When an otherwise healthy box ball or hedge suddenly develops brown or black patches on the foliage and the leaves drop from the stems it is probably due to a disease called box blight. It can be caused by one of a number of different fungi which grow on the surface of the foliage and stems to feed and which penetrate into the tissues with fine thread-like mycelium.
It is often seen in summer and autumn as a dramatic infection that can spread quickly to other box plants or along a whole length of box hedging. Box blight in itself does not usually kill plants down to the roots, but repeated infections year after year often weaken them to the extent that they fade away.
Can clipping make box blight worse
Constant clipping of hedges and topiary at one particular level really exacerbates the problem with blight by forming a ‘table’ layer where the blight and its spores develop. The top and sides of such plants can trap humidity inside the plants which also increases the chances of blight developing. Cutting into the top and sides here and there can reduce the chances of blight getting a foothold. It is also possible to transfer the box blight fungus on the blades of shears, hedge-trimmers and secateurs so make sure that they are cleaned between use with a dilute solution of bleach.
Where blight is already present you can help to limit its spread by digging up and disposing of infected plants or by hard pruning the less badly affected plants. Collect up any fallen leaves around the base of the plants and remove dead foliage from the plants where it is clinging to the stems. Where box is left untrimmed for a number of seasons, it may develop a looser, more open structure that allows better air circulation and reduces the incidence of infection.
How to limit the risk of box blight
Unfortunately older hedges and topiary are particularly vulnerable where the soil has become exhausted which puts the plants under stress, particularly during hot, dry weather. Summer rain storms after drought can help to release and spread the fungal spores up onto younger foliage and the moisture will then increase humidity which allows the disease to develop very rapidly.
Feeding can help to reduce the susceptibility of otherwise healthy box bushes and hedging, though it is better to use fertilisers with fairly low levels of nitrogen as this can cause the foliage to grow soft which makes it more vulnerable to infection from the spores. Sulphate of potash applied around the base of plants just before growth starts in April can also be beneficial by helping to toughen up the new growth before it can be infected.
What can I spray for box blight
There are few fungicides that can be used, but they are often ineffective if used on their own. A combination of pruning, feeding and fungicide may help to control an outbreak of the disease, or at least slow down its spread. The tissues and spores of some types of box blight can survive in the soil for up 6 years, reactivating periodically to re-infest new plants if they are introduced.