How to manage Box Blight

The fungal disease box blight attacks Box (Buxus) and can result in twiggy patches and dieback.

Box blight was first recognised in the mid 1990’s and has now become a real problem in many gardens all over the UK, especially with warm and damp weather.

What is Box Blight?

Box blight is a fungal disease that largely affects Buxus sempervirens (Box) and its cultivars.

There are two types of blight- Cylindrocladium buxicola (box blight) and Pseudonectria buxi (volutella blight). White spore masses on the underside of infected leaves signifies C. buxicola, whereas P. buxi spores are pink.

Both can exist on the plant at the same time but C. buxicola will be the one causing the most damage, and is the box blight I will discuss in the rest of this post.

What are the signs of Box Blight?

Autumn and Winter is usually the time when box blight is most active, although outbreaks can occur in damp and humid conditions in Summer.

A common symptom is dark brown or black spots appearing on the Box leaves. These will merge to cover the entire upper leaf surface and soon after the leaves will fall off, leaving twiggy, bare patches on the Box. If the blight is active then the area of discoloured leaves can have a slimy appearance.

If the box blight has been active for some time then black streaks will appear on the stems. This is not good news- it is a symptom of the more damaging blight C. buxicola and is a sign the fungus is becoming established in the plant.

If left untreated the blight will cause significant leaf loss, stem damage and stem die-back on the Box plants.

How to manage the Box Blight

Box blight is not a death sentence for your plants. Sadly the Buxus can never be cured from box blight but the disease can be managed.

I have found there are three main factors to managing blight- applying a fungicide, careful garden and tool hygiene, and improving the plant health

Treat the Box plants in Spring and Autumn with a commercial grade fungicide. Applications must be delivered by fully qualified operatives who are certificated by NPTC (National Proficiency Tests Council).

If box blight outbreaks occur during the year then treat the affected area as quickly as possible with a fungicide such as Provanto Fungus Fighter. This will slow down the spread of the infection.

If possible, cut out any severely affected areas where black streaks are visible on the stems.

Clear any fallen leaf litter and dead material. Destroy both, ideally by burning. Do not put any Box leaf litter, cuttings or dead material in the compost as the fungal spores can remain viable for at least 6 years.

Fungal spores can remain in soil so removing and replacing the topsoil around the affected plants can help. But be careful not to cause damage to the plant’s root system.

Keep shears, clippers and hedge trimmers disinfected when trimming Box to avoid spreading box blight to healthy plants. Do this by dipping tools into a bucket of water with a splash of garden disinfectant regularly when working, or apply to the blades using a trigger sprayer. I use one small capful (approx. 15ml) of Citrox mixed with up to five litres of water.

How to avoid Box Blight outbreaks

Make sure the Box plant is as strong and healthy as possible to increase infection resistance.

Mulch annually in Spring or Autumn around the base of the plants with a good depth of organic rich compost. This helps improve the nutrient balance and soil structure. Leaf mold can be used to improve the soil structure but does not have any nutritional value so extra feeding will be required.

Regularly feed the Buxus plants with a sustainably sourced seaweed extract such as Shropshire Seaweed. I mix a small capful (15ml) of seaweed extract with a litre of water and spray on to the leaves every four weeks during the growing season.

In addition to this regular feeding I will lightly fork a slow release granular feed like Vitax Q4 Professional around the base of the Box plants in Spring.

Increase air movement through the Box plants by reducing the amount of times the Box are trimmed. Regular clipping encourages denser growth, which is an ideal environment for the fungus to flourish.

If your Box is in a border or flower bed then best to reduce overcrowding and improve air movement by removing plants growing in close proximity of the Buxus.

Avoid overhead watering as spores can be spread in the water droplets and damp leaves create conditions that the box blight fungus thrives in.

Following this advice does not guarantee success at managing box blight but will certainly improve the chances of the Box plants fighting off infections.

Remember to continue with the fungicide treatments and good gardening practices even when your Box plants start to recover, to ensure box blight does not return.