Blight UK | Tomato & Potato Late Blight Guide

The Short Answer

Cause

Phytophthora infestans (oomycete)

Affects

Potato, Tomato

Peak risk

July to September in the UK, in warm wet spells

Can it be cured?

No cure once a plant is infected

Do this first

Remove and bin affected material now — never compost it

Causal Organism

Disease Cycle & Biology

• Caused by the oomycete Phytophthora infestans.
• Spores (sporangia) shed from infected foliage are wind-blown, sometimes long distances; prolonged surface wetness (several hours) is required for infection (RHS).
• The pathogen overwinters primarily in infected potato tubers left in the ground or in cull/waste piles; most garden infections each year arise from wind-blown sporangia originating elsewhere (other gardens, allotments, commercial crops) rather than on-site carryover (RHS, UMN Extension).
• New strains in the UK have the potential to produce soil-contaminating resting spores (oospores), though these have not yet been confirmed in the UK (RHS).
• UMN Extension additionally documents many distinct clonal lineages (e.g. US-23), some host-specific to tomato or potato and others affecting both; and notes hairy nightshade (Solanum physalifolium) and, under glasshouse conditions, Petunia, as additional hosts alongside potato and tomato.
• RHS similarly notes occasional cases on ornamental Solanum (e.g. S. laciniatum) and Petunia.
• Historically responsible for the Irish potato famine of the 1840s (UMN Extension) and still the most economically important disease of potatoes worldwide.

Favourable Conditions

Favourable temperature (UMN Extension)

60°F / 15.5°C 70°F / 21°C

Plus prolonged leaf wetness. Hot, dry weather halts spread (RHS, UMN Extension).

• Warm, wet weather.
• RHS describes "warm, wet weather" generally, while UMN Extension quantifies favourable temperatures more precisely as 60-70F (~15.5-21C) with prolonged leaf wetness -- both sources agree hot, dry weather halts spread.
• UK outbreaks typically begin from June onwards, earliest in the South West (RHS).

Symptoms

Rapidly spreading watery rot with pale leaf-edge sporulation (late blight pattern)

• Watery rot of leaves spreading rapidly, causing collapse, shrivelling and browning within days; lesion edges may appear light green while the pathogen is actively spreading, with fine white fungal growth visible on the leaf underside in humid conditions.
• Brown lesions also develop on stems/petioles.
• On fruit (tomato) or tubers (potato), causes a firm reddish-brown to purple/brown decay beneath the skin that often develops into a soft rot as secondary bacteria invade (RHS, UMN Extension).

How to Tell Blight from Look-alikes

If the growth underneath is greyish-brown and the stems and fruit are clean, it is leaf mould rather than blight — a humidity problem, not a reason to pull the plant. Descriptions from Clemson Cooperative Extension (HGIC) and the RHS.

At a Glance

Affected Crops

Potato

severe

Solanum tuberosum

UK-wide, especially warm wet summers; outbreaks typically from June, earliest in the South West

The most serious and damaging disease of potatoes (RHS). The same pathogen causes tomato blight, so the two crops share the risk and should never follow one another in a rotation.

Tomato

severe

Solanum lycopersicum

UK-wide outdoors especially; less common but still damaging under glass

Retroactively linking tomato (already in graph) to the same Late blight disease entity now that potato -- the other major host -- has been added. RHS/UMN both confirm Phytophthora infestans is the identical pathogen causing blight on both crops.

Treatments

Sanitation / prompt removal of infected material

cultural

For Late blight: RHS: infected material should be deeply buried (below cultivation depth), sent for council green-waste collection, or burned -- NOT composted, since home composting may not reach temperatures high enough to kill resilient oospores. For tomatoes, also clean plant supports/equipment/glasshouse surfaces with disinfectant. Sources: RHS potato-blight and tomato-blight.

General mechanism

Removes spore sources; RHS advises careful bagging of affected parts before/immediately after removal, as spores disperse easily when disturbed.

Crop rotation (multi-year, avoid replanting same-family crops)

cultural_sanitation

For Late blight: RHS: rotate at least 4 years, keeping tomatoes out of ground that grew potatoes (and the reverse) throughout that period, to reduce risk from soil-borne resting spores. UMN Extension advises planting tomatoes where no tomatoes, potatoes, peppers or aubergines have grown for 3-4 years. Sources: RHS potato blight and tomato blight guidance; UMN Extension late blight.

General mechanism

Growing potatoes/tomatoes (or other susceptible crops) in a new position each year, avoiding the same ground for a minimum period, reduces carryover of soil-borne resting spores, cysts and other soil-persistent inoculum. RHS recommends at least 4 years for blight (also avoiding tomatoes in that soil during the rotation) and a minimum of 5-6 years for potato cyst nematode where a problem is known; UMN Extension independently recommends 3-4 years for blight for the tomato/potato/pepper/aubergine group specifically. Distinct from the existing "Reservoir-host, weed, and inoculum-source management" entity, which covers removing volunteer plants/weeds rather than the land-use rotation itself.

Resistant/tolerant cultivar selection

cultural_genetic

For Late blight: RHS lists potato cultivars currently showing good blight resistance (Athlete, Alouette, Carolus, Sarpo Axona, Sarpo Mira) versus highly susceptible ones (Arran Pilot, King Edward, Majestic, Sharpe's Express); resistance is partial and can be overcome by new pathogen strains, not full immunity. For tomato, RHS names 'Crimson Crush' and 'Mountain Magic' as cultivars that performed well in a severe-blight year (Wisley, 2021). UMN Extension corroborates that even resistant varieties will show some symptoms under highly favourable conditions, and points to Cornell's disease-resistant vegetable variety list as a living reference. Sources: RHS potato-blight, RHS tomato-blight, UMN Extension.

