Potato blight is the disease that can end a maincropThe main, usually latest-sown and longest-growing type of a crop, grown for bulk and storage rather than an early harvest. Full definition → in a fortnight. It thrives in the warm, wet conditions Scotland supplies so reliably, and there is no amateur fungicide that stops it. Here is what to watch for, when to act, and the controls that actually give you a crop.
What does it look like?
On leaves: Rapidly spreading, watery rot of the leaves, which soon collapse, shrivel and turn brown.
On stems: Brown lesions on stems.
On tubersA swollen underground stem or root a plant uses to store energy, which can also grow into a new plant, the classic example being a potato. Full definition →: Reddish-brown decay below the tuber skin, progressing to soft rot as bacteria colonise the affected tissue.
What causes it
Infection needs prolonged surface wetness, several hours of it, rather than just a passing shower, which is why a spell of settled damp weather is the real trigger rather than any single wet day.
Potato blight and tomato blight are the same organism, Phytophthora infestans. If you also grow tomatoes, a wet spell that starts an outbreak here is a real risk to them too, especially outdoors, so check them as well. See Tomato Blight →
Life cycle
Overwinters: Survives winter in affected potato tubers left in the ground.
Spread: Wind-blown spores from other gardens and commercial crops are the primary infection source for home gardeners.
Because spores blow in, a clean garden is not a safe garden. This is why forecasting matters more than hygiene alone.
🏴 The Hutton Criteria: Scotland's blight warning system
The UK's national blight warning system was developed in Scotland, at the James Hutton Institute, and is named after it.
A Hutton Criteria is met when there are two consecutive days with a minimum temperature of 10C and at least six hours of relative humidity at or above 90%.
What it replaced: It superseded the Smith Period, a 1950s model unchanged for around 60 years, which required at least 11 hours at 90%+ relative humidity. Blight had been appearing on crops before a Smith Period was recorded, so the older model was missing outbreaks.
Validation: Tested against over 2,000 historic reports of potato late blight outbreaks, the Hutton Criteria showed a significant improvement in predicting outbreaks.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
Chemical control
RHS: 'There are currently no fungicides available for use by gardeners' against blight.
What actually works: No fungicide will save a crop once blight takes hold, so this is won earlier: choose a resistant variety, keep to a rotationMoving each crop family to different ground each year, on a multi-year cycle, so pests and diseases that build up in soil don't get a repeat host. Full definition →, and watch the Hutton Criteria forecasts closely enough to strip foliage before infection spreads.
Grow blight-resistant varieties
Athlete, Alouette, Carolus and the Sarpo varieties (Sarpo Axona, Sarpo Mira) all carry useful blight resistance, though none is completely immune.
Cultural controls: 4-year rotation, earth upDrawing soil up around a plant's base as it grows, most often potatoes, to keep light off developing tubers and support tall stems. Full definition → tubers, harvest early-maturing varieties, remove foliage once ~25% of leaves are infected, dispose of infected material by burial, green waste collection or burning — never compost
Removing foliage at 25% infection is the decisive intervention: it stops spores reaching the tubers. Pair this with the Hutton Criteria: the forecast tells you when to look, the 25% rule tells you when to act.
Use tubers from a blighted crop promptly; don't store them
RHS: 'Use the tubers from blighted crops as soon as possible, and avoid storing them if at all possible.'
This answers the question most gardeners actually have once blight has appeared: can I still use what's in the ground? Tubers from an infected crop are still usable, but don't keep them — eat them soon after lifting rather than storing them through winter as you would a healthy crop.
Monitor blight risk forecasts (Hutton Criteria)
Monitor using BlightSpy forecasts. The underlying UK risk model is the Hutton Criteria.
Scottish grower notes
In Scotland a major early source of blight is discarded potato ‘dumps’ and self-sown volunteers from last year's crop — they sprout early and can trigger an outbreak, so clearing them reduces the risk to your plants. And no potato variety is completely blight-resistant: resistant types lower the risk but don't remove the need to watch the forecast.
Farm Advisory Service (FAS) Scotland. Written for growers, but the cultural advice (clear dumps and volunteers, no variety is fully resistant) applies to gardens too.
Myths about potato blight
7 common claims about potato blight, checked: 4 busted1 disputed1 untested1 hold up
“Spraying potatoes with a copper product like Bordeaux mixture will cure or prevent blight”
There is currently no fungicide of any kind, copper-based or otherwise, sold to home gardeners for use against potato blight in the UK. That includes Bordeaux mixture and similar copper products, which growers once used to coat leaves ahead of an outbreak; even at that, they were a preventative barrier rather than a cure, and did nothing for a plant already showing symptoms.
What to do: Skip the search for a blight spray and put the effort into forecasting risk, choosing resistant varieties and stripping infected foliage the moment it appears, since no product on general sale does the job.
1 source
- RHS: Potato and tomato blightThere are currently no fungicides available for use by gardeners against blight.
“Blight is only a problem in wet years; a dry summer is safe”
Infection needs surface wetness lasting several hours, not a whole rainy season, so a single humid, dewy or drizzly spell can start an outbreak even inside a summer that is dry overall. Wetter summers do carry more risk simply because damp spells happen more often, but that is different from saying a dry-looking year is safe.
What to do: Judge risk spell by spell through the growing season rather than by the summer's general character, and check plants closely after any stretch of warm, damp weather even in an otherwise dry year.
1 source
- RHS: Potato and tomato blightInfection requires several hours of prolonged surface wetness, which is why the disease is worse in wet summers.
