Box Attack

Ideal for formal, evergreen hedges and clipped topiary in borders and pots, Buxus sempervirens, has long been a garden staple. This evergreen favourite is however, now under attack from both box blight disease and box tree caterpillars.

Box Blight
Box blight is a fungal disease caused by the fungus Cylindrocladium buxicola. It thrives in moist conditions and causes both the foliage of box to brown and die and the stems to turn black and then also dieback.

If the weather is damp a white furry growth which are the spores of the fungus can be seen. They generally appear in patches but will spread to the rest of the plant or along the rest of the hedge.

Action
It is best to prune out infected and dead stems and remove dead leaves both from the plants and from underneath the plant. It is also worth scraping off the top few inches of soil underneath the plant. Everything removed should be binned and not composted.

If the problem is spread over a larger area you can reduce the size of the plants by half.

Box blight does not kill the roots of the plant so diseased plants do not necessarily have to be dug out unless the plant is severely infected, then the whole should be removed to prevent any further spread. If plants have been removed, it is best not to plant box in the same place. 

Prevention
Going forward, box should only be clipped in dry weather to prevent spores spreading and only once or twice a year as regular clipping will encourage denser growth which in turn leads to poorer air circulation. Use clean disinfected pruning equipment every time and try to keep airflow as free as possible around the plants.

If the area is damp and humid box blight is more likely to develop. Only water early in the morning and around the base of the plant so the leaves stay dry.

If you have detected and dealt with box blight, it is unlikely that you will have eliminated it in your first attempt, so it is important to continue to monitor the plants and deal with new infections promptly.

Box Tree Caterpillar
Box tree caterpillar, Cydalima perspectalis, can render a box plant leafless.

Symptoms of damage are patches of dieback, which could be confused with box blight but patches of webbing and droppings near damaged areas indicate the presence of box tree caterpillars.

The moths of this caterpillar were first seen in the UK in 2008 and is thought to have entered the UK on imported plants. Box tree caterpillars when newly hatched are greenish-yellow with black heads. When they are older they have thick black and thin white stripes along their bodies and can be up to 4cm long.

Box tree caterpillars are most active from spring to autumn but they can overwinter as small caterpillars in webbing spun between leaves.

Action
To control small colonies of caterpillars, try simply picking them off by hand. For larger infestations, there is little else to do but use an insecticide or biological control. Remember not spray insecticides near flowering plants in order to prevent harm to beneficial pollinators.

Prevention
The best prevention to avoid both box tree caterpillars and box blight is to check plants in nurseries to avoid bringing infected box plants home and quarantining box for up to six weeks before planting in order to check for any developing infestation of either the caterpillars or the fungus.