Tomato Blight is a serious disease that affects tomatoes in Scotland. Here is what it looks like, the conditions it needs, and the controls that actually work.
What does it look like?
On leaves: Rapidly spreading, watery rot of leaves, which soon collapse, shrivel and turn brown.
On stems: Brown lesions that can cause collapse.
On fruit: Watersoaked patches turning brown on green fruit; mature fruits decay rapidly.
What causes it
Infection needs prolonged surface wetness, several hours of it, rather than a passing shower, so a settled damp spell is the real trigger.
Tomato blight and potato blight are the same organism, Phytophthora infestans. If you also grow potatoes, a wet spell that starts an outbreak here is a real risk to them too, so check them as well. See Potato Blight →
Life cycle
The disease doesn't usually start from last year's tomato material at all: the primary source each year is normally affected potato plants, since the pathogen overwinters inside infected potato 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 → left in the ground. Most garden infections then arrive as wind-blown spores from elsewhere, other gardens, allotments or commercial potato crops, rather than from anything carried over on-site. A newer detail worth flagging honestly: some blight strains can now produce resting spores capable of contaminating soil directly, though this hasn't yet been confirmed in UK tomato crops specifically.
Growing under cover
Growing under cover reduces blight risk — but it is not a shield, and in Scotland the caveat matters more than the benefit.
RHS: 'Tomatoes grown in greenhouses are less likely to be affected but can still suffer badly at times', because 'wind-blown blight spores are less likely to reach the plants'.
RHS: if spores do enter through doors or vents, 'the high humidity inside usually leads to very rapid development of symptoms'.
Mitigation: Keep greenhouse foliage dry.
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: The same is true here: no fungicide helps, so keep greenhouse foliage dry, grow a resistant variety such as 'Crimson Crush' or 'Mountain Magic', and destroy infected material rather than composting it.
Keep foliage dry and manage greenhouse humidity
Keep greenhouse foliage dry. That is the mitigation for the high-humidity risk that follows once spores get into a greenhouse.
Grow blight-resistant varieties
'Crimson Crush' and 'Mountain Magic' both carry strong resistance, and both 'performed very well at Wisley in 2021, when blight was extremely widespread'.
Cultural controls: dispose of infected material by deep burial, green waste collection or burning — never compost; minimum four-year 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 →; monitor blight forecasts
The Hutton Criteria forecast applies here identically: same organism, same trigger conditions as potato blight.
Use affected fruit promptly; make unripe green fruit into chutney or sauce rather than trying to ripen it indoors
RHS: 'Use the fruit from affected crops as soon as possible. Once a plant has developed high levels of the disease on the leaves and stems it is likely that many of the fruit will also have been infected, even if they don't yet show symptoms.'
This answers the question most gardeners actually have once blight appears: can I still use the fruit? The advice is not to try ripening green fruit indoors, since much of it may already be infected without showing it — a large share is likely to rot instead. Using it straight away, for example as chutney or green tomato sauce, is the safer option.
Myths about tomato blight
6 common claims about tomato blight, checked: 5 busted0 disputed0 untested1 hold up
“Growing tomatoes in a greenhouse or polytunnel gives full protection from blight”
A greenhouse or tunnel cuts the number of wind-blown spores that reach the plants, which does lower risk, but it is not a barrier. If spores get in through a door or vent, the humid air inside speeds up how fast symptoms develop once infection starts, sometimes leading to worse outbreaks than an equivalent outdoor crop. Cover changes the odds, it does not remove the disease as a possibility.
What to do: Treat cover as risk reduction, not immunity: ventilate on warm days, keep foliage dry, and check plants regularly even under glass or polythene.
In Scotland: Since outdoor tomato cropping is marginal across most of Scotland, almost every grower here is already relying on this partial protection by default. That makes ventilation and dry foliage a higher priority than they would be somewhere the reduced-spore benefit is a genuine extra, rather than the baseline growing method.
1 source
- RHS, Tomato blightGreenhouse-grown tomatoes are less likely to be affected because wind-blown spores are less likely to reach them, but can still suffer badly, and high humidity inside speeds up symptom development once spores get in.
“Blight-resistant tomato varieties such as Crimson Crush or Legend are immune to blight”
Resistant varieties are bred to hold up better under blight pressure, not to be immune to it. Even 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 → that performed strongly in a widely blight-affected trial year are described as carrying resistance rather than immunity, and can still develop the disease if mild, wet conditions persist for long enough. The practical difference is that a resistant variety may keep cropping while a susceptible one collapses, not that it will never show symptoms at all.
