Whitefly and Virus in a Tomato Plot
A farmer in the Chobe area planted 0.2 ha of tomatoes. Three weeks after transplanting, many plants show upward-curling, yellow-edged leaves and stunted growth; when the leaves are disturbed, small white flies rise in clouds. A few plants have collapsed with dark stems near soil level. The farmer has been spraying a broad-spectrum insecticide twice a week for two weeks, but the problem is worsening. Neighbouring fields of beans and maize look healthy. The farmer wants to spray more often and at a higher dose. The season is hot and dry, and the tomatoes are rainfed with hand watering.
1. Diagnose the problem and explain why the farmer's spraying has not worked.
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The curling, yellow-edged leaves, stunting and clouds of white insects point to whitefly-transmitted virus disease (such as tomato leaf curl), not to a simple insect attack; the dark-stemmed collapsed plants suggest a separate soil-borne stem disease. Spraying broad-spectrum insecticide twice weekly has failed for several reasons: once a plant is infected with a virus, no insecticide cures it; frequent broad-spectrum sprays kill the natural enemies of whitefly and can cause whitefly populations to rebound; whitefly develop resistance under heavy selection pressure; and spraying does not remove virus sources in nearby weeds or old crops. Increasing dose and frequency will worsen resistance, cost and beneficial-insect loss without curing infected plants.
2. What should the farmer do now, and what should be done next season?
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Now: rogue out and destroy (burn or bury) severely infected and collapsed plants to reduce virus and inoculum; stop the frequent broad-spectrum sprays and, if whitefly pressure is high, use a recommended selective product with a different mode of action, rotating actives and respecting intervals, applied to the underside of leaves where whitefly feed; remove weeds that harbour whitefly and virus; support plant vigour with correct watering and nutrition, since there is no cure. Next season: use virus-tolerant or resistant varieties, raise seedlings in a clean nursery away from old tomato crops, use a reflective or insect-proof cover for seedlings, plant a border or trap crop, avoid planting next to infected old crops, and rotate away from tomato and other solanaceous crops for at least two seasons in that plot.
3. How would you explain the role of the neighbouring beans and maize to the farmer?
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Explain that the healthy beans and maize are not the source of the tomato problem; the virus and whitefly are specific to tomato and related plants, and whitefly may move from old tomato, weeds or nearby infected crops. However, the neighbours' crops matter in two ways: they can act as a barrier or as a reservoir depending on what they are, and if the farmer plants tomato near them in future, whitefly may move between fields. The farmer should therefore coordinate with neighbours about old tomato crops and weed clearing, and avoid planting tomato downwind of an infected field. This illustrates that pest management is an area-wide problem, not only a within-field one.