Q What is box blight?
A Box blight is a fungal disease affecting the leaves and stems of box plants.
Caption: Sparse patches with brown leaves are a symptom of box blight
Q How do I recognise it?
A The most obvious symptom is browning leaves which then die and fall, leaving sparse patches. Twigs and branches may be killed though whole plants rarely die.
Q What causes it?
A Two fungi are particularly associated with box blight, and others may also be involved. The most damaging is Cylindrocladium buxicola, which initially produces dark-brown or black spots on the leaves and black streaks on the stems, followed by grey patches under the leaves. Less aggressive Volutella buxi produces pink spores on the undersides of affected leaves in damp conditions. Both fungi invade the tissues of the plant, interfering with water transport, and killing the cells.
Q Why has it become such a problem?
A The more aggressive fungus has only relatively recently appeared in Britain. It was first recorded in 1994 in a Hampshire nursery but has now spread through most of the country. Its origin is uncertain, though it may have come in from Central America where it is widespread.
Q When do attacks occur?
A The fungi that cause box blight are frost tolerant and need damp conditions to spread, so tend to be most virulent in autumn and winter. Drier areas of the country may suffer less than those with a wetter climate.
Q How does it spread?
A The infection is probably spread from plant to plant by splashing water, either from rain or watering. The spores are sticky, so could also be carried by birds and animals or by gardeners on tools or clothing. Longer distances can be covered by moving infected plants or soil containing infected remains such as fallen leaves.
Q What plants does it attack?
A So far the disease has only been identified on box (buxus) species, though it could also affect closely related plants such as sarcococca. Many box species are known to be susceptible, including our native box B. sempervirens. In the garden the problem is likely to be worst on tightly clipped box because this will hold onto moisture, allowing the fungus to spread. Clipped box also has lots of cuts for the fungus to enter, and the areas of damage will be very obvious.
Q Could I mistake it for anything else?
A Brown patches can also be caused by drought – especially if the plants are in containers – by wind scorch or by animals spraying or urinating on the bushes. Look for the other symptoms under What causes it? to confirm the diagnosis.
Q How serious is it?
A Even the more virulent form of box blight is rarely fatal, and new growth usually appears. However it can be seriously disfiguring — dwarf hedges typically have a dead-looking top, and green sides with brown patches here and there. Both the National Trust and National Trust for Scotland have had to remove all the box from some of their gardens, and even the Prince of Wales’s garden at Highgrove has been affected.
Q How do I control an attack?
A The best thing to do is get rid of the infected plants entirely, especially if cylindrocladium is the culprit. If this is too drastic, cut out all the a ected areas back to healthy wood. Then collect up any fallen leaves, bits of twig etc — some gardeners use a garden blower vac to be extra sure. Burn or bin all the infected material – do not compost it. Thinning out the branches to allow in more air can help, though this is rarely practical with topiary or hedges where box is most often used. Thoroughly clean any tools used on infected plants with bleach or disinfectant.
Q Are there any suitable sprays?
A No chemicals are specifically recommended for box blight, though general-purpose fungicides recommended for ornamentals could be tried. There is some evidence that products containing myclobutanil (eg Bayer Garden’s Systhane Fungus Fighter or Do Systemic Fungus Control) may be of some benefit against cylindrocladium.
Q How can I prevent it?
A Avoid bringing infected plants into the garden. If you already have a lot of box it is safer to propagate new plants yourself from cuttings. If you do buy new plants, quarantine them for three weeks keeping the leaves moist, and checking that no symptoms appear before planting them out. Avoid watering box from above and wetting the leaves; water the soil instead. Cylindrocladium can infect plants directly through the leaf surface, unlike volucella, which can only enter where the leaf or stem is damaged. However it is still likely to be worthwhile following the traditional advice and not pruning until Derby Day (in the first week of June), or the nearest dry, sunny day, when fungal spores find it harder to spread.
Q Are there any resistant varieties?
A No varieties of box are known to be resistant. If you cannot control the disease, or want to be sure not to get it, then you could consider alternative plants.
Q What are the best alternatives to box?
A For dwarf hedges and small topiary Ilex crenata is a good option. For larger hedges and topiary Lonicera nitida is a popular alternative. Other plants to consider include dwarf berberis eg Berberis buxifolia ‘Pygmaea’, varieties of Euonymus fortunei, dwarf hebes, lavender and lavender cotton (Santolina chamaecyparissus).