Tomato mosaic virus — symptoms and treatment

Severity: high

What is Tomato mosaic virus

Tomato mosaic virus (ToMV) is a tobamovirus that causes mottling and distortion of leaves. It is highly contagious — and in a way that sets it apart from most tomato troubles, because it needs no insect, no wind and no rain. It travels on contact: on hands, on a knife, on a stake, on seed. Tobamoviruses are tough and persist on surfaces and debris, so one careless afternoon of pruning can seed a row. There is no chemical treatment available, which makes hygiene the whole of the defence.

Symptoms

The signature is in the colour of the leaf rather than any spot on it. The foliage develops a mosaic — irregular patches of light and dark green mottled across the leaflet, with no defined margin between them, most obvious on younger leaves and in good light. Alongside the mottling the leaves distort and curl: puckered, narrowed, strap-like or crinkled. The fruit is affected too — fruit size is reduced, and cutting one open can reveal internal browning invisible from outside.

• Early: light and dark green mosaic mottling on the leaves, clearest on new growth.
• Progressing: leaf distortion and curling; puckered or narrowed leaflets.
• Advanced: reduced fruit size; internal browning within the fruit.

Mottling with distortion is the pairing to look for. Nutrient deficiency yellows leaves in a pattern tied to the veins, not in irregular light-and-dark patches, and does not deform the leaf. Unlike TYLCV, this is not about upward-curling yellow margins and stunting — and it arrives with no whitefly in sight.

Causes and conditions

ToMV is spread mechanically, by physical contact. The virus is carried in the sap, and the small wounds made in ordinary handling — pruning, tying in, pinching out side shoots, even brushing along a row — are enough to transfer it from an infected plant to a healthy one. Hands, tools, stakes and clothing all carry it, and it persists on those surfaces and in crop debris. It is also seed-borne, which is how it enters a garden with no tomato history, and it can be carried on tobacco products handled before touching plants. It needs no vector and no particular weather.

Treatment

Sanitation and resistant varieties — cultural

Timing: pre-planting and throughout. No chemical treatment is available — nothing will cure an infected plant, so this is containment, not rescue.

• Use virus-free seed. Seed is a primary entry route; clean seed keeps the virus out.
• Disinfect tools with 10% bleach. Do this between plants, not just between sessions — the pruning knife is the main way this virus travels.
• Plant TMV-resistant varieties. The most reliable protection where the virus recurs.
• Remove infected plants. Take them out promptly so they cannot be the source your hands and tools carry down the row.

Prevention

• Grow resistant varieties, widely offered and labelled in seed catalogues.
• Wash hands thoroughly before handling plants — particularly after handling tobacco products.
• Disinfect knives, secateurs, stakes and ties between plants rather than working down the row with one blade.
• Work through healthy plants before suspect ones, and clear crop debris after harvest.

Frequently asked questions

Can I cure a plant with Tomato mosaic virus? No. No chemical treatment is available, and no spray or feed will clear a virus from a plant's cells. Remove infected plants and protect the healthy ones through hygiene and resistant varieties.

Is the fruit safe to eat? Yes. The virus does not affect people. The fruit will be smaller and may show internal browning when cut — trim that out, and what remains is edible.

Is it contagious to other plants? Very. It spreads plant to plant by contact — hands, tools, stakes, clothing — and it can affect other plants in the tomato family, such as peppers and aubergines.