Can a plant virus be cured, and what do I do with the plant?

This is one of the few places in gardening where the answer is genuinely closed. A virus replicates inside the plant's own cells; there is nothing to kill that is not the plant. Commercial growers deal with viruses by producing clean stock through tissue culture and heat treatment, which is not something that can be done to a plant in a border.

Step by step

1. Check the pattern before concluding virus.Virus mottling is irregular, blurred at its edges, unrelated to the veins, and different on every leaf and every shoot. Anything that produces a neat, symmetrical, repeating pattern — yellow between green veins, a stable cream margin, bleaching on the sunny side — is something else and probably fixable.
2. Look for the supporting signs.Distortion and puckering of new leaves, narrow strap-like leaves, ring spots or concentric pale rings, colour-breaking streaks in flowers, stunting out of proportion to conditions, and a plant that has never grown properly despite good treatment.
3. Rule out the mimics deliberately.Hormone weedkiller drift causes twisting and mottling and looks alarmingly like virus, but affects several unrelated plants at once and improves in later growth. Nutrient problems follow the veins. Some variegation is deliberate and stable. Cold damage in spring mottles and then grows out.
4. Accept that eye diagnosis is not certain.Laboratory testing exists and is used commercially. For a garden plant it is rarely worth it, which means the honest position is a strong suspicion rather than proof — so weigh the value of the plant against the risk to what is around it.
5. Remove the plant if the diagnosis is convincing.Lift it with its roots, bag it, and put it in council green waste or burn it. Do not home compost it and do not leave it in a corner.
6. Do not propagate from it, ever.Cuttings, divisions, offsets and grafts all carry the virus. This is how virus spreads through a plant collection, a dahlia society or a family — via generously shared cuttings from an attractive plant nobody realised was infected.
7. Clean up after handling it.Some viruses, tobacco mosaic among them, transfer on hands and on tools with remarkable ease. Wash your hands, disinfect secateurs, and do not smoke around solanaceous plants such as tomatoes and peppers.
8. Deal with the vector.Most garden viruses are moved by aphids, some by thrips, some by nematodes in the soil and some by whitefly. Controlling those insects is the only preventive measure available, and it is why an aphid problem is more serious than the direct damage suggests.
9. Replant sensibly.For aphid-transmitted viruses, an unrelated replacement in the same spot is fine. For the small number of soil-borne, nematode-transmitted viruses — some affecting raspberries and strawberries — avoid replanting the same crop there and buy certified virus-free stock.

Tips

• Buy certified stock for the crops where it exists — raspberries, strawberries, potatoes and fruit trees are all sold as certified virus-free, and it is worth paying for.
• A virused plant that still flowers is still a reservoir. The decision is about the garden, not about that plant.
• Colour-breaking in tulips — flame-streaked petals — is the famous historical example of a virus valued for its symptom. It also weakens the bulb steadily, which is why those tulips died out.
• Hostas showing blue-green ink-bleed mottling along the veins usually have hosta virus X, which spreads via sap on tools when dividing.

Common mistakes

• Buying a treatment for it — Nothing sold to gardeners treats a plant virus, and anything claiming to is treating something else or nothing at all.
• Cutting the affected shoots off and keeping the plant — The virus is systemic — it is in the roots, the crown and every shoot, including the ones that look normal.
• Passing on cuttings from an attractive but odd-looking plant — Virus-infected plants are often propagated enthusiastically precisely because the mottling looks like variegation.
• Composting the plant — Some viruses persist in plant debris and in soil. A cool heap and a spreading mulch is a distribution system.

Questions people ask