Early Blight
Alternaria solani; Alternaria tomatophila
Target-shaped dark brown lesions on tomato and potato leaves, starting on older foliage and working upwards.
Background
Early blight is a fungal disease of tomatoes, potatoes and related crops. Despite the name, it is not necessarily early in the season — it follows stress and dense foliage rather than time of year. Typical lesions are dark brown with concentric rings, giving a target-spot appearance. On potato tubers, dark sunken lesions can develop during harvest if foliage has been affected.
How to identify
- Dark brown leaf spots 5-15mm across with concentric rings (target pattern)
- Yellow halo often around the lesion
- Lower older leaves affected first
- Heavy infection causes defoliation from the ground up
- Tomato fruit lesions: dark sunken leathery areas, often at the stem end
Life cycle
Spores germinate in free water at 20-30C. Infection to new spore production in 5-10 days. Overwinters on infected crop residue and weedy solanums.
Weather triggers
- Temperature: 18-32°C
- Humidity: >75%
- Rainfall trigger: >5mm
- Warm humid weather with overhead watering is worst. Alternating wet and dry days favour disease.
Peak season (southern hemisphere)
Red = active season · Dark red outlined = this month
Affected vegetables & crops
Click any crop to see current prices and growing info.
Climate zones at risk
Organic & low-impact control
- Copper-based sprays as a protectant (Bordeaux, copper oxychloride)
- Remove and bag affected lower leaves promptly
- Mulch to prevent soil splash
Chemical control
- Mancozeb and chlorothalonil are registered protectants for commercial use; availability for home gardens has become restricted — check current labels
- Rotate with copper to reduce resistance
Always read product labels — registrations change.
Prevention
- Stake and prune tomatoes for airflow
- Mulch heavily to prevent soil splash onto lower leaves
- Remove volunteer potatoes and solanaceous weeds
- Rotate tomato and potato beds on a 3-4 year cycle
Companion planting
No reliable companion plant prevents early blight.
Biosecurity
Widespread.
Sources
- NSW DPI — Tomato early blight
- Queensland DAF — Early blight in potato
- Agriculture Victoria — Early blight
Accuracy confidence: high. We update this library as new extension guidance is published.