“Blight-resistant varieties like the Sarpo types are immune to blight”
Named resistant cultivarsA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition → carry real, useful resistance, but none is immune; they can still develop blight if favourable wet conditions persist for long enough. Resistance is also not permanent: some varieties once sold as resistant no longer perform that way because the pathogen itself has changed over time.
What to do: Grow a resistant variety to lower the odds and buy extra time, but keep checking for symptoms rather than assuming a resistant crop can be left unwatched.
In Scotland: A Scottish agricultural advisory source states plainly that no current variety is totally resistant, for either leaf or tuber blight.
2 sources
- RHS: Potato and tomato blightResistant cultivars such as the Sarpo varieties are not completely immune and may still develop blight in favourable conditions.
- Farm Advisory Service (FAS) Scotland: Potato diseasesNo current variety is totally resistant to blight, though varieties differ in their level of leaf and tuber resistance.
“If tubers from a blighted crop look clean with no blemishes, they're safe to store as usual”
Tuber blight begins as decay just below the skin before it turns into visible soft rot, so a tuber can already carry the infection while still looking sound on the outside at harvest. That is why the standard advice for a blighted crop is to use the tubers promptly and avoid storing them at all where possible, rather than storing them through winter as usual.
What to do: Treat tubers lifted from a blight-affected crop as a use-soon batch rather than a long-term store, even when none of them show visible marks when you dig them up.
1 source
- RHS: Potato and tomato blightTuber blight shows as reddish-brown decay below the skin, progressing to soft rot, and advises using tubers from blighted crops as soon as possible and avoiding storage where possible.
“Rotating where you grow potatoes each year protects the crop from blight”
Rotation is recommended for blight, but for a narrower reason than most gardeners assume: it targets resting spores that can persist in the soil, not the wind-blown spores that account for the great majority of garden infections. Because most new outbreaks arrive on the wind from other gardens, allotments and farms rather than from last year's potato bed, rotation closes off one route while leaving the dominant one wide open.
Our read This is a real difference from a soil-borne disease like clubrootA soil-borne fungal disease of the brassica family that swells and distorts roots, and survives in soil for years. Full definition →, where staying off infected ground for years is the main defence. For blight, rotation is a minor supporting measure, not the primary one.
What to do: Keep a rotation for general soil health and other diseases, but rely on forecasting, resistant varieties and prompt removal of infected foliage for the bulk of blight protection, since rotation alone won't stop spores blowing in from elsewhere.
In Scotland: Discarded potato dumps left over winter are flagged as a key early blight source for Scottish growers, so clearing those matters at least as much as which bed potatoes go into.
2 sources
- RHS: Potato and tomato blightRecommends a rotation of at least four years to reduce risk from soil-borne resting spores, while noting the great majority of garden infections come from wind-blown spores from other gardens and commercial crops.
- Farm Advisory Service (FAS) Scotland: Potato diseasesIdentifies discarded potato dumps as the key early source of blight inoculum.
“A homemade baking soda (sodium bicarbonate) spray controls potato blight”
A university-compiled bibliography of sodium bicarbonate research covers a long list of true fungal diseases, powdery mildews, apple scab, early blight and postharvest rots among them, but contains no study on late blight specifically. That gap is telling: potato blight is caused by an oomycete, not a true fungus, so the mechanisms documented for bicarbonate against fungal pathogens were never tested against the organism that actually causes this disease.
Our read Absence of a study is not the same as a disproven claim, but it means there is no research base to point to for this specific use, only research on a biologically different group of pathogens.
What to do: Don't rely on a baking soda spray to protect a potato crop from blight. There is no evidence it works against this particular organism, and blight can wreck a crop within days once conditions are right.
1 source
- Washington State University Extension: Literature on Baking Soda (Linda Chalker-Scott)Compiles decades of sodium bicarbonate fungicide research across many true fungal diseases, with no entries addressing late blight or Phytophthora.
“Earthing up potatoes properly protects the tubers from blight”
Both RHS and Garden Organic confirm earthing up (or mulchingA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → thickly) reduces tuber infection by creating a soil barrier against blight spores washed down from infected foliage. It reduces, rather than eliminates, tuber infection risk.
What to do: Keep potatoes well earthed up or mulched, especially once foliage shows blight symptoms, to cut down spore contact with developing tubers underneath.
2 sources
- RHS — Potato blightEarthing up potatoes well provides some protection to the tubers from blight spores washed down into the soil from lesions on the leaves or stems.
- Garden Organic — Potato blightEarthing-up potatoes, or mulching the soil with a thick layer of hay, straw or other organic material, can reduce the levels of tuber infection.
Frequently asked questions
What should I do first if I find potato blight in my garden?
Start with grow blight-resistant varieties. After that, cultural controls. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Is there a spray that works?
No. There are no amateur chemical treatments available for potato blight. The controls that work are cultural: the measures listed above.
How much of this advice is actually proven?
Every control listed above is evidence-based, not just repeated folklore. None of it is a guess.
Sources
2 sources, recorded with what each was used for
- RHS1: causal organism, symptoms on leaves/stems/tubers, warm wet conditions and surface wetness requirement, June onset, overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → in tubers, wind-blown spore spread, no amateur fungicides, resistant cultivars, cultural measures, forecast monitoring
- FAS Scotland14: potato dumps and self-sown volunteers as a key early blight source in Scotland; no fully blight-resistant variety