What to look for

Late blight is caused by Phytophthora infestans, an aggressive oomycete, or fungal-like pathogen, that can rapidly destroy tomato and potato crops during cool, wet weather. High-tunnel crops can also become infected.

Scout for:

• Irregular, water-soaked or greasy brown leaf lesions that enlarge rapidly.
• Pale green or yellow tissue around lesion margins.
• Gray-white sporulation around lesion edges on leaf undersides during humid conditions.
• Dark lesions on stems and petioles.
• Firm brown lesions on tomato fruit.
• Purple-brown surface lesions and reddish-brown decay beneath the skin of potato tubers.

Other diseases can resemble late blight, so seek confirmation before destroying a planting.

Management

If maintaining production through the remainder of the season is economically important:

• Scout fields and high tunnels frequently.
• Avoid overhead irrigation and working in plantings while foliage is wet.
• Remove lower tomato leaves where appropriate to improve airflow.
• Maintain preventive fungicide coverage during cool, wet weather.
• Apply protectants before forecasted rain when possible and restore coverage after substantial rainfall as permitted by the label.
• Rotate targeted fungicides among FRAC groups.

The New England Vegetable Management Guide’s tomato late blight recommendations include the protectants chlorothalonil, mancozeb, and copper. Listed targeted options include cyazofamid (Ranman), cymoxanil-containing products such as Curzate, mandipropamid + difenoconazole (Revus Top), fluopicolide (Presidio), ametoctradin + dimethomorph (Zampro), and oxathiapiprolin-containing products such as Orondis Opti. Consult the Guide’s potato disease-control section for potato-specific products, rates, and preharvest intervals.

Copper is an important preventive option for organic growers, but it must be applied before infection and with thorough coverage. Conventional growers- do not rely on mefenoxam (Ridomil Gold) alone because resistant late blight populations occur. Always confirm that a product is labeled for the crop, rotate FRAC groups, observe preharvest and restricted-entry intervals, and follow the pesticide label.

Because much of this season’s yield has already been determined, growers should weigh the cost of additional applications against the value of the remaining crop. Where continued management is not justified, harvest sound tomato fruit and terminate the planting. In potatoes, killing and removing infected top growth can help limit movement of the pathogen into tubers. Good thing is late blight doesn’t overwinter in Connecticut.

Report suspected late blight: If you suspect late blight, please send photos of the upper and lower leaf surfaces, stems, fruit, or tubers, along with the crop and town, to shuresh.ghimire@uconn.edu or 959-929-1031 — text or iMessage.

Continue protecting cucurbits

Cucumber and cantaloupe growers should continue preventive fungicide programs for cucurbit downy mildew and scout all cucurbit crops closely. Maintain protectant coverage, include a downy mildew-targeted fungicide, and rotate FRAC groups. See last week’s Vegetable Pest Message for details.

New England Vegetable & Fruit Conference Returns December 15-17, 2026

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