Verticillium wilt

What you are looking at

One branch wilts. The leaves on it flag on a hot afternoon and pick up again by morning, then a few days later they yellow from the margins and between the veins, brown, and hang there dead. Everything else on the plant looks perfectly well.

The following summer another branch goes the same way, and then another. A Japanese maple lost to this usually takes three or four years about it, dying by degrees while the gardener watches and waters.

On tomatoes, dahlias and strawberries it works from the bottom of the plant upwards instead, yellowing the older leaves first, stunting the growth and ruining the crop.

One cut settles it

Take a wilted branch, ideally one at least as thick as a pencil, and cut across it. Then split it lengthways.

Verticillium leaves a stain in the wood, brown or olive or faintly green, showing as a broken ring or as streaks running along the grain just under the bark. Healthy wood is pale and even.

That stain is the fungus in the water-conducting vessels, which is also the whole explanation of the symptoms. It gets in through the roots from the soil, grows up inside the plumbing, and blocks it. Where the plumbing is blocked, the leaves cannot drink, so the branch fed by those vessels wilts while its neighbour is unaffected.

Wood with no stain means the wilting has another cause, and that is worth knowing before writing a tree off.

Nothing will cure it, and something can still be done

No product available to a gardener or a grower treats verticillium wilt. The fungus lives in the soil as microsclerotia, resting bodies tough enough to sit in the ground for fifteen years and longer, and nothing applied to a plant reaches them.

What does work is vigour. The plant defends itself by walling off infected vessels, and a plant with water, food and an uncompacted root run does that far better than one competing with grass in dry soil. Trees kept properly for a decade after diagnosis are common, and so are trees dead in two seasons because nobody watered them.

The other thing that works is choosing correctly afterwards. Conifers, grasses, bamboos and ferns cannot get it at all. Among broadleaves, birch, hornbeam, beech, oak, apple, pear and hawthorn are rarely troubled. Replacing a dead acer with a birch is a real answer; replacing it with another acer is not.

What to do about it

Confirm it by cutting a wilted branch and looking for the stain.

Water, mulch and feed the plant properly, and clear grass and weeds from the root area.

Cut affected branches back into clean unstained wood.

Take out a plant that has lost several branches, lift as much root as you can, and burn or bin the whole thing.

Do not compost any part of it and do not shred it for mulch.

Replant with something outside the host range, and never with the same species.

For vegetables, rotate, use grafted tomatoes on resistant rootstocks, and grow in containers of fresh compost where a greenhouse border is infected.

What it gets mistaken for

Honey fungus kills from the roots and collar upwards, with a sheet of white fungal growth under the bark at ground level and black bootlaces in the surrounding soil. Verticillium leaves the base of the plant sound.

Phytophthora root rot kills the roots and the collar on wet ground, and the whole plant declines together rather than one limb at a time.

Drought wilts everything at once and recovers completely after a soak.

Silver leaf stains wood too, and it silvers the foliage first and belongs almost entirely to plums and cherries.

Clematis wilt collapses a whole clematis stem within a day, usually just as the buds are about to open, and is a different fungus with a much simpler answer: cut it to the ground and it usually reshoots.

Fusarium wilt does very nearly the same thing as verticillium on many of the same plants, and the management is identical, so telling the two apart matters to a laboratory rather than to a gardener.