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The August my tomatoes went black, the potatoes two beds along went the same way about four days later, which is how I learned the hard way that they are the same disease. I made every mistake available. I dithered for a week hoping it would pass, then dug the lot up in a panic while the soil was still covered in spores, and half of what I stored turned to brown slime in the sack by October. Nothing teaches you the right sequence quite like opening a sack of rotten potatoes in a cold shed. This guide is that sequence, in the order it actually needs doing.

What Potato Blight Is and Why It Spreads So Fast

Potato blight, or late blight, is caused by Phytophthora infestans, a fungus-like organism rather than a true fungus, which is why many ordinary garden fungicides do nothing against it. It is the same organism that causes tomato blight, and it moves freely between the two crops, which is why keeping them apart matters. It is also the disease behind the Irish potato famine, which tells you something about how destructive it can be when conditions suit it. Spores blow in on the wind, land on wet foliage, and germinate in warm humid weather. Two consecutive days of minimum temperatures above 10C with long spells of high humidity is the classic trigger, conditions a British summer produces reliably from July onwards. Once infection starts in a damp crop, it can go from a few spots to a collapsed bed in under a week.

How to Recognise Potato Blight

It starts on the leaves as dark brown or blackish patches, often at the leaf tips and edges first, frequently with a pale yellowish-green halo around them. In humid weather, look at the undersides of affected leaves in the morning and you may see a faint white fuzzy growth, which is the organism sporulating. From there it moves fast: whole leaves brown and shrivel, dark streaks appear on the stems, and the plant takes on a scorched look, as though someone has gone over it with a blowtorch. There is often a distinctive musty, rotten smell to a badly affected bed on a still day.

Below ground, infected tubers develop sunken dark patches on the skin, and cutting one open shows a reddish-brown rusty marbling spreading in from the surface. Blighted tubers frequently go on to develop soft, foul-smelling secondary rots in storage, which is where most of the damage is actually done. Do not confuse blight with the ordinary yellowing and dying back of haulms at the end of the season, which is simply the crop finishing, or with the slower spotting of early blight, a different and far less serious disease. Blight is fast and it stinks. If in doubt, look for how quickly it is spreading, because that is the tell.

What to Do the Moment You See It

Act the same day. Cut every bit of foliage and stem off at ground level, and do not leave the cuttings lying on the bed. Bag them and put them in the household waste or take them to the tip. Do not compost blighted haulms, because a garden heap does not get hot enough to reliably kill the spores, which is worth remembering even if you are otherwise keen on composting at home. Removing the foliage stops the disease travelling down the stems into the tubers, and that is the whole point of doing it quickly.

Then leave the tubers in the ground. This is the part people get wrong, myself included that first year. Spores from the dead foliage are sitting on the soil surface, and if you dig immediately you drag your potatoes up through them and infect the crop on its way out. Wait two to three weeks. The spores on the surface die off, the skins on the tubers set firmer, and what you lift then is far more likely to store. When you do dig, do it on a dry day, leave the potatoes on the surface for a few hours to dry, and inspect every single one. Store only clean, firm, unmarked tubers, because one infected potato will rot an entire sack. Anything with a sunken dark patch gets eaten straight away if sound inside, or binned.

The Truth About Spraying

For years UK gardeners protected potatoes with copper-based or mancozeb sprays applied before infection. Those products are no longer available to amateur gardeners here, and in any case they were preventatives that had to be on the leaves before spores arrived, never cures for an infected plant. Commercial growers still spray, on a schedule, with products and equipment not sold to home gardeners. So if you are searching for a potato blight treatment or the best fungicide for blight, the honest answer for a UK home garden is that there is not one worth buying. Home remedies do not work against this organism either. That is not a counsel of despair, because the cultural methods below genuinely do work, but it does mean the money is better spent on the right seed potatoes than on a bottle of anything.

The Early Potato Escape Route

The most reliable defence in a British garden is timing, not chemistry. Blight arrives from mid-July onwards in most years. First earlies planted in March are ready to lift from late June, which means the crop is out of the ground and in the kitchen before the disease turns up at all. In twenty years I have almost never lost a first early to blight, and I have lost plenty of maincrops. Second earlies mostly get away with it too. Maincrops are the vulnerable ones, sitting in the ground through August and September precisely when conditions favour the disease. So if blight is a recurring problem on your plot, the simplest fix is to grow more earlies and fewer maincrops, which is worth thinking about when you order your seed potatoes in winter. My guide to when to plant potatoes covers the timings for each type.

Blight Resistant Varieties

If you want maincrop potatoes and you garden somewhere wet, resistant varieties are the real answer. The standouts by a distance are the Sarpo family, bred in Hungary and trialled in Wales: Sarpo Mira and Sarpo Axona have the strongest late blight resistance available to gardeners, scoring top marks in commercial evaluations, with the bonus of drought tolerance and vigorous foliage that smothers weeds. They are floury maincrops, excellent for mash, chips and baking, and they store for a long time. Cara, Valor and Setanta also carry useful resistance without being quite in the same class. Availability moves around between suppliers and seasons, so check who has them when seed potato stock lands from around December, and order early because the resistant varieties sell out first in a year following a bad blight summer.

