Box blight is a fungal disease of box resulting in bare patches and die-back, especially in topiary and parterres.
What is box blight?
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 leaves, but is less serious.
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
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.
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
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. Ongoing trials of a variety of compact, evergreen shrubs at RHS Garden, Wisley are showing promise as alternatives. The following all have small leaves and can be clipped into formal hedging styles
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 tebuconazole (Provanto Fungus Fighter Concentrate) and tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect) carry label recommendations for use against this disease and can be applied up to six times per year. Triticonazole (Fungus Clear Ultra) 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 species of Calonectria.
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.
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