Mosaic virus

What you are looking at

The leaf carries irregular blocks of dark green against pale green or yellow, with no line between them and nothing growing on the surface. Rub it and nothing comes off. Hold it to the light and there is no hollow chamber underneath.

New leaves come out puckered, blistered or narrowed, sometimes to a strap a centimetre wide where the plant should be making a broad blade. The plant stays small while the one next to it grows away.

Fruit shows it too. Courgettes and cucumbers come lumpy and mottled with hard patches, tomatoes ripen unevenly, and the crop is not worth much.

Whatever you feed or water it, none of this improves. That permanence is the strongest single clue, because almost every other cause of mottled leaves responds to something.

A group of diseases rather than one

Mosaic is a pattern, not a species. Cucumber mosaic virus has the widest host range of any plant virus and turns up on well over a thousand species from courgettes to delphiniums. Tobacco and tomato mosaic viruses run through the tomato family. Zucchini yellow mosaic ruins squashes. Arabis mosaic, turnip mosaic and a long list of others do the same job on their own hosts.

They travel by different routes and the routes decide the control.

Most are carried by aphids. An aphid probes an infected plant, picks the virus up on its mouthparts within seconds, and delivers it to the next plant it tries. That happens far too quickly for an insecticide to prevent, which is why controlling aphids here means encouraging their predators rather than spraying after the event.

Some travel by contact. Tobacco mosaic is the notorious one, spread on hands, clothes, tools and cigarettes, and stable enough to survive in cured tobacco and in dried plant debris for years.

Some go through seed, some through tubers, cuttings and divisions, and a few through soil nematodes.

Nothing cures a virus

There is no treatment. A virus is inside the plant’s own cells, and nothing available to a gardener or a commercial grower removes it. Any product sold on that promise is selling something else.

What that leaves is worth being clear about. Take the plant out, stop it infecting others, and prevent the next one. That is the whole of the advice, and it is better than the hedged non-advice that usually fills the gap.

One footnote. Some virus infections have been kept deliberately. The broken tulips that made fortunes in seventeenth-century Holland were infected with tulip breaking virus, and several variegated abutilons and honeysuckles sold today are virused plants propagated for the pattern. Those are curiosities rather than a reason to keep a mottled courgette.

What to do about it

Pull out badly affected plants, bag them and bin them. Do not compost them.

Start with the worst ones where a whole row is showing symptoms.

Deal with the aphids, mainly by keeping the predators that eat them.

Weed thoroughly, and particularly clear chickweed and groundsel, which carry cucumber mosaic virus through the winter.

Wash hands and tools between plants in a greenhouse, and keep tobacco away from tomatoes.

Buy certified virus-free raspberry canes, strawberry runners and seed potatoes.

Choose resistant cultivars where the catalogue offers them.

Never take cuttings, divisions or tubers from an affected plant.

What it gets mistaken for

Magnesium and iron shortages yellow the leaf between the veins in a regular pattern, on a predictable part of the plant, and they respond to a feed or a change of pH within a fortnight.

Herbicide damage twists, cups and narrows new growth in a way that closely resembles a virus. The tell is the pattern across the garden. A drifting herbicide hits everything downwind, and a virus takes individual plants and leaves their neighbours untouched.

Spider mite stipples a leaf with fine pale dots of even size and leaves webbing at the shoot tips.

Genetic variegation is stable, symmetrical, present from the first leaf, and usually rather beautiful. A mosaic is irregular, appears partway through the season, and comes with distortion and stunting.

Downy mildew makes angular yellow patches penned in by the veins, with growth on the underside. A mosaic ignores the veins and has nothing underneath.