One such pest is tomato yellow leaf curl virus. This virus differs slightly in various parts of the world and the SA strain has been named tomato curly stunt virus (ToCSV). Symptoms resemble those of bunchy top virus. The individual leaves of infected plants look almost identical, but bunchy top makes the plant look more compact and stunted in comparison to curly stunt, where the plants continue to grow and stunting is less severe.
Bunchy top virus is transmitted by leaf hoppers, usually in hot weather and eases off when things get cooler and hopper activity is less. Mostly, the disease affects only a small number of plants and is much less of a threat than curly stunt.
Curly stunt virus is devastating and can render the crop a complete failure if sufficient plants are infected at a young stage. The virus is transmitted by the whitefly species Bemisia tabaci, which prefers tropical and subtropical conditions. The disease is thus confined to these climatic regions. The cold winters of temperate climates are able to break the fly’s life cycle.
Many weeds are host to curly stunt virus, including most members of the nightshade family, such as Datura (thorn apple, stinkblaar). It can also infect beans. In areas where curly stunt occurs, it’s worthwhile controlling broadleaf weeds in the vicinity of tomato lands well in advance of planting. Peppers can have the virus and not display symptoms, but are a source of possible infection. Whiteflies are not partial to peppers.
Suitable insecticides can be applied to control the silverleaf whitefly. Use imidacloprid as a drench after transplanting and then employ unrelated products later on so as to take out any individuals which may have resistance to the insecticide. This is important, because you don’t want to lose an effective product through overuse and build up a population of resistant whiteflies.
Resistant genes have also been extracted from wild species of tomatoes. However, you need to remember that, not only do some genes work better on certain types of virus, they confer resistance – and not immunity. There will still be some yield loss which will be proportionate to the stage that the plant became infected. In addition, the fruit will become blemished.
Controlling the whitefly, then, is your priority, even if you are using varieties with resistant genes. Disease symptoms begin to manifest three to four weeks after infection. This means that, even when you remove infected plants at the beginning of an outbreak, others will still manifest symptoms later. Have someone inspect the lands, removing plants with symptoms and placing these in clear plastic bags with the opening folded over to bake in the sun. This effort is well worthwhile and drastically reduces the rate of spread in the land. In essence, it’s part of a multi-pronged effort using all possible control measures simultaneously.
Apart from the chemicals used, spotted wilt virus is also contained to a large extent by removing broad-leafed weeds in the vicinity of the land well in advance of planting. The rouging of plants with symptoms as soon as they occur, together with insecticidal sprays, will greatly reduce the problem, and is money well spent. In the case of spotted wilt, the vector is the western flower thrip.
With spotted wilt, very effective resistant genes have been incorporated into a great many varieties. Any farmer can find a resistant variety to suit their requirements. In this case, the resistance can probably be classified as immunity, although companies are loathe to make such claims to safeguard themselves in case the disease mutates to overcome the resistant gene. This has already happened in at least one country.
Contact Bill Kerr at [email protected]. Please state ‘Vegetable production’ in the subject line of your email.