Tomato Fruit Set, Rain and a Tank-Mix Reflex Before Harvest
A farmer in Jalgaon has 0.2 ha of tomato, transplanted 45 days ago, with fruit set and first harvest expected in 12 days. Weather has been warm at 24–28°C with 85–90% relative humidity and light rain. Lower leaves show brown spots 5–10 mm across with concentric rings and a yellow halo; spots are spreading upward and several leaves have dried. Whiteflies are present on the underside of upper leaves, but no leaf curling or yellow mosaic is visible. The farmer has mancozeb 75% WP, carbendazim 50% WP and imidacloprid 17.8% SL in the shed. He wants to tank-mix all three at full label dose tomorrow morning and harvest within the week if prices are good. He has no protective clothing beyond a cotton shirt and no gloves.
1. Identify the most likely disease and explain how the symptoms and weather support it. Differentiate it from late blight and Septoria leaf spot.
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The most likely disease is early blight, caused by Alternaria solani.
Supporting evidence. The spots are 5–10 mm across, brown, with concentric rings and a yellow halo. They start on older lower leaves and move upward. Warm temperatures of 24–28°C, high humidity and light rain favour early blight. The concentric rings are the key field sign.
Differentiate from late blight. Late blight, caused by Phytophthora infestans, produces irregular, water-soaked, grey-green lesions that turn brown and greasy. In cool, wet weather it forms white fuzzy sporulation on the underside. Fruit develops firm brown rot. It does not usually show neat concentric rings.
Differentiate from Septoria leaf spot. Septoria spots are smaller, about 2–3 mm, with grey centres, dark margins and tiny black pycnidia. Yellowing is more general and there are no concentric rings.
Differentiate from bacterial spot. Bacterial spots are angular and water-soaked, without rings. A hand lens helps. If uncertain, send a sample to a local plant protection laboratory before spending on sprays.
2. Build a treatment plan using only the products on hand. Include dose and interval, pre-harvest interval, resistance management, and explain why tank-mixing all three is a bad idea.
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Use mancozeb as the protectant. Mancozeb 75% WP is a contact protectant. It can slow spread but will not cure existing lesions. Use the label dose, typically around 2–2.5 g/L, with a sticker, covering both leaf surfaces. The label pre-harvest interval for tomato is commonly about 7 days; the farmer must check his own label. Harvesting in 5 days would violate that interval.
Do not rely on carbendazim alone. Carbendazim is a benzimidazole, and Alternaria populations commonly develop resistance. It is not the best choice for early blight. If it is used, it must be rotated with a different mode of action, not repeated.
Do not tank-mix all three. The mixture increases cost, residue risk and selection for resistance. Imidacloprid does nothing for the fungal disease and is harmful to pollinators. Tank-mixing is only acceptable when the label specifically permits it.
Practical plan. Spray mancozeb now and repeat after 7–10 days if rain and humidity continue. If disease pressure is high, use a labelled systemic or translaminar product such as difenoconazole or azoxystrobin in rotation. Respect the pre-harvest interval. Remove badly infected lower leaves, improve airflow, avoid overhead irrigation, and stake plants so leaves dry faster.
3. Advise on the whitefly population and the harvest decision. Include the minimum safety measures the farmer must follow.
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Whitefly. Imidacloprid can reduce whitefly, but it is not justified when there is no leaf curl or yellow mosaic and the population is low. It is harmful to bees and should not be sprayed on flowering crops during the day. Use yellow sticky traps, remove weeds that harbour whitefly, and conserve natural enemies. Spray only if a threshold is crossed, in the evening, at label dose, and observe the label pre-harvest interval.
Harvest decision. If mancozeb was sprayed and the label pre-harvest interval is 7 days, do not harvest after 5 days. A short delay is better than a residue violation and a rejected consignment. Pick a legal harvest date before spraying, and write it on the spray record.
Minimum safety measures. Wear long sleeves, trousers, rubber gloves, boots, goggles and a mask. Spray with the wind, never eat, drink or smoke during spraying, and bathe and wash clothes afterwards. Keep children and animals away. Store pesticides locked and away from food. A cotton shirt and no gloves is unacceptable. The farmer should also keep a simple spray diary with product, dose, date and pre-harvest interval.