Discolouration of the leaves: the leaves show light green spots or have stunted growth. The veins become light green first. These symptoms lead to a decrease in production.
These symptoms are caused by viruses. These viruses are transmitted from plant to plant by small insects such as whiteflies. Only one of these insects is sufficient to transmit the virus disease. Young plants have to be free of these insects in order to prevent the disease at a later stage. Infection of the plants at a later stage leads to less influence on the yield.
Viruses cannot be controlled by chemicals. But the insects (e.g. whiteflies) can be controlled by pesticides.
- Seedlings can be sprayed once a week with imidacloprid 1-2 ml/l
- After transplanting, the use of imidacloprid can be continued for two weeks. The pre-harvest interval (PHI) for imidacloprid is 14 days.
- Imidacloprid can be alternated with thiamethoxam, chlorfenapyr or pyriproxyfen once a week. Thiamethoxam is applied at 13 g/20 l (PHI 3 days).
Chlorfenapyr is applied at 0.5 g/l (PHI 1 day)
Pyriproxyfen is applied at 1-2 ml/l (PHI 7 days). Please note that this insecticide is an insect growth regulator and does not kill the natural enemies of the white flies.
- Good ventilation will keep the population of the insects low, therefore use wide spacing.
- Young plants with virus symptoms should be removed and destroyed.
- Older plants with symptoms can be sprayed with milk solution: 1 litre milk in 10 litres water.
When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.