What to do: Grow a resistant variety to shift the odds in your favour, but keep watching for symptoms and still practise good hygiene, dry foliage, rotation, and prompt removal of infected material, rather than treating resistance as a substitute for those steps.
In Scotland: The strongest resistance data available comes from a trial at Wisley in Surrey, a milder and drier climate than most of Scotland, so read a good resistance rating as real evidence rather than a guarantee of the same performance in a cool, damp Scottish summer.
1 source
- RHS, Tomato blightResistant cultivars including Crimson Crush and Mountain Magic performed very well at Wisley in a year when blight was extremely widespread, but resistant varieties are not usually completely immune and may still develop the disease if favourable conditions persist.
“Copper spray or Bordeaux mixture protects tomatoes from blight”
No fungicide, copper-based or otherwise, is currently available for UK gardeners to use against blight. Copper sprays have a long history as a blight treatment and are still sometimes recommended informally, but that approval gap means spraying one on tomatoes offers no legitimate protection under current UK garden-use rules, whatever the product label or folk advice claims.
What to do: Do not rely on any spray, copper-based or otherwise. Put the effort into what actually reduces risk: dry foliage, ventilation, a resistant variety, good rotation, and prompt removal of infected material.
2 sources
- RHS, Tomato blightThere are currently no fungicides available for use by gardeners against blight.
- RHS, Potato and tomato blightConfirms the same finding, that no fungicides are currently available for gardeners to use against blight.
“Keeping tomatoes and potatoes apart in the garden stops blight from spreading between them”
Blight spores are wind-blown and can travel long distances, and the great majority of garden infections come from outside the plot altogether, other gardens, allotments, or commercial potato crops, rather than from a neighbouring bed on the same site. A few metres of separation between a tomato bed and a potato bed does nothing to stop spores that are already arriving from streets or fields away. The genuine cross-crop risk is an infected potato crop, or old tubers left in the ground over winter, being present anywhere on the plot, not the distance between the two crops.
What to do: Do not rely on physical separation as protection. Remove potato volunteers and leftover tubers promptly wherever they turn up, and check both crops together whenever a mild, wet spell raises blight risk, regardless of how far apart they are planted.
1 source
- RHS, Tomato blightSpores are easily shed from infected foliage and may be wind-blown for long distances; the primary source each year is usually affected potato plants overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → as infected tubers, but the great majority of garden infections arise from wind-blown spores originating from other gardens, allotments and commercial potato crops.
“Green tomatoes from a blighted plant can be brought indoors and ripened safely”
Once a plant has developed heavy blight on its leaves and stems, much of the fruit is likely to already be infected internally, even where it still looks green and unmarked. Setting that fruit aside to ripen indoors mostly wastes the effort, since infected tomatoes tend to rot rather than ripen once picked. The fruit is still usable, but only if it is used straight away rather than stored.
What to do: Pick and use blight-affected green fruit immediately, in a cooked dish such as chutney or green tomato sauce, rather than leaving it to ripen on a windowsill.
1 source
- RHS, Tomato blightUse the fruit from affected crops as soon as possible; once a plant has developed high levels of the disease on the leaves and stems it is likely that many of the fruit will also have been infected, even if they don't yet show symptoms.
“Watering tomatoes at the base instead of overhead reduces blight risk”
Blight infection needs prolonged surface wetness on leaves and stems, several hours of it, rather than a brief shower, and keeping foliage dry is the specific advice given for reducing risk once spores are around. Overhead watering wets the leaves directly and keeps them damp for longer, working against that advice, while watering at the base avoids adding that extra surface moisture in the first place.
Our read No institutional source runs a direct side-by-side comparison of watering methods. This verdict combines two separately stated facts, the surface-wetness infection trigger and the dry-foliage recommendation, rather than quoting a single line that names overhead watering by name.
What to do: Water tomatoes at the base of the plant, ideally in the morning, and avoid wetting the foliage, especially under cover where humidity is already higher.
1 source
- RHS, Tomato blightInfection needs prolonged surface wetness, and greenhouse foliage should be kept as dry as possible to reduce risk once spores are present.
Frequently asked questions
What should I do first if I find tomato blight in my garden?
Start with keep foliage dry and manage greenhouse humidity. After that, grow blight-resistant varieties. 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 tomato 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: organism, leaf/stem/FRUIT symptoms, warm wet conditions, life cycle and overwintering via potatoes, greenhouse reduces spore arrival but high humidity accelerates symptoms if breached, keep foliage dry, no amateur fungicides, 'Crimson Crush'/'Mountain Magic' Wisley 2021, disposal and four-year rotation
- RHS1: corroborates organism and the no-amateur-fungicides finding