One honest caveat, the same one that applies to tomatoes: the blight organism keeps evolving, and new strains have already eroded the resistance of some older varieties. Resistance is a very strong advantage rather than a permanent guarantee. Pair it with the habits below and you have covered everything a gardener realistically can.

Reducing the Risk Next Year

A few practical habits make a real difference. Earth up generously, because a deep covering of soil stops spores washing down from the foliage onto the tubers, and shallow-covered potatoes are the ones that rot. Space plants properly so air moves through and foliage dries quickly after rain, and water the soil rather than the leaves. Keep potatoes and tomatoes well apart, and never plant either where blighted potatoes grew the previous year. Clear every last tuber at harvest, because the disease overwinters mainly in living potatoes left in the ground, and next spring those volunteers come up carrying it. Rotate where you grow them, and never plant supermarket potatoes or your own saved tubers from a blighted crop, since certified seed is the one place not to economise. Get all this right and you will still lose the odd year to a filthy August, but you will keep cropping when others do not. More on the basics in how to grow potatoes and common potato problems.

Frequently Asked Questions

A Sensible Place to Start

If blight has just hit your potatoes, do three things today: cut the foliage off at ground level, bag it and bin it, and then leave the tubers alone for a fortnight. That sequence saves more crops than anything else in this guide, and it is exactly what I failed to do the year I lost half a sack to rot in storage. Then plan next year around timing rather than treatment: more first earlies lifted in June, a resistant maincrop like Sarpo Mira if you want potatoes for storing, generous earthing up, and not a single tuber left in the ground at harvest. Blight is a weather disease and some summers will beat you whatever you do, but those few decisions turn it from a disaster into an inconvenience.

This article contains affiliate links. If you buy through one of my recommended links I may earn a small commission at no extra cost to you. I only recommend products I genuinely think are worth buying.

I lost an entire outdoor row to blight one August in Oxfordshire, the kind of wet muggy August we get every few years, and it happened so fast I genuinely thought someone had poured weedkiller over the bed. One evening the plants were fine, three days later they were black and stinking. That row taught me more than any success ever did, and almost everything I now do with tomatoes, from where I plant them to which varieties I choose, traces back to that week. So this is not a cheerful guide, but it is an honest one, and the honest version is the one that saves your crop.

What Tomato Blight Actually Is

Tomato blight is late blight, caused by a fungus-like organism called Phytophthora infestans, the same organism responsible for potato blight and, historically, the Irish potato famine. It is not strictly a fungus, which matters because it means many ordinary fungicides do nothing against it. The spores blow in on the wind and thrive in exactly the conditions a British summer serves up on repeat: warm days, cool nights, high humidity and wet foliage. Outdoor tomatoes are hit hardest because rain lands on the leaves and the wind carries the spores straight in. Greenhouse plants are much safer, simply because the spores struggle to reach them, though an open door or vent on a muggy day can still let blight in.

How to Tell If It Is Actually Blight

Half the plants people declare dead from blight have something far less serious, so before you pull anything up, look properly. Blight has a distinct signature. It starts as irregular brown or grey-green watery patches on the leaves, often with a paler yellowish halo around the edge, and it spreads fast, in days not weeks. In humid weather you may see a faint white fuzzy growth on the undersides of affected leaves, which is the organism itself sporing up. From the leaves it moves to the stems, leaving dark brown or black streaks, and finally to the fruit, which develops firm brown leathery patches, usually starting on the shoulders where spores land from above. The overall impression, and this is the tell every experienced grower recognises, is of a plant that looks scorched, as though it has been hit by frost or a blowtorch.

What it is often confused with matters just as much. Blossom end rot is a sunken brown patch on the bottom of the fruit only, with no leaf damage, and it is a watering and calcium problem, not a disease. Simple leaf yellowing from the bottom up is usually feeding or watering, covered in my guide to caring for tomato plants. Early blight, despite the shared name, is a different and much slower disease showing as brown spots with concentric rings like a target, and it rarely destroys a plant the way late blight does. If the damage is slow and patchy, you probably do not have late blight. If it is fast and spreading and the plant looks scorched, you do.

Early Blight and Late Blight Are Not the Same Thing

This trips people up constantly, so it is worth being clear. Late blight is the fast, plant-killing disease this article is mostly about, the Phytophthora one. Early blight is caused by a completely different organism, Alternaria, and shows as dark spots with a bullseye pattern of concentric rings, usually on the older lower leaves first. Early blight is a nuisance that reduces vigour and can be managed by removing affected leaves, improving airflow and keeping the plant well fed. Late blight is the emergency. If your searches keep turning up American advice about spraying for blight, bear in mind much of it addresses early blight, which behaves differently, and that the fungicides they name are often not available to UK home gardeners anyway.

