Initially noted by the development of dark spots on the leaf, which spread until the whole leaf turns brown and then drops. Black streaks spread down the stems then smaller stems are also killed.
The fungus is thought to propagate and spread in wet conditions.
This is noted by masses of pinkish spores on the underside of the leaf.
The causal fungus thrives in wet conditions and the overall effect is similar to that of Cylindrocladium.
The most important control measure for both diseases is to buy or propagate from healthy plants, to ensure that the diseases are not brought into the garden.
The control of Volutella and Cylindrocladium is aided by good cultural practice.
If conditions exist which favour the promotion and spread of fungal diseases, both these and others are more likely to thrive.
The regular cutting of box hedging and topiary gives rise to dense foliation and a warm, humid microclimate within the plant.
Hedges clipped from the smaller-leaved, closer-foliated Buxus sempervirens Suffruticosa are particularly vulnerable to disease unless a more open habit is achieved through thinning.
Warmer summers and warmer, wetter winters in recent years might have exacerbated this condition and reports of the increasing incidence of these fungal pathogens have been received from Holland, Belgium and France, which countries enjoy a similar climate in some regions to the U.K.
A Suggested Course of Action
Remove and burn all infected material during a dry period (to prevent the further spread of spores). Clean up and remove all cuttings and rubbish as a regular practice. Sterilise cutting tools.
For clean hedges and topiary start a regime of thinning once a year before the weather warms to 12 degree C. As this is unpredictable, an autumn prune might be advised.
How much to prune
In the USA, where some of their climate zones experience much hotter temperatures than the UK, pruning is carried out sufficiently to enable sight through the plant.
However, despite our recent climate changes we are still in a temperate zone and a suggested yardstick is that it should be possible to run extended and open fingers through the foliage like a coarse comb.
How to achieve this
If there are only one or two pieces of topiary this can be done with secateurs, parting the foliage and selectively cutting so that the surface of the topiary has few, and small, gaps.
If there is a large number, take the shears with the points slightly apart say 2cm plunge them point first and at right angles to the topiary face, say 10cm deep, and snip. Repeat over the surface of the plant in regular pattern 15cm or so between cuts in any direction. Remove cuttings.
Try this out on an unimportant plant first to get the technique right.
For hedges it is suggested that the same technique is employed but in a single line down the middle. Once again, perfect the technique on an unimportant section first.
Whatever cuts are made will ultimately repair to give a uniform surface.