Box blight

What you are looking at

Dark brown spots appear on the leaves, sometimes ringed with a darker edge, and run together until the leaf is brown all over. Within days those leaves fall, still whole, leaving twigs that carry nothing. The patch has a defined edge rather than fading out, and it widens week by week.

Look at the young green stems next to the bare twigs. Narrow black streaks running lengthways along them are what confirms the diagnosis, and they belong to no other box problem.

In warm still weather after rain, turn a browning leaf over. A fine white bloom on the underside is the fungus sporing, and touching it moves it.

Why hedges get it and specimens often do not

Spores germinate when a leaf stays wet for several hours at around 20 degrees. Everything that makes a good clipped hedge works towards that. Tight annual shearing produces a dense outer shell of twiggy growth, the middle of the plant stops seeing moving air, and dew sits on the interior leaves until late morning. An open specimen dries by nine o’clock and is infected far less often.

The spores themselves are sticky and heavy. They travel a few metres in splashing rain and no further on their own, which is why blight arrives in a garden on a plant, a pair of shears or the sole of a boot rather than on the wind. A hedge trimmer taken between three gardens in a week is the commonest route of all.

Fallen leaves under the plant keep the fungus alive for five years or more. Clearing them changes what happens the following June more than anything done to the foliage above.

What no amateur product will do

No fungicide available to a gardener in Britain gives control of box blight. Professional growers use programmes that are not sold in garden centres, and even those are protective rather than curative. Anything on a shelf marketed for this will disappoint, and the money is better spent on a leaky pipe or on replacement plants.

That leaves cultural change carrying the whole page, which sounds thin and is not. A hedge watered at the base, clipped once in dry weather, thinned to let air through, and kept clear of its own fallen leaves can be held for years with blight present in the garden. The same hedge sheared twice a year and watered overhead will be bare in three summers.

What to do about it

Stop wetting the leaves today. That single change removes the condition the fungus needs.

Cut out the marked growth, going 15 cm past the last damaged leaf into clean wood, and bag it. Then rake up everything underneath, skim off the top centimetre of soil where the plant is badly affected, and mulch over the cleaned ground.

Wash and disinfect the shears before touching the next plant.

Drop to one clip a year and take it in a dry settled spell. Open the plant out rather than tightening it.

Where a hedge has already failed twice, replant with Japanese holly. It takes the same shaping, holds the same dark green, and this fungus does not touch it.

What it gets mistaken for

Volutella blight, the older and milder box disease, turns leaves straw-coloured to pink-bronze and leaves them hanging on the plant. Its spore pustules are pink or salmon rather than white, and it does not streak the stems black.

Box tree caterpillar leaves silk, green-black droppings and chewed leaf edges. Blight leaves whole leaves on the ground. A plant can have both, and caterpillar wounds make infection easier.

Winter bronzing is a cold and wind response on the exposed face of a hedge. The foliage turns orange-brown, stays attached, and greens up again in April.

Drought and root damage brown a whole plant or a whole branch at once, from the tip back. Blight works leaf by leaf from inside the plant outwards.