General mechanism

Breeding or selecting cultivars carrying resistance genes reduces disease severity without eliminating need for other controls (resistance is rarely full immunity). Documented for basil downy mildew (e.g. Prospera, Devotion, Obsession, Amazel/Pesto Besto) and basil Fusarium wilt (e.g. Aroma-2, Prospera, Obsession; some lemon/purple basils naturally more resistant).

Adjust watering timing (water in morning, not evening)

cultural_watering

For Late blight: UMN Extension (home garden guidance): keep foliage as dry as possible using drip irrigation/soaker hoses and morning watering so leaves dry quickly in the sun, plus staking/caging and spacing plants for airflow. RHS makes the equivalent point for tomatoes specifically ("keep foliage as dry as possible"). Source: UMN Extension late blight; RHS tomato-blight.

General mechanism

Evening watering creates favourable damp conditions for nocturnal slug/snail activity and movement; watering in the morning instead reduces the moist, dark conditions these molluscs rely on to feed and move at night.

Can I Save the Crop?

There is no cure. Once blight is established in a plant it cannot be reversed, so every decision below is about limiting the loss and protecting what is still underground or still ripening.

Blight on Potatoes

• What you see: dark brown patches on leaves and stems, spreading fast in humid weather; a firm reddish-brown rot under the skin of lifted tubers.
• What is at stake: the tubers. The haulm is expendable once it is badly infected — the crop underground is not.
• What to do: cut the haulm off 2–3cm above ground once roughly a quarter of the leaf area is affected, then leave the tubers 2–3 weeks so the skins set before lifting.
• Do not store them. Blighted tubers rot in store and take sound ones with them. Use what is firm straight away.
• Next season: earth up well, so spores washed off the foliage cannot reach the tubers, and pull out any volunteer potatoes.

Blight on Tomatoes

• What you see: watery brown lesions on leaves and stems, and a firm brown decay on green or ripening fruit that often turns soft as bacteria move in.
• What is at stake: the fruit, which is exposed rather than protected underground — so blight on tomatoes usually ends the plant's season.
• What to do: remove affected leaves early; once stem lesions appear the plant will not recover, so take it out and ripen any usable fruit indoors.
• Under glass: clean supports, canes and glasshouse surfaces with disinfectant afterwards, and keep foliage dry by watering the soil in the morning.
• Next season: grow a resistant cultivar such as 'Crimson Crush' or 'Mountain Magic', and never plant tomatoes in ground that grew potatoes.

Prevention Checklist

• Choose cultivars with current blight resistance — Sarpo Mira, Sarpo Axona, Carolus and Alouette for potatoes; 'Crimson Crush' or 'Mountain Magic' for tomatoes.
• Rotate at least four years before potatoes or tomatoes return to the same ground.
• Space plants for airflow, and train cordon tomatoes to a single stem.
• Water the soil in the morning rather than the evening, and keep water off the foliage.
• Earth up potatoes so spores washed off the leaves cannot reach the tubers.
• Pull out volunteer (self-set) potatoes — they carry the disease between seasons.
• Never compost blighted material: bin it, bury it deeply, or use council green waste.
• Watch regional alerts during the July–September risk window.

Common Mistakes to Avoid

• Composting blighted material. Blight's resting spores (oospores) are resilient structures that home composting often fails to destroy — bin, bury deeply, or use council green-waste collection instead.
• Skipping rotation, or rotating too short a period. The RHS recommends at least four years before growing potatoes or tomatoes on the same ground again, since soil-borne resting spores persist.
• Trusting older 'resistant' varieties. Newer blight strains have overcome the resistance some older cultivars (like Cara and Valor) once had — check current RHS-recommended resistant varieties rather than relying on reputation.
• Storing blight-affected tubers. Use tubers from a blighted crop as soon as possible rather than storing them — secondary bacterial rot spreads readily among stored potatoes.

References

1. RHS (Royal Horticultural Society) — Potato blight

   Tier 1. Primary source for identification, disease cycle, favourable conditions, and treatment guidance.

2. Clemson Cooperative Extension (HGIC) — Tomato diseases and disorders

   Lesion descriptions used to separate late blight from early blight and Septoria leaf spot.

3. RHS (Royal Horticultural Society) — Tomato blight

   Tomato leaf, stem and fruit symptoms, and how leaf mould differs from blight.

4. University of Minnesota Extension — Late blight

   Favourable temperature range, leaf-wetness requirement and rotation length. Not linked because the page blocks automated access, so the exact URL could not be re-checked.

RHS and UMN Extension are independent sources and differ slightly on one point: the RHS recommends at least 4 years of crop rotation, UMN Extension 3-4 years. Both figures are given above rather than averaged into a single number that neither source states.

Hero photo: Scot Nelson, CC BY-SA 2.0, via Wikimedia Commons · Symptom photo: Scot Nelson, CC0 (public domain), via Wikimedia Commons · Disease cycle photo: Björn Andersson (SLU), CC BY-SA 4.0, via Wikimedia Commons · Conditions photo: tanakawho, CC BY 2.0, via Wikimedia Commons · Potato photo: Scott Bauer, USDA-ARS, public domain, via Wikimedia Commons · Tomato photo: Maasaak, CC BY-SA 4.0, via Wikimedia Commons · Crop rotation photo: Sten Porse, CC BY-SA 3.0, via Wikimedia Commons · Resistant cultivar photo: Alupus, CC BY-SA 3.0, via Wikimedia Commons · Sanitation photo: Ksd5, CC0 (public domain), via Wikimedia Commons · Watering photo: PGskot, CC BY-SA 4.0, via Wikimedia Commons