Box blight

Box blight is a devastating fungal disease that attacks the leaves and stems of box (Buxus) plants. The affected leaves quickly turn brown, often with a yellow discolouration, and fall off the plant. Even quite moderate attacks can lead to extensive defoliation and creates large bare patches.

The disease may also attack stems, particularly young ones, producing black marks and causing them to die back.

Closely trimmed plants, such as topiary, are generally more prone to infection than unclipped ones.

Box blight spreads very quickly in warm, humid conditions.

• Box – Buxus species
• Pachysandra terminalis
• Sarcococca

Box blight is caused by two different fungi – Cylindrocladium buxicola and Pseudonectria buxi. These may attack plants on their own or together in combination. Cylindrocladium can also attack two closely related plants in the same family – Pachysandra terminalis and Sarcococca (winter box).

The spores are dispersed in water, which is why it is worse in humid conditions, and may even be spread in wet, windy conditions and by birds and other animals.

The disease survives from year to year as resting spores or fungal mycelium on fallen leaves and other Buxus growth, and can remain viable for six years. These produce spores that travel to the leaves when conditions are right.

Cylindrocladium buxicola is the more virulent of the two diseases, causing more serious damage, and can infect unwounded plants. Pseudonectria buxi needs wounds for infection and, as a result, is more prevalent on plants that are regularly cut back and trimmed.

Try to keep plants growing as strongly as possible to help them fight off disease attacks. However, when extensive outbreaks of box blight occur, this is unlikely to stop it happening.

As box blight spreads very quickly in warm, humid weather, check plants regularly during periods when these conditions are present.

As it’s not practical to pick off and dispose of affected leaves, prune out infected growth, back to healthy growth, and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it.

It may be better to dig up, remove and dispose of badly affected plants.

Carefully pick up all fallen leaves and stems (especially after trimming) – aiming to reduce or prevent spreading any spores – to reduce disease carry-over through the winter. Removing the top couple of inches of soil or compost, and replacing it with fresh, will also help remove the resting spores, which can remain viable for around six years.

You can help protect new growth and prevent the disease – and to a slightly lesser extent, control existing diseases – by spraying with a good garden fungicide. Bayer Garden Fungus Fighter has a label recommendation for use against box blight, and FungusClear Ultra should also give good disease prevention.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to possible danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Wherever possible, when buying plants check thoroughly for symptoms of box blight. Keep new plants in a holding area for a few weeks before planting them out to monitor them and confirm they are disease free.

As box blight thrives in humid conditions and the spores are spread in a layer of water, always water the soil or compost around box plants, rather than watering over the plants.

And, as regular tight trimming produces denser foliage and less air movement through the plants, reduce the frequency of cutting to help increase ventilation.

No species or varieties of Buxus show any resistance to box blight.

• Fungicides
• Secateurs