Late blight is the tomato disease growers fear most, because it can turn a healthy plant into a collapsed, blackened mess within about a week once the weather favors it. Check the undersides of your leaves right now, both low and high on the plant, for water-soaked olive-brown blotches and a faint white mold, and pull any plant with lesions on the stem before you do anything else.
What you are looking at
Late blight is serious, and it moves fast: a single infected leaf can turn into a collapsed, blackened plant within about a week when conditions cooperate. Unlike most tomato diseases, it does not wait for the plant to weaken first. It can start on a vigorous, otherwise healthy plant, and it can start high in the canopy rather than at the bottom.
Water-soaked leaf lesions
The first sign is usually a pale to dark olive-brown blotch on a leaflet, with a soft, water-soaked look rather than a dry, papery one. There is no yellow ring around it, which is one of the first things that separates it from early blight. These lesions can appear on lower leaves or upper ones, since the spores that cause late blight arrive on the wind rather than splashing up from the soil. A lesion that covers a whole leaflet within two or three days is a strong sign you are dealing with this disease rather than a slower one.
White fungal growth on leaf undersides
Flip a suspect leaf over on a cool, humid morning and look at the underside edge of the lesion. A thin, fuzzy white to gray growth there is the pathogen actually producing spores, and it is the single most reliable field sign of late blight. It disappears once the leaf dries out in the sun, so check early before the dew burns off.
Blackened stem lesions
As the disease advances, dark brown to almost black lesions appear on stems and leaf petioles. They start firm and turn greasy-looking, and once one girdles a stem, everything above that point wilts and dies within a day or two. This is the point at which the plant cannot be saved.
Greasy brown fruit rot
On fruit, look for large, firm, brown to olive-green blotches, often starting near the stem end. The surface looks greasy and water-soaked rather than sunken or fuzzy with mold, and the rot can cover most of a green tomato while the fruit is still hard.
What it is not
Three other tomato problems get mistaken for late blight often enough that the mix-up is worth walking through, because the response to each is completely different. Treating a nuisance disease like an emergency wastes plants you could have kept, and treating an emergency like a nuisance costs you the whole row.
Septoria Leaf Spot
Septoria produces many small, round spots with dark margins and tan or gray dry centers, each with tiny black specks visible in the middle once you look closely. It starts on the lowest, oldest leaves and works upward slowly, over weeks rather than days, and it never produces the white mold late blight does. Septoria Leaf Spot: The One People Mistake for Blight is a nuisance you manage by removing affected lower leaves and improving airflow, not an emergency that calls for pulling the plant.
Early Blight
Early blight forms lesions with visible concentric rings, like a target, surrounded by a narrow yellow halo. It also favors the oldest leaves first and spreads gradually rather than overnight. The yellow halo and target rings are the tell: late blight lesions are plain brown blotches with neither.
Verticillium Wilt
Verticillium causes one-sided wilting and a V-shaped yellow patch spreading in from the leaf edge, with no lesion on the leaf surface at all. Cut a wilting stem lengthwise and look for a light brown ring just under the skin. That vascular discoloration points to Verticillium Wilt on Tomatoes: The Slow Wilt That Comes Back Every Year rather than late blight, which never produces that ring because it does not infect the vascular system.
The mistake that costs the most is calling late blight something slower than it is. If you wait a week to see whether a lesion spreads the way septoria does, you may not have a planting left to treat.
Where it comes from and when it strikes
Late blight is caused by Phytophthora infestans, an oomycete rather than a true fungus, though it behaves like one in the garden.
How it survives and spreads
The pathogen does not persist for long as a free organism in bare soil. It overwinters mainly in infected potato tubers left in the ground, in volunteer potato or tomato plants that sprout from missed tubers or dropped fruit, and in plant debris left over a mild winter. From there it spreads by producing enormous numbers of microscopic spores called sporangia, which travel on wind currents. This is why late blight can turn up in a garden that grew no tomatoes the previous year and had no obvious local source, since the spores can travel a long way before landing on a new host.
The weather that lets it take hold
Infection needs free moisture sitting on the leaf surface, so extended stretches of dew, fog or rain matter more than any single storm. The disease favors cool, damp weather, closer to the conditions of a rainy early spring or a wet autumn than the heat of midsummer, which sets it apart from many other tomato diseases that prefer warm, humid conditions. Once spores land on a wet leaf, they can germinate and infect within hours, and a run of overcast, humid days with little drying sun is when an outbreak accelerates fastest. If you grow more than one crop susceptible to this family of pathogens, check your notes on tomato diseases from past seasons, since a garden that had one bad late blight year is more likely to see spores arrive again when a neighbor grows potatoes or tomatoes nearby.
