Late blight is the most serious tomato disease a home gardener will face, and it is the one that demands the fastest response. It can destroy a whole tomato crop — and a potato crop — in a matter of days, turning green leaves black and fruit to rot. It is the same disease that caused the Irish potato famine, and it is still a major problem wherever tomatoes and potatoes are grown in cool, wet weather. The good news is that it is preventable with good habits, and the bad news is that once it is established, there is very little that will save the crop.
This guide explains the symptoms, the conditions that favour late blight, cultural prevention, what to remove, what not to compost, and the urgent steps to take if it appears. If you are not sure the spots on your tomatoes are late blight, the plant disease checker can help you compare, and our early blight on tomatoes guide covers the slower, less serious disease it is most often confused with.
Symptoms
Late blight starts as water-soaked, grey-green spots on the leaves, usually on the upper leaves first, often after a period of cool, wet weather. The spots grow quickly, turning dark brown or black, and in humid conditions there is a white, downy growth on the underside of the leaf, at the edge of the spots — the fuzz is the best identification clue. The leaves blacken and die, the stems develop dark, greasy-looking lesions, and the whole plant can collapse within days. The fruit shows firm, dark, greasy-looking patches that spread and rot, and the fruit is inedible.
The key difference from early blight is the pace and the pattern: early blight has concentric-ring spots on the lower leaves and progresses slowly over weeks, while late blight has water-soaked spots with no rings, a white fuzz on the leaf underside, and it can destroy a whole plant in days. If your tomatoes have blackened leaves and greasy-looking fruit after wet weather, act immediately — do not wait to see if it gets better, because it will not.
Conditions that favour it
Late blight is favoured by cool, wet weather — temperatures of roughly 15–20°C with rain, dew or high humidity, and leaves that stay wet for many hours. It spreads by spores carried on the wind, and it can travel long distances, so an outbreak in one garden can infect tomatoes and potatoes across a whole neighbourhood. It overwinters in infected potato tubers left in the soil, and it can also survive on tomato debris. It does not survive long in the soil without a living host, which is why clearing debris and removing volunteer potatoes matters so much.
The disease is most common in late summer, when the weather turns cool and wet, and it is worst in crowded, shaded beds where the leaves stay wet. Overhead watering, rain splash and poor airflow all make it worse, and a plant that is already stressed is more vulnerable.
Cultural prevention
Cultural prevention is the only reliable control, and it starts before the disease appears. Space tomato plants generously so air moves freely between them, and stake or cage them so the leaves are off the ground. Water at soil level, in the morning, so the leaves stay dry, and avoid overhead watering entirely. Mulch to prevent soil splash, and prune for airflow, removing crowded stems and the lower leaves.
Choose resistant varieties where they are available — many modern tomato varieties are bred for late blight resistance, and this is the single most effective step in a region where the disease is common. Your local extension service or seed supplier can advise on what suits your area. Rotate tomatoes, potatoes, eggplant and peppers to fresh ground each year, and do not grow them in the same bed more than once in three or four years. Do not plant tomatoes near potatoes, since the disease moves easily between them. Clear away all tomato and potato debris at the end of the season, and remove volunteer potato plants in spring, since they can carry the disease over winter.
What to remove, and what not to compost
If late blight appears, remove the affected plants immediately — the whole plant, not just the leaves — and put them in the bin or a sealed bag. Do not compost them, and do not leave them on the ground, because the spores spread rapidly to neighbouring plants. Do not compost any infected material unless your heap reliably stays hot enough to kill the spores — and if you are unsure, the bin is the safe choice. Check your local extension service for composting and disposal guidance in your area, since in some regions there are specific rules for disposing of late blight material.
At the end of the season, clear away all tomato vines, leaves and fallen fruit, and all potato haulm and tubers, and do not leave them in the bed over winter. Wash your hands and tools after handling infected plants, and avoid working with tomatoes when the leaves are wet.
Treatment, in order
1. Act immediately. Late blight does not wait. The moment you see water-soaked spots with white fuzz on the leaf underside, remove the affected leaves, and if the plant is badly affected, remove the whole plant. Check the neighbouring plants daily, and remove any that show the first signs. The goal is to slow the spread long enough to harvest what is ripe.
2. Protect the healthy plants. Copper-based fungicides can protect healthy leaves if applied early and thoroughly, before the disease arrives — they are protectants, not cures, and they do not cure leaves that are already infected. In a region where late blight is common, a preventative copper spray in wet weather can help, but it needs reapplying after rain, and copper accumulates in the soil, so use it sparingly. Always read the label, apply in the evening, and check your local extension service for products approved in your area.
3. Harvest early. If late blight is spreading in the area, pick green tomatoes that are full-sized and let them ripen indoors, away from the disease. A tomato picked green and ripened indoors is better than a tomato left on a dying plant. Do not store fruit that shows any sign of blight — it will rot and can spread the disease.
4. Know when to stop. If the disease is widespread and the weather stays cool and wet, the honest answer is to remove the crop, clear the bed, and plant something else. No spray will save a badly infected tomato crop, and keeping it only spreads the disease to the neighbours' tomatoes and potatoes. Remove the plants, bag them, and clean the bed.
Frequently asked questions
Will late blight kill my tomato plants? Yes, and quickly. A badly infected plant can collapse within days, and the fruit rots. The disease is the most serious tomato disease in cool, wet weather, and it needs an immediate response.
Is late blight the same as early blight? No. Early blight has concentric-ring spots on the lower leaves and progresses slowly over weeks. Late blight has water-soaked spots, a white fuzz on the leaf underside, and it can destroy a whole plant in days. See our early blight on tomatoes guide for the comparison.
Can I eat tomatoes from a plant with late blight? No — fruit showing blight spots is inedible and should be discarded. Green fruit picked before the disease reaches it, and ripened indoors, is safe to eat if it shows no sign of blight.
Should I compost tomato plants with late blight? No. Put infected plants in the bin or a sealed bag. The disease spreads rapidly, and the spores can survive in a cool compost heap. Check your local extension service for disposal guidance in your area.
How do I prevent late blight next year? Choose resistant varieties, space plants for airflow, water at soil level, rotate the whole tomato family, clear away all debris, remove volunteer potatoes, and do not plant tomatoes near potatoes. Check your local extension service for resistant varieties and for late blight warnings in your area.
Late blight is a disease where speed and honesty matter more than any spray. Learn the signs, check the plants after wet weather, remove the affected plants at once, and choose resistant varieties next year — and the crop will stand a far better chance. For the slower tomato disease, see our early blight on tomatoes guide, and for the related fruit problem, our blossom end rot guide. For general help, use the plant disease checker.