Spotted wilt – the ever-increasing threat

Issue date : 04 July 2008

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Spotted Wilt was restricted to the southern Cape until the western flower thrip was introduced into the country. Thrips are this disease’s only vector. Spotted wilt has become economically important as major vegetables are susceptible to it. Both the English name spotted wilt and the Afrikaans name kromnek describe the disease in tomatoes.

The first symptom is that the plant becomes slightly lighter in colour and loses some of its healthy sheen. Next, the shoots or growing points start to bend slightly downwards, which is the source of the name. By this stage the plants develop an even lighter colour and this is followed by light brown spots on the leaves – hence the English name. fruits usually develop large circular lesions rendering them unmarketable. When there is a high thrip population, the disease can spread rapidly and wipe out the crop.

Thrips are not easily noticed unless we deliberately look for them. In this case the vector is a flower thrip. If flowers are present, this is the first place to look. The thrips will enter right into the flower and also cause the “zippering” effect on fruit, manifested as a line from the blossom end to the calyx. It’s much easier to clean out the thrips before flowering begins. You can train yourself to detect infected plants at a very early stage. When inspecting, mark any plants you suspect and check in a day or two to see whether they’ve developed symptoms. Early detection is the key to success. Uproot infected plants and place in a large plastic bag, tie it to seal it, and leave in the sun to destroy the thrips, and “cook” and kill the plants.

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Combining this with simultaneous thrip control, we can contain the disease. Do this daily until the outbreak stops. Remember other plants will be infected many days before symptoms appear. We often find two or three infected plants together. Perhaps more importantly, we should monitor the source of the disease. The disease has many host crops and a number of host weeds. Some may show virus symptoms, but they may be masked in others. Long before planting, remove or kill all weeds in the vicinity of the crop. This is extremely important. is the greatest source of infection and is usually totally neglected. disease can be contained with care and timely management.

Thrips don’t easily leave a source of food to go far afield. I have had the virus develop in some tunnels and have prevented it moving to tunnels 40m away. here is a good resistance gene in many varieties which works very effectively. The plant either gets spotted wilt or is unaffected. In some areas, resistant varieties with this gene are a prerequisite due to disease-susceptible weeds and plants which can’t effectively be removed. Many farmers are in a position to virtually eliminate the disease by having a weed-free surrounding area. – Bill Kerr ((016) 366 0616 or e-mail [email protected]) |fw