Tomato Diseases: How to Identify & Treat (USA & Canada)

Updated 2026-07-11 · Pest & disease

Most tomato problems are diagnosable from where the symptom starts and how it spreads. Get the identification right and the response is usually simple — and often structural (variety, spacing, watering) rather than another spray. Start with resistant varieties and clean culture; reach for fungicide only when the diagnosis calls for it.

Early blight vs late blight — tell them apart

These two are confused constantly but need different responses:

• Early blight (Alternaria): starts on the oldest, lowest leaves as dark spots with concentric "target" rings and a yellow halo, creeping upward. Favoured by warm, humid weather and stressed plants. Manage with rotation, mulch, staking, removing lower leaves, balanced nitrogen, and protectant fungicide (chlorothalonil or copper) if it advances.
• Late blight (Phytophthora infestans): the destroyer — large, greasy, grey-green blotches on leaves, stems and fruit, with white mould on leaf undersides in cool, wet weather. It can kill a planting in days and spreads regionally. Destroy affected plants, follow blight-forecasting alerts, grow resistant varieties (e.g. "Ph"-gene / late-blight-tolerant types), and use a protectant program under risk.

Leaf spots: Septoria and bacterial spot/speck

• Septoria leaf spot: many small circular spots with dark margins and tiny black specks (pycnidia) in the centre, on lower leaves — a very common wet-season defoliator. Rotation, mulch to block soil splash, airflow and fungicide manage it.
• Bacterial spot/speck: small dark, sometimes greasy or haloed spots on leaves and fruit; thrives in warm, wet, splashy conditions. Use clean/treated seed and transplants, copper-based sprays, and avoid working plants when wet — bacteria have no fungicide cure, so prevention is everything.

Wilts: Fusarium, Verticillium and the resistance letters

If plants wilt in the day and recover at night, then decline — often one-sided, with yellowing lower leaves and brown streaking inside the stem — suspect a soil-borne wilt (Fusarium or Verticillium). There is no cure once infected.

The answer is genetic and structural: buy resistant varieties — the letters after the name (V, F, FF, N, T) flag resistance to Verticillium, Fusarium races, nematodes and tobacco mosaic. Combine with long rotation out of tomato/potato ground and good drainage.

Blossom-end rot is not a disease

A sunken, leathery brown patch on the bottom of the fruit is blossom-end rot — a calcium delivery problem driven by uneven watering, not an infection, so fungicides do nothing. Fix the water: keep soil evenly moist (drip irrigation and mulch are ideal), avoid big wet–dry swings, don’t over-fertilise with nitrogen, and correct soil pH/calcium from a soil test. It typically corrects itself once moisture steadies.

The prevention checklist that beats spraying

• Resistant varieties first — the cheapest disease control you will ever buy.
• Rotate 2–3 years out of tomato/potato ground.
• Space and stake for airflow; prune lower leaves off the soil.
• Water at the base (drip), not overhead, and never handle wet plants.
• Mulch to stop soil splash, and clean up debris at season end.