Plant Problem Diagnostic
Late Blight: How to Identify and Manage It
The short answer
Late blight is a potentially devastating disease of tomato and potato that can destroy a planting quickly. Leaf infections appear as large brown blotches with a gray-green margin, infected tomato fruit develops a firm brown rot, and potato tubers become discolored and soft. The pathogen produces spores that travel long distances on the wind, so this is a disease to take seriously. The least-toxic priorities are prevention: plant resistant varieties, keep leaves as dry as possible with drip or soaker-hose watering done in the morning, and remove volunteer potatoes and infected debris.
Because late blight can spread through a neighborhood and cause total crop loss, confirming it matters. If you suspect late blight, contact your local Extension office promptly; many regions run alert systems, and a protectant fungicide program applied before infection may be needed under favorable conditions.
How to identify late blight
Look for rapidly enlarging brown to gray-green blotches on leaves and stems, often with a pale border, sometimes with white fuzzy growth on leaf undersides in humid weather. Fruit shows firm brown rot.
- Large brown blotches with a gray-green edge on leaves and stems
- Firm brown rot on tomato fruit; soft, discolored potato tubers
- Spreads very fast in cool, wet, humid conditions
Least-toxic management, in order
- 1Resistant varieties — Plant tomato and potato varieties with late-blight resistance where available.
- 2Remove inoculum — Destroy volunteer potatoes, cull piles, and infected plants that can harbor the pathogen.
- 3Keep leaves dry — Water with drip or soaker hose in the morning so foliage dries quickly.
- 4Scout often — Check plants frequently, especially during cool, wet weather, so you catch it early.
When to escalate
Late blight can cause total crop failure and spread to neighboring gardens and farms, so it warrants prompt action. Contact your regional Extension office to confirm it and get current guidance; protectant fungicides must be applied before infection when conditions favor the disease, and infected plants should be removed to protect others.
⚠ Late blight is a community disease that can wipe out tomato and potato plantings. If you suspect it, report to and confirm with your local Extension office quickly, and remove infected plants.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- University of Minnesota Extension — Late blight of tomato and potato — Leaf blotch and fruit/tuber symptoms, wind-dispersed spores, resistant varieties, dry foliage, and preventive fungicide timing
- Cornell Cooperative Extension — Late Blight — Late blight severity, potential for total crop failure, and Extension decision-support and alert systems