There are two organisms which cause Blight in the Genus Buxus; both are fungal and they can occur together. Volutella has been around for many years. It is caused by Volutella buxi resulting in browning of the leaves and dieback of branches leaving bare patches in hedges and topiary specimens. It spreads in wet humid conditions - pinkish spores appear on the undersides of leaves and are carried in water splashes to new sites. Fallen leaves carry the disease over the winter and new spores are produced in the spring. They enter the plant through clipping cuts and other damage to leaves and stems.
Not all brown or dead patches are caused by Box Blight, it could be caused by dog or tom cat urine if it is at the bottom of the plant, as they 'territory mark' the same spot repeatedly and this scorches the leaves. If the dead area occurs suddenly and not gradually this could be the reason - the blight tends to develop slowly.
Late cutting can lead to frost damage later causing the outer leaves to die giving a white surface on hedges and topiary. This may be mistaken for Blight, but the underlying foliage will be green and will regrow. To avoid this any trimming should be carried out no later than September to allow new growth time to harden off.
The other Box Blight, which is more serious, has only been recognised in the British Isles since the mid 1990s, but is causing great concern. The fungus Cylindrocladium buxicola produces symptoms similar to Volutella with defoliated plants. The leaves develop dark brown spots which enlarge to affect all of the tissues, patches of greyish fungal growth appear on the underside and asexual spores (conidia) are soon released to spread the infection. Black streaks appear on the stems. The spores are capable of penetrating the cuticle so wounding is not necessary for infection to occur.
The disease requires moist conditions to thrive - the asexual spores die in dry conditions. Viable spores have been found in decomposing leaves after nearly a year, so any debris should be burnt if possible - never add to the compost heap.
A recently discovered threat to box is the caterpillar of Cydalima perspectalis a native of east Asia which can strip the foliage. It first appeared in Europe in 2007, but some have been found in Buckinghamshire and Hertfordshire. Pick off the caterpillars or prune out infested material with defoliation and webbing. Insecticides containing pyrethrum, deltamethrin or cyhalothrin may work. Any sightings should be reported to the RHS at Wisley.
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