WHAT IS IT?
Box blight is a fungal disease of box resulting in bare patches and die-back, especially in topiary and parterres.
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 can suffer from. A second blight, called Volutella blight, also affects leaves, but is less serious.
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.
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 under surfaces 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
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.
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 AN ALTERNATIVE 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:
Berberis darwinii 'Compacta' Berberis thunbergii ‘Atropurpurea Nana' AGM Elaeagnus × submacrophylla 'Compacta' Euonymus fortunei (various cultivars) Ilex crenata Lonicera nitida 'Maigrün' L. nitida 'Baggesen's Gold' AGM Luma apiculata 'Glanleam Gold' Osmanthus delavayi AGM Pittosporum 'Arundel Green' AGM P. 'Collaig Silver' P. 'Oliver Twist' P. tenuifolium 'Golf Ball' (PBR) Podocarpus 'Chocolate Box' P. 'Young Rusty' Rhododendron Bloombux ('Microhirs3'PBR) Taxus baccata 'Repandens' AGM Ugni molinae,U. molinae 'Butterball'
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 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.