Late Blight
Pathogen: Phytophthora infestans · Affects Tomato, Potato
Late blight is the fastest-moving disease in the vegetable garden — the same pathogen behind the 1840s Irish potato famine can still collapse an unprotected tomato or potato planting within 7–10 days of the first lesion, especially after a run of cool, wet nights. Because it spreads by windborne spores, a single infected plant can seed an entire field before symptoms are obvious from a distance.
Symptoms
- Water-soaked dark lesions on leaves and stems
- White fungal growth on leaf undersides in humid conditions
- Brown, greasy patches spreading rapidly across foliage
- Firm brown rot on tubers and fruit
Causes
- Cool, wet weather (10–20 °C) with prolonged leaf wetness
- Spore spread via wind, rain splash and infected seed tubers
- Dense planting and poor air circulation
Treatment
- Remove and destroy infected foliage immediately — do not compost
- Apply copper-based fungicide or chlorothalonil on a 7-day schedule
- Switch to systemic mefenoxam once outbreak is confirmed
- Improve airflow by pruning lower leaves and staking plants
Prevention
- Plant certified disease-free seed tubers and resistant cultivars
- Rotate solanaceous crops on a 3-year cycle
- Water at the base of plants early in the day
- Scout weekly during cool wet spells
Frequently asked questions
Can late blight be cured once a plant is infected?
No — there is no curative treatment. Infected tissue must be removed and destroyed, and remaining plants protected with a preventive fungicide program before they are exposed.
How fast does late blight spread?
Under cool, humid conditions a single lesion can produce spores that infect an entire planting within 7–10 days, which is why prevention and daily scouting matter more than cure.
Preventive Fungicide Program
A rotating schedule of protectant contact fungicides for foliar diseases on vegetables, fruit and ornamentals.
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