Plant disease
Late blight
The fast-moving, plant-killing blight behind the Irish potato famine.
Late blight is in its active season now — scout your plants this week.
How to identify late blight
Late blight starts as greasy-looking, dark brown or blackish blotches on leaves and stems, often edged with a pale yellow-green halo. They are not the tidy round spots of early blight — they look water-soaked and spreading, like a bruise.
In cool, humid weather, flip a spotted leaf over and you may see a fine white fuzzy growth ringing the edge of the blotch on the underside. On fruit it shows as firm, greasy brown patches that quickly rot the whole tomato or potato.
This is the one leaf disease worth treating as an emergency. It can take a healthy plant to dead in under a week and blows on the wind to your neighbors' gardens, so identify it fast and act the same day.
Attacks: Tomatoes, Potatoes
Life cycle: Spores spread on wind and rain and germinate on wet leaves in cool, damp weather (nights around 50-60F with long dew or fog). It races through a planting in days and can overwinter in leftover potato tubers and cull piles.
Signs of late blight
What you actually see on the plant — usually before you spot the pest itself.
- Greasy dark brown or black blotches on leaves and stems, spreading fast
- White fuzzy growth on the leaf underside at the edge of the blotch in humid weather
- Firm, greasy brown rot on green or ripening fruit
- Whole plants collapsing within days in cool, wet spells
Organic control, least-toxic first
Start at the top and only move down if you need to. Physical and cultural fixes come before any spray.
- Pull and bag infected plants the same day
This is the priority step, not a last resort. Pull out affected plants, seal them in a bag for the trash, and do NOT compost them — the spores survive and spread on the wind to the rest of your garden and your neighbors'. Acting fast is the whole game with late blight.
- Plant resistant varieties next time
Late-blight-resistant tomatoes and potatoes exist and are the best long-term defense in wet regions. The seed packet or catalog will name the resistance.
- Space and stake for fast drying
Wide spacing, staking or caging, and pruning lower leaves keep foliage drying quickly, which slows spores that need long leaf wetness to take hold.
- Apply a copper fungicide preventively
In a wet season or once blight is reported nearby, a registered copper fungicide can protect healthy plants, but it only prevents new infection — it does not cure sick leaves. Start before symptoms and reapply on the label's schedule and after rain.
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One rule for any product you spray: follow the label. The label is the law, and it is the tested, safe rate for your plants — homemade mixes and dish-soap sprays are not, and can scorch foliage.
Prevent it next season
- Choose late-blight-resistant tomato and potato varieties in cool, wet regions
- Never save or plant volunteer potatoes; dig and destroy every leftover tuber, where the disease overwinters
- Water at the base in the morning so leaves are not wet overnight
- Give plants full sun and wide spacing so foliage dries quickly
- Scout daily in cool, humid spells and pull the first infected plant immediately
Questions about late blight
What is the difference between early blight and late blight?+
Early blight makes tidy brown target-ring spots on the older lower leaves and moves slowly. Late blight makes greasy, spreading dark blotches with a white fuzz underneath and can kill a plant in days. Late blight is the emergency of the two.
Can I compost plants with late blight?+
No. Bag them for the trash. The spores survive and spread on the wind, so composting infected plants can reinfect your whole garden and your neighbors'.
Will copper spray cure late blight?+
No. Copper only protects healthy tissue from new infection — it cannot heal leaves that are already blighted. Use it preventively before symptoms and always follow the product label.
Plan a garden that fights back
Healthy, well-spaced plants shrug off pests that flatten a crowded bed. PlotToTable sizes your beds, spaces every crop, and flags the pests that hit what you grow.