Symptoms of box blight include browning of the leaves and patches of defoliation. Black streaks maybe visible on the stems and small white spores on the underside of the leaves.
Discoloration of box leaves and defoliation could also be the result of nutrient deficiency. This is why a correct diagnosis is important as the box plants may just require a good feed rather than the more expensive fungicide treatments.
A simple test for box blight is to place the suspect leaves in a sealed plastic bag with a damp tissue. A positive indication of the Cylindrocladium buxicola fungus (see photo below) is white spore masses appearing on the underside of the leaves after a few days. If the spores are a pink colour then this is Volutella blight.
Cylindrocladium buxicola is the more aggressive of the two but both fungal infections do need to be treated in order to prevent significant damage to the box topiary or hedging.
Apply a fungicide spray to affected areas as soon as possible to slow the spread of the disease. Suitable fungicide sprays are available in most good garden centres. If the blight infection is widespread then an application of a commercial strength fungicide maybe necessary. A spraying license is required to do this.
Have a read of this previous post I wrote if you need more tips and advice on treating box blight- Fungicides and Good Gardening Practices are Ways to Beat Box Blight