Late blight is a fast-moving fungal-like disease that attacks tomato and potato plants, and it can wipe out a crop in days once it takes hold.1
What causes late blight
Late blight, sometimes called tomato blight or potato blight, is caused by an organism called Phytophthora infestans.1 It belongs to a genus that causes all kinds of plant problems, and it targets tomatoes and potatoes because both belong to the nightshade family, Solanaceae, sharing the same genus, Solanum.1
It's caused by an oomycete, a fungus-like water mold, rather than a true fungus, that spreads rapidly through leaves and fruit.1,,4 The same pathogen infects both tomato foliage and fruit, and potato foliage and tubers, causing rot and collapse.4,,5
This disease has serious historical weight. Late blight caused the Irish potato famine in the 1840s.1 Potato late blight is still infamous today for that famine, which is estimated to have caused around one million deaths in the mid-19th century.7
Why warm, wet weather matters
Late blight thrives in warm, wet conditions, and outbreaks can cause almost total yield loss in susceptible tomato varieties grown outdoors.4 Tomatoes grown under cover fare better, though they can still be hit hard at times.4
Spores spread fastest in warm, humid stretches, taking hold above roughly 10°C once relative humidity stays above 90% for about 48 hours. Growers call this window a Smith Period, and outbreaks tend to follow within seven to ten days of one occurring. Keep an eye on your local weather during any stretch of mild, damp days, especially in early summer onward.4
How to spot it early
Look for dark brown, water-soaked lesions on leaves, often with a pale halo. Affected foliage typically turns brown and shrivels, and a fine white mold can appear on the underside of leaves in humid conditions.4
A rotten smell is another telltale sign, alongside the visible leaf damage. On fruit and tubers, the disease causes decay and rot rather than surface spotting alone.4,,5
Check plants regularly once warm, wet weather sets in, since this disease does not appear every year but can be devastating when conditions favor it.1 Potato late blight remains the most serious and damaging disease affecting the crop.5
Steps to protect your plants
Start with resistant varieties where you can find them, since breeding for resistance is one of the most reliable long-term defenses against this disease.8 Good airflow around plants also helps foliage dry out faster after rain or watering, reducing the humid conditions the pathogen needs.
Good airflow and watering at the base rather than overhead help keep foliage dry and reduce the humidity the pathogen needs.
If you spot suspicious lesions, remove and destroy affected foliage promptly rather than composting it, since the pathogen spreads through infected plant material.7 Keep tools clean between plants to avoid spreading spores on damp foliage.
Copper-based fungicides can offer some protective cover, but they work best applied before symptoms appear, not after infection is established.
Budget-friendly options for home gardeners
Choosing resistant varieties at planting time costs no more than buying any other seed or seedling, and it's the cheapest form of long-term protection available to home gardeners.8 Spacing plants properly and watering at the base are free changes that reduce humidity around foliage.
Removing and disposing of infected material promptly costs nothing but attention, and it limits how far spores can travel to healthy plants nearby.7 Biological approaches to pest and disease control are also being explored as ways to reduce reliance on chemical treatments.7
Common mistakes to avoid
Don't compost infected foliage, fruit, or tubers; remove and destroy affected material instead, since the pathogen spreads through infected plant matter.7
Don't crowd your tomato and potato beds; good airflow helps foliage dry out faster, reducing the humid conditions the pathogen needs, especially during a Smith Period of sustained warm, damp weather.
Copper fungicides work as a protective measure applied before infection, not as a cure once lesions and rot are already visible.
A note on regional risk
Late blight doesn't show up everywhere every year, but risk levels shift with the weather season by season and region by region.1,,3 Monitoring local outbreak reports and weather patterns during warm, humid stretches gives you a head start on protecting your plants before symptoms appear.3
Quick-reference checklist
- Learn to recognize dark, water-soaked leaf lesions and white mold on leaf undersides early.
- Watch for warm weather above about 10°C paired with 90%+ humidity lasting roughly 48 hours.
- Choose resistant tomato and potato varieties when planting.8
- Space plants for airflow and water at the base, not overhead.
- Remove and destroy (never compost) any infected foliage, fruit, or tubers.7
- Clean tools between plants during humid weather.
- If using copper-based fungicide, apply before symptoms appear and always follow the product label; keep away from children, pets, and waterways.
- Check local blight risk reports during warm, wet stretches of the season.3