Box blight

Calonectria pseudonaviculata

also called buxus blight

A fungal blight of box: dark leaf spots, straw-brown patches of dieback, and black streaks running up the young stems.

Fungus Serious Rain splashSpread on tools & handsAirborne spores Spreads to neighbors

What you'll see

Dark brown or black streaks run up the green shoots — the surest sign it is blight and not drought.
Leaves develop dark-ringed spots, then turn straw-brown and drop, leaving bare twiggy patches.
In damp weather a fine white bloom of spores shows on the underside of infected leaves.
Bare patches spread outwards and coalesce, often worst in the dense middle of clipped plants.

Why it happens

A fungus that lives on the leaves and stems of box, splashed from plant to plant on water and carried in on new stock or on shears. The sticky spores cling to anything wet and survive on fallen leaves and in the soil for years.

It thrives in warm, still, humid spells — overhead watering, close planting and clipping in muggy weather all favor it.

Treating it

Cut out affected growth well past the streaking and bin or burn it — never compost box clippings.
Clear every fallen leaf from inside and beneath the plant, where spores overwinter.
Stop overhead watering; water the roots only and let the foliage stay dry.
Disinfect shears between plants and after any diseased cutting.
On a valued specimen a copper-based fungicide can slow it, but it will not cure an established case.

Keeping it away

Buy only clean plants and isolate new box for a few weeks before planting.
Space plants for airflow and thin congested growth so the center dries quickly.
Clip in dry settled weather, not in a humid spell, and clear up every trimming.
Mulch under the plants to stop rain splashing old spores back onto the leaves.
Where blight keeps returning, replace box with a look-alike such as Ilex crenata or a low hebe.

Who gets it

Box (Buxus) above all, both clipped hedging and topiary, with the fine-leaved edging varieties often hit hardest.

Boxwood