Late blight
Late blight, the potato and tomato blight, is a water mould that can wipe out plants within days in warm, humid weather. Leaves develop brown patches with a white fringe underneath; tomato fruit turns brown and firm; potato tubers rot.
Fungal diseases
What it looks like
Speed is its signature.
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Brown patches Dark, water-soaked patches, often from the leaf edge. -
White fringe A fine white growth beneath the patches in humid weather. -
Blackened stems Stems collapse with dark lesions. -
Brown fruit Tomatoes develop firm brown marbling and rot.
How to confirm it
Weather and speed confirm it.
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Warm, wet spell
Outbreaks follow humid spells above 10 °C (50 °F).
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Rapid spread
Whole plants go down within days.
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White underside
The fringe of spores in the morning.
What to do
Act fast to save what you can.
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Remove infected growth
Cut off affected leaves and stems at once and bin them.
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Cut potato haulm
Cut down blighted potato foliage and wait two weeks before lifting.
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Pick tomatoes
Harvest unaffected green fruit to ripen indoors.
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Do not save seed
Never keep seed or tubers from infected plants.
Prevent it next time
- Grow blight-resistant varieties, such as 'Crimson Crush' and 'Sarpo' potatoes.
- Grow tomatoes under cover where summers are wet.
- Remove volunteer potatoes.
- Space plants and keep leaves dry.
What not to do
Do not compost blighted plants or tubers; the pathogen survives on living tissue.
In the growing guides
Where the guides mention it, with plant-specific advice.
Sources
- Potato and tomato blight, Royal Horticultural Society
- Tomato-potato late blight in the home garden, Penn State Extension