What to Do the Moment You See Late Blight

Speed is everything, because you are no longer trying to cure the plant, you are trying to save the crop and protect everything nearby. First, if you have caught it very early on just one or two leaves, remove them immediately, bag them and bin them, do not compost them. Sometimes, in a dry spell that follows, that buys you time. More often the weather stays muggy and it spreads regardless.

Once it is into the stems, accept the plant is finished and act decisively. Cut off and harvest every fruit that is still firm and unblemished, even the green ones, because green tomatoes ripen perfectly well indoors on a windowsill or in a drawer with a banana, and they make excellent chutney besides. Then pull up the whole plant, roots and all, bag it and put it in the household waste or take it to the tip. Do not compost blighted material and do not leave it lying on the plot, because it sheds spores onto everything around it. Wash your hands and tools before touching healthy plants. The goal at this stage is containment: one blighted plant dealt with fast is a disappointment, a whole bed left to rot is a disaster that seeds next year’s trouble too.

The Truth About Sprays and Cures

This is the section people least want to read and most need to. There is no home garden spray in the UK that cures late blight once a plant is infected. The old copper-based preventative sprays that gardeners used for decades are no longer available to amateurs here, and even those were preventatives, applied before infection to slow it, never cures. So when you search for a tomato blight cure or the best fungicide for blight and find products promising miracles, treat them with deep suspicion, because the science is simply not on their side for late blight. Home remedies like milk sprays, bicarbonate and the rest may help very slightly against some other tomato problems, but they will not stop late blight. Anyone selling you a cure is selling you hope, not results. The honest defence is not a bottle, it is everything in the next two sections.

How to Stop Blight Getting a Foothold

Since you cannot cure it, prevention is the whole game, and a few habits genuinely cut your risk. Grow under cover if you can: a greenhouse, polytunnel or even a simple rain cover over outdoor plants keeps the leaves dry and the spores off, and dry leaves are the single biggest factor. Water the soil at the base, never the foliage, and water in the morning so any splashes dry quickly, the same discipline that helps with most tomato troubles, covered in how often to water. Space plants generously and remove lower leaves so air moves freely and the canopy dries fast after rain. Keep tomatoes and potatoes well apart, since blight jumps between them, and never plant tomatoes where blighted potatoes grew the year before. And keep plants strong with steady feeding, because a vigorous plant shrugs off a light early hit far better than a stressed one, which is where good tomato growing from the start pays off.

The One Thing That Actually Works: Resistant Varieties

After that August wipeout, the change that made the real difference was not a spray or a clever trick, it was switching varieties. Modern blight-resistant tomatoes have been bred specifically to withstand late blight, and while no variety is completely immune, a good resistant one will often sail through a wet summer that flattens the old favourites beside it. The standout for UK gardeners is Crimson Crush, an F1 variety bred for genuine blight resistance that still produces large, properly flavoured tomatoes rather than the tough tasteless fruit that early resistant varieties gave you. Others worth knowing are Mountain Magic and the cherry variety Lizzano. If you garden outdoors in a wet part of the country, or you have been burned by blight before, a resistant variety is the best decision you can make. Thompson and Morgan stock Crimson Crush and other resistant varieties as both seeds and young plants, and it is the one seed choice I would not compromise on after what I have seen blight do.

One honest caveat: the blight organism is always evolving, and a variety that resists today’s strains may meet a new one in future, so resistance is a strong advantage rather than a cast-iron guarantee. Combine a resistant variety with the good habits above, and you have done everything a gardener realistically can.

Blight in the Soil and Next Year

A common worry is that blight lurks in the soil ready to strike again, but late blight does not generally survive winter in UK garden soil on its own. It overwinters mainly in living plant material, most often infected potato tubers left in the ground, which is why clearing every last potato at harvest matters. So you do not need to sterilise your soil or abandon the bed. What you should do is practise good potato hygiene, rotate where you grow tomatoes and potatoes, clear all debris at the end of the season, and start next year with resistant varieties and dry-leaf discipline. Blight is a weather disease as much as anything, so some summers will always be worse than others, but a gardener who does these things will keep cropping tomatoes through years that leave their neighbours with black stalks.

Frequently Asked Questions

A Sensible Place to Start

If blight has just hit you, do not waste money on a cure that does not exist. Harvest every firm tomato to ripen indoors, pull and bin the infected plants fast, and keep your tools and hands clean around what is left. Then make next year different: grow a resistant variety like Crimson Crush, keep the leaves dry by watering at the base and giving plants room to breathe, and keep your tomatoes clear of the potato patch. Blight is a genuinely hard disease and some wet summers will test you whatever you do, but these few decisions are the difference between the odd lost plant and a black, empty bed. After that August of mine, I have rarely lost more than a plant or two, and never a whole crop again.