Box blight is especially bad news for both Common Box and Dwarf Box plants (for the Latin speakers amongst you - Buxus sempervirens and its cultivars), although no Buxus species is immune. The word blight has been in the English language for about 500 years and its synonyms are very descriptive and equally unpleasant: plague, scourge, bane and - my personal favourite - curse. Suffice to say that, unlike a bit of mildew or a swarm of aphids, this is not a problem that you can afford to ignore. There are two types of box blight to look out for and I'll deal with each in turn.
Common Box and Dwarf Box plants both suffer from Common Box Blight, also known as Box Dieback. This is an old enemy of the classic box hedge and whilst there are actually several different fungi that can cause this, the culprit is usually a little chap who goes by the name of Volutella buxi.
The leaves of your box plants will turn brown in the early stages and this is when you really want to catch it because, for now, the damage is really just cosmetic. Left unattended, however, Common Box blight gets bored of consuming the leaves and spread down the twigs themselves to the stem, where it can cause horrible looking cankers. These distorted openings in the bark will eventually expose the plant's heartwood to decay and further infection, which means that you could end up having to cut back a large chunk of your plants.
The surest way to know that Volutella buxi is the cause of the trouble is to inspect the undersides of the leaves in wet weather - you will see a pinkish mould there, which is releasing spores that spread the fungus.
In the mid 1990's, another, much more virulent type of Box Blight appeared. Again it affects both Common Box and Dwarf Box plants. It has spread over Western Europe, although no one can say for sure where it originated. It goes by the name Cylindrocladium buxicola and a case of this is a real problem for box hedge owners. It strikes from mid to late autumn, when you will see brown or black blotches on the upper leaf surfaces, which gradually spread and consume the leaf.
In damp weather, you may see a grey mould covering the bottom of the leaves, although this is not always present or very visible. In time, the disease will make its way down the stem of box plants, leaving noticeable black stripes. As this happens, large parts of the plant will wilt and die. Although larger plants will usually survive this for some time, there is no hope for them in the long run, as the fungus has now taken up residence inside the plant. Ultimately it will kill its host and in the meantime it will continue to re-infect neighbouring plants. It is not restricted to any one variety and so has no problem moving between Common Box and Dwarf Box plants.
With both diseases, the best preventative measure is to regularly tidy up (and burn) the fallen leaves that collect under your hedge, especially during winter. This prevents spores from taking shelter on the dead leaves and so gaining a head start when the spring comes. In the case of Common Box Blight, it is sufficient to prune out and burn the affected stems - be sure to disinfect your tools with Dettol in between each cut.
However, if you have a case of Cylindrocladium buxicola, the advice of the RHS is to remove and destroy the affected plants, making sure to clean up any leaves as you do so. In both cases, fungicides that contain penconazole (Scotts Fungus Clear is recommended) can be sprayed onto the remaining plants to help prevent the spread of spores but do note that, at the time of writing this article, there are no fungicides that have been proven to kill these fungi once they have taken hold.