Box blight is actually caused by two separate fungi, sometimes acting together.
Cylindrocladium buxicola can be recognised from the spots it causes on leaves which become larger areas of dead (necrotic) tissue. As the condition progresses, it causes defoliation. Other symptoms include wispy grey fungus on the underside of the leaves and bark charcoal or black streaky staining on the woody parts of the plant. As the disease advances leaves are lost from the plant. Cylindrocladium buxicloa has only recently been identified (the first research paper on it was only published in 2000) and it is currently being studied at Wisley and elsewhere.
The second box blight fungus is Volutella buxi which has been present in the UK for a long time. An infected plant typically develops pink pustules on underneath the leaves. These are most likely to develop in humid weather. The leaves yellow and fall off the plant and in severe cases cankers can develop on the branches and trunk.
Both diseases are believed to lie dormant on dead leaves on the ground, producing spores in spring as temperatures rise. This leads to the most obvious and easiest form of control (which is non-chemical). Dead leaves should be raked up and burned. All other infected material such as branches should also be cut off and destroyed. Don't forget to disinfect tools afterwards.
There are no garden chemicals available specifically to deal with either blight, but copper fungicides and those containing penconazole seem to have some effect. Our belief is that good planting, in soil containing plenty of organic matter, top dressing with blood, fish and bone in the spring, and a precautionary spray with a fungicide such as Scotts RoseClear3 in early April will go a long way towards preventing outbreaks of box blight. We would also advise never clipping a box hedge in warm, damp weather, as Volutella certainly enters the plant through open wounds and it is most active in humid conditions. So forget the old adage about clipping your box hedge on Derby Day.
If this has not put you off, why not take a look at our box hedging plants