Cultural control and hygiene
Hold any commercially sourced plants in isolation for at least three weeks to confirm they are free of infection before planting out. Commercial nurseries may use fungicides which suppress but do not kill the fungus and this isolation technique will allow time for any suppressed disease to become visible.
Inspect plants for early symptoms as Cylindrocladium spreads very rapidly in warm and humid conditions and is difficult to manage.
Reduce the frequency of clipping to create more ventilation throughout the plants (regular pruning creates dense foliage and less air movement).
Avoid overhead watering as Cylindrocladium thrives in humid conditions.
As the disease is widespread, if possible, take cuttings from disease-free plants.
If the disease does break out, remove and destroy affected plants. If they are mature and highly valued, cut out all affected parts, clean up fallen leaves (including stripping and replacing surface topsoil to ensure complete removal) and treat with a fungicide. Spores can remain viable on fallen leaf litter for at least 6 years.
Unfortunately there is not known to be a resistant Buxus at present.
C. buxicola attacks Buxus balearica, B. bodinieri, B. glomerata, B. harlandii, B. microphylla, B. macowanii, B. riparia, B. sinica and B. sempervirens. There are differences of susceptibility among Buxus species and varieties. In general the varieties of B. microphylla are more tolerant than varieties of B. sempervirens. The dwarf form, B. sempervirens 'Suffruticosa', popular for parterres, is very susceptible. Based on work done in Belgium, B. microphylla 'Belvédère', 'Rococo', 'National' and 'Trompenburg' are tolerant to the disease. Other plants in the Buxaceae family are susceptible such as Pachysandra terminalis and Sarcococca species.
Choose alternative hedging and topiary plants
To be completely safe, choose an alternative hedge or topiary plant. The following all have small leaves and can be clipped into formal hedging styles:
There are no fungicides available to amateurs with specific recommendations for use against box blight. However, the fungicides myclobutanil (Bayer Garden Systhane Fungus Fighter* and other products), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra) are labelled for control of other diseases on ornamentals and could therefore be used legally on box (at owner's risk) to try and control box blight.
Research undertaken by the RHS has investigated myclobutanil and found it to be ineffective at controlling Cylindrocladium blight. In Germany and Belgium, professional formulations of difenoconazole and tebuconazole have good protectant activity. Only tebuconazole is effective against both genetic types of Cylindrocladium.
*NB: The following products are being withdrawn:
Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate and Bayer Garden Fungus Fighter Disease Control). These products cannot be sold after the 30th November 2015 and remaining stocks must be used or disposed of before the 30th November 2016.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)