Box blight

Box blight is a fungal disease of box resulting in bare patches and die-back of box, especially in topiary and parterres.

Box blight

Quick facts

Common name Box blight
Scientific name Cylindrocladium buxicola (syn. Calonectria pseudonaviculata)
Plants affected Buxus spp. (box)
Main symptoms Brown leaves and bare patches
Caused by Fungus
Timing All year round, most active in rainy seasons

What is box blight?

Box blight is a disease of box leaves and stems caused by the fungus Cylindrocladium buxicola (syn. Calonectria pseudonaviculata). Box blight doesn't kill the roots of box plants. Cylindrocladium buxicola is divided into two genetic types which differ in their sensitivity to some fungicides (triazoles). It largely affects 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 leaves, but is less serious.

Symptoms

Correct diagnosis is important because of the effort involved in managing box blight. Try to find 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

    Cylindrocladium buxicola infects young stems causing black streaks and dieback. Credit RHSIn wet conditions the white spore masses of C. buxicola may be seen on the undersurfaces of infected leaves. Credit RHSBox blight causes leaves to go brown and fall, leading to bare patches. Credit RHS/Kay DavisIt is not uncommon for box blight to affect whole side or tops of hedges, particularly in damp, shaded areas. Credit RHS/pathology

    Control

    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 three 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 cuttings from healthy box in your garden will reduce the risk of introducing the disease.

    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.

    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. The following all have small leaves and can be clipped into formal hedging styles:

    Berberis buxifolia 'Pygmaea'
    Berberis × stenophylla 'Corallina Compacta' , 'Irwinii', 'Nana'
    Berberis thunbergii ‘Atropurpurea Nana’ AGM
    Euonymus fortunei (various cultivars)
    Hebe cupressoides 'Boughton Dome'
    Ilex crenata
    Lavandula angustifolia cultivars
    Lonicera nitida
    Osmanthus delavayi AGM
    Rosmarinus officinalis
    Santolina chamaecyparissus AGM
    Taxus baccata 'Semperaurea' AGM 

    Chemical control

    Fungicides are unlikely to be effective against box blight unless combined with the other strategies to control.
    If you have a problem with box blight, the fungicides Bayer Fungus Fighter (tebuconazole) and Bayer Fungus Fighter Plus (tebuconazole and trifloxystrobin) carry label recommendations for use against this disease and can be applied up to six times per year. Triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) is labelled for 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. Tebuconazole is effective against both genetic types of Cylindrocladium.

    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.

    Download

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

    Links

    Chemicals: using a sprayer
    Chemicals: using safely and effectively
    Chemicals: storing and disposing safely

    Biology

    Cylindrocladium buxicola survives as resting structures or mycelium on fallen leaves for up to 6 years and produces 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, C. buxicola has been spread widely by human activity, especially on infected plants from nurseries. Cylindrocladium buxicola can infect unwounded plants and causes serious damage. C. buxicola 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.

    Further reading

    RHS research into box blight

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