Tomato Late Blight Control

Symptoms

Large irregular water soaked greasy looking patches appear on leaves, usually starting at the tips and margins, and quickly turn dark brown to black. In cool moist weather a white downy fungal growth appears on the underside of the spots. Stems develop dark brown streaks and fruits show greasy brown firm rot. The whole plant can collapse and give off a foul smell, and the disease spreads with frightening speed in cool wet cloudy weather.

Damage

Late blight, caused by Phytophthora infestans, is the most feared disease of tomato and potato and can destroy an entire crop within a week under cool wet conditions. Losses range from severe to total when the weather stays cloudy, foggy and humid with temperatures around 10 to 20 degrees. The same pathogen attacks potato, so the two crops should not be grown close together. Because it moves so fast, prevention and early protective spraying matter far more than waiting to react.

Organic & cultural control

There is no strong organic cure once late blight sets in, so prevention is everything. Use disease free seedlings, give wide spacing and staking so plants dry fast, avoid evening irrigation and overhead watering, and ensure good drainage. Remove and destroy the first infected plants immediately by burying or burning them away from the field. Copper based bio fungicides give some protection if applied before the disease arrives, and avoid planting tomato near potato fields.

Chemical control

Begin protective sprays of mancozeb 75 WP or chlorothalonil before the disease appears when cool cloudy humid weather is forecast. Once late blight is seen, switch to systemic or translaminar fungicides such as cymoxanil plus mancozeb, dimethomorph plus mancozeb, mandipropamid or metalaxyl plus mancozeb, and repeat at short intervals during risk weather. Do not rely on metalaxyl alone because resistance has developed. Alternate fungicide groups, follow the product label dose, observe the pre harvest interval and consult your KVK or agronomist before spraying.

Always follow the product label, the recommended pre-harvest interval, and your local KVK or agronomist. Pesticide approvals change, so verify before use.

Prevention

Grow tolerant varieties where available and use only healthy disease free seedlings. Keep tomato and potato fields well apart and follow crop rotation. Plant on raised beds or ridges with good drainage, space plants well and stake them so air moves freely. Watch weather forecasts closely and start protective sprays before cool wet spells. Destroy all infected plants and debris and do not leave cull piles of potato or tomato near fields.