upright habit.

Regular cleaning and tidying not only keeps a garden looking neat, but can reduce pests and diseases too. Beneficial jobs include collecting fallen leaves, promptly removing diseased growth and debris, cleaning tools, and weeding. Still, too much tidying can discourage wildlife, so it’s a fine balance.

garden hygiene

Non-chemical control

If your garden is free of box blight, or you are yet to plant box, there are steps that you can take to minimise the risk of it arriving and to prepare the garden for its possible arrival. For more detailed information, see our page box blight: keeping it out.

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. Taking

A method of growing new plants from parts of an existing plant, such as sections of root, stem, leaf or bud. When prepared correctly and planted in the right conditions, they can produce roots and eventually become independent plants. There is a wide range of different methods for taking cuttings, depending on the plant and time of year.

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

The introduction of outdoor air into a space, for example, a greenhouse. This airflow can reduce temperature, replenish the gases absorbed by plants and keep down fungal diseases like grey moulds.

Avoid overhead watering as box blight thrives in humid conditions. Use

Mulch is a layer of material, at least 5cm (2in) thick, applied to the soil surface in late autumn to late winter (Nov-Feb). It is used to provide frost protection, improve plant growth by adding nutrients or increasing organic matter content, reducing water loss from the soil, for decorative purposes and suppressing weeds. Examples include well-rotted garden compost and manure, chipped bark, gravel, grit and slate chippings.

Managing an outbreak

If the disease does break out steps should be taken to reduce its spread. Further steps can be found on our page box blight: managing outbreaks.

When conditions are dry;

• Dig up and destroy badly affected box plants or less important plantings, especially if trying to safeguard prized plantings
• Cut back or cut out affected parts of less severely attacked box plants or valuable specimens you are trying to save
• Remove fallen leaves from the centre and around the base of affected plants and strip off the surface of the topsoil. Infected material should be bagged to avoid dropping debris around the garden and binned (not composted)

Feed plants that have been cut back with a general purpose fertiliser to aid recovery.

Choose alternatives to box

To be completely safe, choose an alternative hedge or topiary plant. Read our guide to choosing box alternatives, and then browse our box alternatives selection page to find a plant that works for you.

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners 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. Homemade products are not recommended as they are unregulated and usually untested.

Preventative fungicide applications can be made 7 to 10 days before trimming and 7 to 10 days after trimming where there is a high risk of box blight. However, fungicides are unlikely to be effective against box blight unless combined with the other control strategies outlined above.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

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.

C. pseudonaviculata was first recognised in the UK in the mid 1990s and has also broken out in continental Europe, New Zealand and more recently in USA and Western Asia (Iran), but its origin is unknown. C. henricotiae was identified more recently and is currently only recorded in the UK and parts of mainland Europe.

Further reading

RHS research into box blight