Fusarium wilt

What you are looking at

The lowest leaves yellow first. The yellowing works up the plant while the top still looks well, and the dead leaves hang on the stem rather than dropping cleanly.

The plant wilts in the heat of the afternoon and stands back up overnight. That goes on for days or weeks, each day a little worse, until one morning it has not recovered.

Often one side goes before the other. A single branch, one half of the plant, or even one half of a leaf either side of the midrib.

Cut the main stem across, 10 to 20 cm above the soil. A ring of tissue just inside the bark is stained brown or reddish-brown, and in a Fusarium plant that stain usually runs a long way up the stem.

The roots and the stem base are firm and sound. Nothing is rotting at ground level.

Blocked plumbing

The fungus lives in soil and enters through the fine roots, often through a wound made by a nematode or by cultivation. From there it grows into the xylem, the tissue that carries water from the roots to the leaves.

Inside the xylem it multiplies and spreads upward, and the plant responds by plugging its own vessels with gums and gels to wall the infection off. Between the fungus and the plant’s own blockade, the water supply is throttled.

That is why the plant wilts in the afternoon and recovers at night. In the heat the leaves lose water faster than the damaged plumbing can replace it. At night demand falls and the plant catches up. As more vessels block, the recovery gets shorter, and then it stops.

The brown stain in the cut stem is the blocked and discoloured vascular tissue itself.

Fusarium or verticillium

These two produce almost the same picture in the plant and the same brown ring in the stem, and they are the classic pair to separate.

Temperature is the most useful difference. Fusarium is a warm-weather disease, running hardest in soil above about 24 deg C, so it worsens through high summer. Verticillium prefers cooler conditions, often shows in early summer, and can stall when the real heat arrives. A wilt that appeared in June and eased in August was probably verticillium; one that got steadily worse through August was probably Fusarium.

The staining differs in degree. Fusarium tends to stain darker and reach higher up the stem, while verticillium staining is often faint and confined to the lower stem.

Host range is the other difference and it runs the other way. Verticillium has a very wide host range, so it moves between unrelated crops. Fusarium is narrow, which is the subject of the next section.

The strain that killed your tomatoes is harmless to everything else

Fusarium oxysporum is a single species divided into more than a hundred formae speciales, each of which attacks one host or a narrow group of related plants and nothing else.

Fusarium oxysporum f. sp. lycopersici kills tomatoes and cannot touch a cucumber. The one that kills basil does nothing to lettuce. The one that ended the Gros Michel banana trade is a banana pathogen and is covered on its own page.

For a gardener this is unusually good news, and it is the practical difference between this and the diseases it gets confused with. A bed that has lost a tomato crop to Fusarium is available for beans, brassicas, roots and onions straight away. Honey fungus and Phytophthora cinnamomi offer nothing comparable, since both have host lists in the thousands.

What the specificity does not do is let you clear the ground. The fungus persists for many years as thick-walled resting spores, so the tomatoes will meet it again whenever they come back round. That is what resistant cultivars and grafted rootstocks are for.

What to do about it

Cut a stem across and look for the brown ring before deciding anything.

Buy cultivars labelled F, FF or FFF, which is the whole answer for tomatoes.

Graft onto a resistant rootstock where infested ground has to be used, and keep the union above the soil.

Grow in containers of fresh compost for a small crop.

Pull out infected plants with their roots, bag them and take them away.

Plant anything unrelated in the same bed without hesitation.

Rotate to reduce the load, while expecting rotation alone to fall short.

Sow early enough to crop before the real heat where the climate allows it.

Feed with nitrate rather than ammonium nitrogen on ground with a history.

Test before liming, and lime for the soil rather than for the disease.

Wash tools, boots and canes between beds.

What it gets mistaken for

Verticillium wilt looks nearly identical and runs in cooler weather, with fainter staining lower down the stem.

Bacterial soft rot dissolves the tissue into foul-smelling slime.

Root rot from overwatering browns and softens the roots and the collar.

Drought wilts a plant in dry soil and it recovers on watering.

Clubroot swells brassica roots into knobbly clubs.

Root-knot nematodes bead the roots and open the wounds this fungus enters through, so the two often occur together.