What to do about it now
Act on the day you find it, not the next weekend. Late blight moves fast enough that a few days of hesitation can turn one infected plant into a lost row.
Remove what is already infected
Pull any plant with stem lesions immediately. A stem lesion means the disease is already inside the plant’s tissue, and there is no treatment that reverses that, the same way a wilt cannot be cured once it has set in. Bag the whole plant, roots and all, and dispose of it in household trash or by burying it deep. Do not compost it and do not leave it in a pile at the edge of the garden, since the pathogen can keep producing spores on dead tissue for a short time.
Slow it down on what is still healthy
Stop overhead watering and switch to soil-level irrigation, since wetting the foliage is exactly what the pathogen needs to infect new leaves. Space out remaining plants if you can, and prune lower leaves and suckers to open up airflow so foliage dries faster after rain or dew. A fungicide containing chlorothalonil or copper, applied to plants that are not yet showing symptoms, can slow new infections, but it will not cure a lesion that has already formed. Reapply after rain, following the label’s schedule.
What about the harvest
Green or ripe fruit without lesions is safe to pick and eat; the disease affects quality and shelf life on infected fruit, not food safety on unaffected ones. Fruit already showing the greasy brown blotches should be discarded, since the rot continues after picking and the tissue underneath is usually already breaking down. If a plant is close to producing a full crop when you find the first signs, some growers pick all the mature green fruit immediately and let it ripen indoors, away from the infected plant, rather than risk losing the harvest along with the foliage.
Keeping it out next season
Because Phytophthora infestans does not linger long in bare soil without living tissue to survive on, rotation matters less here than it does for a soil-borne disease, but it still helps by starving out volunteer plants and leftover debris. Pull any volunteer potato or tomato plants that sprout from last year’s dropped tubers or fruit, since those are a common overwintering source.
Resistant cultivars
Several cultivars carry bred-in resistance, marked on the seed packet with a letter code. ‘Iron Lady’, at roughly 75 to 80 days to maturity, combines resistance to late blight, early blight and septoria. ‘Mountain Magic‘, at roughly 75 days, resists late blight and fruit cracking in a cherry-sized fruit. ‘Defiant PhR‘, at roughly 70 days, carries the PhR code specifically for Phytophthora resistance. ‘Plum Regal‘ is a paste tomato bred for late blight resistance as well. Days to maturity on a seed packet are counted from transplant, not from seed, and run a week or two either way depending on the season, so cross that figure against your own last-frost window from your state extension service rather than treating it as a fixed date.
| Cultivar | Days to maturity | Resistance code |
|---|---|---|
| Iron Lady | 75-80 days | LB, EB, Septoria |
| Mountain Magic | 75 days | LB, cracking |
| Defiant PhR | 70 days | PhR (Phytophthora) |
| Plum Regal | 75 days | LB (paste type) |
Sanitation and spacing
Clean stakes and cages with a diluted bleach solution before storing them over winter, since spores and other pathogens can ride over on equipment. Lay down mulch at planting to cut down on soil splash, space plants for airflow, and prune the lowest leaves once the plant is established so foliage dries quickly after rain. Buy transplants from a source that inspects for disease rather than picking up leggy, unlabeled seedlings at a discount table, since infected transplants are one of the most common ways this disease arrives in a garden that had none the year before.
Common questions
Can I save a plant if I catch late blight early enough?
You can usually save the planting, not the plant. Remove and destroy the infected plant right away and treat the rest of the row with a protectant fungicide before they show symptoms, since that is what actually changes the outcome once one plant is confirmed infected.
Does late blight come back in the soil every year the way verticillium does?
Not in the same way. The pathogen needs living tissue to survive winter, so it mainly overwinters in infected potato tubers, volunteer plants, or debris rather than persisting indefinitely in bare soil the way a wilt-causing fungus does.
Can late blight spread from my potatoes to my tomatoes, or the other way around?
Yes, potatoes and tomatoes share the same pathogen and can infect each other. Planting them near one another, or leaving volunteer potatoes near a tomato row, gives the disease an easy bridge between the two crops.
Will spraying copper fungicide cure a plant that already has lesions?
No. Copper and chlorothalonil sprays only protect healthy tissue that has not been infected yet. They do not reverse a lesion that has already formed, so any part of the plant already showing symptoms is lost regardless of what you spray afterward.