The box blight fungus (Cylindrocladium buxicola) survives and reproduces in infected leaves and stems, including fallen leaves and dead stems. Fungal spores within these infected tissues act as pockets of the disease (i.e. the inoculum) that can invade previously uninfected leaves and stems.
Infection has probably progressed further than it shows, so it pays to cut out more than you think you need to. It is important to cut out infection when the disease strikes, even if it is not the normal time of year to trim box. However, if the plants are wet such as after rain, wait until conditions are dry before beginning cutting back, otherwise you risk spreading the disease further.
Cutting back or cutting out
Removing some or all of the affected areas will reduce the amount of inoculum and improve the chances of healthy recovery. Prioritise your plantings; removing infected, less important, plantings may safeguard prized plantings.
How can I tell what is diseased? Typically you are looking for patches on your box plants where the leaves have gone brown or have fallen, leaving bare stems. Infected stems will have distinctive black streaks and dieback (i.e. are no longer green under the bark). For more on symptoms and photos of the disease see our page on box blight.
Choose from one of four options, depending on the severity of the infection and your priority;
Option 1: remove minor or isolated infections
(Good where infections have been detected early, for topiary specimens, or where fungicides will be applied.)
- Cut out a little more than you think is necessary as some infections are likely to be latent (not yet visible)
- Do not touch healthy box plants immediately after working on an infected area
- Consider applying a fungicide (see below) before cutting and again after two weeks
Option 2: reduce height by about a half
(Good for hedges and parterres. Also where infection is spread over large areas. This approach creates a more open structure in the plants that discourages box blight.)
- Try to cut stems so no black streaks remain. This may require reducing width as well as height
- Consider applying a fungicide (see below) before cutting and again after two weeks. Reduction can be less severe if you intend to use fungicides
- Clean pruning tools regularly during process
Option 3: cut back to stumps
(Good where infection is severe but is in a position where you want to try to save the box feature (e.g. part of a parterre) or where you do not wish to apply fungicides. This more drastic approach aims is to remove all infected tissues to maximise the chances of removing all inoculum.)
- Cut back all plants showing symptoms to ground level, leaving just the stumps
- Plants can be cut back at any time, but beware of the risks of transporting the pathogen around the garden in wet conditions
Option 4: remove affected plants
(Good where infection is very severe, the box is not of high value, or it threatens other box plantings.)
- Remove plants at any time, but beware of the risks of transporting the pathogen around the garden in wet conditions
- Avoid planting box in the same place for several years (spores can remain viable on fallen leaf litter for at least 6 years)
Remove fallen leaves and infested soil
- Lay down a tarpaulin or sheeting before clipping clearing up clippings and debris
- Remove dislodged leaves and other debris from the canopy after cutting back
- Clear away as many fallen leaves from under and around the plants as possible (even a few remaining leaves can lead to new infections)
- Remove a layer of soil under the plants to get rid of infected leaf fragments. Replace with fresh soil (do not overfill)
- Apply a mulch
Disposing of diseased material
For ways of getting rid of infected clippings, leaf litter or entire plants, see our page on disposing of diseased material.
Bayer Fungus Fighter Plus and Bayer Fungus Fighter Concentrate have label recommendations for box blight and can be applied up to six times per year. If treatment is needed in spring, consider saving some applications in case it recurs in the autumn. Triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) is labelled for control of other diseases on ornamentals and could therefore be used legally on box (at the owner's risk) to try and control box blight. Follow label instructions for use (avoid spraying when rain is expected, but also in bright sunshine or when plants are dry at the roots).
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely