Box Blight volutella is a defoliating fungal disease of box Buxus sempervirens that comes via other infected plants or contaminated soil.
The spores can survive for more than a year in fallen leaves, and enter the plant through clipping cuts and other damage to leaves and stems.
The symptoms: are brown spots on the leaves, which cause rapid leaf drop, and black streaks on smaller stems.
Affected Buxus sempervirens
It spreads in wet humid conditions - pinkish spores appear on the undersides of leaves and are carried in water / rain splashes to new sites.
Good garden hygiene is the best treatment as prevention is better than cure.
After pruning, all affected branches should be burnt along with any fallen material around the base of the plant to remove any dormant spores.
Similarly, clean / disenfect pruning tools with household bleach, methylated spirits or jeyes fluid to prevent tranfer of the spores onto un-affected plants.
Do not plant out new stock in diseased soil or near infected plants.
Similarly check to see if new stock is affected, if it is take / send it back to the supplier.
Good air circulation will reduce the humidity around the plants.
Avoid overhead watering as this creates an ideal situation for the spores to adhere to the leaves and branches.
There are no chemical controls available to the amateur gardener to control this disease.
Note: Not all brown or dead patches are caused by Box Blight!
Areas of leaf scorch may be caused by dogs or cats as they urinate on the lower branches when they mark their territory
This is the most likely reason for sudden appearances of leaf scorch, blight tends to develop more slowly.