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DiseaseLeavesStemsFruitTubers

Late blight

A water mould that destroys potato and outdoor tomato foliage in warm, wet spells, and then rots the tubers or fruit. It moves in days rather than weeks, which is why prevention is the only strategy that works.

Caused by: Phytophthora infestans · also called potato blight, tomato blight

Nothing available to a home grower will fix this

Once blight is visible on a plant there is no home treatment that will save that plant. Everything worth doing is either preventive or damage limitation — protecting the crop underground, or stopping the spread to plants not yet infected.

source

GrowingHQ editorial positionhigh confidence

Stated plainly because the alternative — listing sprays and remedies that do not work on an established infection — costs growers money and, worse, costs them the days in which lifting the tubers would still have saved the crop.

  • GrowingHQ editorial synthesis — GrowingHQ

Checked 2026-08-13

Described view

Two green tomatoes side by side at the same scale: one with the flat, dry, leathery, sharply-bounded sunken patch of blossom end rot at the flower end, and one with the greasy, spreading, brown-marbled lesion of late blight that has no clear edge and is not confined to the flower end

What brings it on

Outbreaks follow spells of warm, humid weather. In the UK the standard trigger is a Smith period: two consecutive days with a minimum temperature of at least 10 °C and at least eleven hours of relative humidity above 90%.

source

Sourced guidancehigh confidence

  • Agriculture and Horticulture Development Board — AHDB: Potato late blight — Smith period criteria

Checked 2026-08-13

Usually seen: June, July, August, September

Preventing it

Ordered by how much difference each measure actually makes, not by how often it is repeated.

  • Grow tomatoes under cover where possible. Blight spores need leaf wetness to infect, and a greenhouse or polytunnel keeps rain off the foliage.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato and tomato blight

    Checked 2026-08-13

  • Choose cultivars bred for blight resistance. Resistance is partial rather than absolute, and it delays infection rather than preventing it.
    Sourced guidanceDepends on conditionshigh confidence

    Applies when: Resistance is specific to the blight strains prevalent at the time, and the dominant strains change.

    • Royal Horticultural Society: Blight-resistant potato and tomato cultivars

    Checked 2026-08-13

  • Grow early potato varieties and lift them before the main blight season. Dodging the disease is more reliable than fighting it.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight

    Checked 2026-08-13

  • Space plants and remove lower leaves so air moves through the crop and foliage dries quickly after rain.
    source

    Horticultural consensusmoderate confidence

    • Penn State Extension — Pennsylvania State University: Vegetable disease management — airflow and canopy

    Checked 2026-08-13

Dealing with it once it is there

  • On potatoes, cut off and remove all top growth as soon as blight is confirmed, then leave the tubers in the ground for about two weeks before lifting so spores on the surface are no longer viable.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight — managing an outbreak

    Checked 2026-08-13

  • Do not compost affected material in a garden heap, and never save tubers or fruit from an infected crop.
    source

    Sourced guidancehigh confidence

    • Royal Horticultural Society: Potato blight — disposal

    Checked 2026-08-13

Widely repeated, and not supported

Recorded so GrowingHQ can address them. These are not alternative approaches — they are things the evidence does not back.

  • “Blight can be cured by removing the affected leaves.”

    By the time lesions are visible the pathogen is already established through the plant's tissue and sporulating. Removing leaves slows spread to neighbouring plants a little; it does not save the plant it was taken from.

    Horticultural consensusCommonly said, not supportedmoderate confidence
    • Royal Horticultural Society: Potato and tomato blight

    Checked 2026-08-13

What it affects

Signs that lead here

Each of these lists the other things that produce the same sign, and how to tell them apart.

On sprays and treatments

GrowingHQ does not tell anyone what to apply, at what rate, or how often. Pesticide approvals differ by country, change without notice, and the product label is legally binding — it is also the document you are holding. What is described here is cultural and physical control, which is what a home grower can act on without a label in front of them.

Plant-specific guides

Where this is one of several causes that look alike on a particular plant, GrowingHQ publishes the differential rather than the general answer.