Mosaic (viral disease)

Symptoms

A characteristic mosaic appears on the leaves — irregular, interweaving light and dark green or yellow patches and streaks. The leaves are often deformed, wrinkled, curled and smaller, and whole plants become stunted and fruit poorly. The symptoms are most visible on young leaves, and the fruit of cucumber, courgette or tomato may be blotchy and misshapen. The disease affects many vegetables, perennials and ornamental plants. The disease picture depends on the specific virus, the host species and the temperature — the same pathogen produces different symptoms on different plants. A distinguishing feature is that the mosaic is systemic (it affects the whole plant and develops from the youngest leaves) rather than localised like fungal leaf spots — there are no clearly delimited spots with a margin, no mycelial coating and no fruiting bodies. Besides the mosaic, there is vein clearing (a lightening of the tissue along the veins), striping and blister-like bulges of the leaf blade. Severe narrowing and thread-like tomato leaves (the so-called 'shoestring' symptom) is associated mainly with CMV (cucumber mosaic virus), occasionally with tobamoviruses, whereas TMV/ToMV mostly produce mosaic and mottling of the leaves. On the fruit you can observe blotching, concentric rings, wart-like bulges (cucumber) and uneven ripening (tomato), and on the flowers of ornamental plants — colour breaking, i.e. streaks and flecks of discolouration on the petals. In high temperatures (above ~28–30°C) the symptoms may become masked and temporarily disappear, only to return once it cools down — this does not mean the plant has recovered.

Treatment

There is no effective treatment for viral diseases — an infected plant cannot be cured. Diseased specimens should be removed and destroyed as soon as possible to limit the source of infection, and after handling them you should wash your hands thoroughly and disinfect your tools. No fungicide or insecticide reverses a viral infection — insecticides only reduce the number of vectors (aphids) but do not cure a plant that is already infected. The key is early roguing, i.e. removing individual diseased plants before the virus spreads to their neighbours: the earlier, the smaller the outbreak. Do not compost infected material (some viruses survive) — remove it from the crop and destroy it in accordance with local regulations. TMV is exceptionally persistent: it survives for months in dry plant debris, on tools, hands, clothing and in tobacco products, which is why tools and surfaces are decontaminated with virucidal/degreasing products (e.g. solutions based on trisodium phosphate or diluted sodium hypochlorite), and hands are washed with soap or wiped with skimmed milk — not with water alone. Care operations (pruning, pinching, pricking out) are carried out first on healthy plants and last on suspect ones, disinfecting tools between plants.

Prevention

The basis is controlling aphids and other virus-carrying insects and removing weeds that serve as their reservoir. Use healthy seed and propagation material, disinfect tools between plants, and choose resistant or tolerant varieties. The single most effective measure is resistant and tolerant cultivars — choosing cultivars with resistance suited to the crop (e.g. to ToMV/TMV in tomato or to CMV) prevents infection or the development of symptoms. Being honest about the vectors: non-persistent viruses (stylet-borne — like CMV) are carried by an aphid within a few seconds of a probing puncture — before an insecticide can kill it — which is why spraying with insecticides alone poorly halts the spread of CMV and is only a supporting method; reflective mulches (silver, aluminised) and barriers that disorient incoming aphids work more effectively. TMV requires separate protection, as it has no insect vector and spreads mechanically: hygiene is what counts — washing hands with soap before work and avoiding smoking and contact with tobacco near susceptible plants (tomato, pepper, cucumber). It is worth using certified, virus-free seed and cutting material and, where recommended, thermal disinfection of seed (e.g. with hot water against ToMV/TMV in tomato seed). As a supplement: control of reservoir weeds and self-seeded plants, destruction of post-harvest residues, crop rotation and ongoing removal of diseased plants (see treatment).

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.