Dealing with viral diseases in tomatoes

Tomato growers should look out for the following: potato virus Y, grey mould, tomato spotted wilt virus, and tobacco mosaic virus.

Avirus is a small, infectious pathogen that reproduces only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to micro-organisms, including bacteria. About 5 000 virus species have been described in detail, although there are millions of types.

The following are viruses that can infect tomato plants.

• Potato virus Y (PVY)

Mainly a problem in Mpumalanga’s Lowveld region, PVY also attacks potato and pepper plants, hence its name. Infected leaves initially show yellow or transparent veins, later developing a mosaic pattern and sometimes necrotic (brown) spots.

Occasionally, infected plants show no symptoms of the disease. PVY is spread by aphids.

Control: Control aphids chemically; avoid planting near other susceptible crops; plant resistant cultivars; practise crop sanitation; control weeds.

• Grey mould

Found in all of South Africa’s tomatoproducing areas, this disease leads to fruit rot. It can be a problem when tomatoes are grown at moderate temperatures and under high humidity or prolonged leaf wetness (dew). All above-ground parts of the plant may be attacked.

The pathogen invades the plant through pruning wounds, growth cracks and/or through dead or dying tissue, such as old flowers or dead leaves.

Light brown lesions are formed, and the fungus is often clearly visible as a grey-brown growth within the lesion.

The disease occurs in both field and greenhouse tomatoes, but is more common in greenhouses, where high humidity frequently occurs.

Control: In the case of greenhouse tomatoes, reduce humidity levels by venting; remove infected plants or plant debris from the production area; optimise ventilation between plants. In the case of field tomatoes, plough in the crop residue. In both cases, apply chemical control.

• Tomato spotted wilt virus ( TS WV)

Widespread in South Africa, this occurs on various host plants. It is a serious problem in the Western and Eastern Cape. Young leaves curl downwards and can show rings; older leaves are bronzed or show a mosaic pattern. Infected fruit display characteristic concentric rings. Infected plants are stunted. TS WV is spread primarily by the western flower thrips and the onion thrips.

Control: Apply chemicals; plant resistant varieties; apply crop sanitation; control weeds.

• Tobacco mosaic virus ( TMV)

TMW is also widespread in South Africa. First reported on tobacco, TMV attacks a wide range of host plants. Leaves show abnormal growth (malformation, stunting) and a green or yellow mosaic pattern. Browning occurs on leaves, stems and fruit. Fruit ripens unevenly, is small, and shows browning of the fruit wall when sliced open.

TMV is spread mechanically, often by workers (hands and clothes), implements or infected water. It is also transmitted by seed, pollen or roots.

Control: Apply strict sanitation in the production area; plant resistant cultivars; control weeds.

• Source: ‘Production Guideline for Summer Vegetables’. 2013. Agricultural Research Council (arc.agric.za > Search ‘Production Guideline for Summer Vegetables’).

TMV IS SPREAD MECHANICALLY BY WORKERS (HANDS, CLOTHES), IMPLEMENTS OR INFECTED WATER