There are two fungi that cause Blight in the genus Buxus, namely Volutella buxi and Cylindrocladium buxicola.The latter was first noticed in the British Isles in 1994 believed to have been introduced from central America.Both produce similar symptoms,in that,infected plants become defoliated.In the case of Cylindrocladium buxicola,the leaves develop dark brown spots which enlarge to affect all the tissues,patches of greyish fungal growth appear on the undersides of the leaves.Spores from these grey patches are released and spread the infection further.Bushes become defoliated and black streaks appear on the stems.The spores are able to penetrate the leaf ‘s breathing holes or stomata, so wounding is not necessary for the fungus to gain entry as it is with Volutella.
As yet there are no chemicals available on the amateur gardening market.
Box blight thrives in humid conditions so where possible irrigate from below. In high temperatures (33 degrees plus the spores are killed)!
Good hygiene is very important as fungal spores can remain active for up to three years.Prune out any affected branches and burn along with any fallen leaves from around the base of the plant .
Do not stress your box too much by over clipping,the saying do not clip box before Derby day (which is during the first week of June)holds true.One cut a year should be sufficient.Box blight has not been noted in unclipped specimens.
Do not overfeed your box with high nitrogen fertilisers.The resulting soft lush growth provides easy access for the fungus.It is better,to feed with a seaweed fertiliser/tonic so as to produce a strong plant that is able to resist the fungus.
Good ventilation is a key point to prevent box blights which is perhaps is difficult with a topiary specimen as the aim is to produce dense foliage, however a more airy bush can be produced by pruning out some of the growth.
Other reasons for brown patches on Box plants.
Brown or dead patches can also be caused by dog or cat urine, as they often mark their territory in the same place repeatedly resulting in scorching this area.If the dead areas occur slowly it is more likely to be box blight.