Here is the reassuring part first: tomatoes grown under glass catch blight far less often than the ones outdoors. The RHS is clear on why, wind-blown spores are much less likely to reach a plant inside a greenhouse. The catch comes in a warm, wet August, when you leave the door and vents open and the spores drift in anyway. Then the very humidity that helps your plants grow turns the greenhouse into the fastest place for blight to spread.
This guide shows you how to identify tomato blight in under a minute, what to do the moment you find it, whether the fruit is still worth saving, and the varieties and habits that keep it away next year. Late blight moves quickly, so the speed of your response decides how much of the crop you keep.
How to tell if it is blight
Tomato blight is Phytophthora infestans, the same organism that causes potato blight, and it announces itself on the leaves first. Look for a watery brown rot that spreads fast: leaves collapse, shrivel and turn brown within a day or two, and in damp air you may see a faint white fungal growth on the undersides. Brown lesions then appear on the stems and leaf stalks, and the fruit develops watersoaked patches that turn brown and firm before the whole tomato decays. The speed gives it away. A nutrient problem creeps; blight runs.
Why a greenhouse protects you, and the one way blight still gets in
Blight needs prolonged surface wetness, several hours of it, to take hold, which is why it tears through outdoor crops in a wet summer and why your watering habits matter so much. A greenhouse keeps rain off the leaves and keeps drifting spores out, so under cover you start well ahead. The risk arrives on warm, humid days through open doors and vents, and the national warning system tells you exactly when those days come.
That system is the Hutton Criteria, run by Blightwatch. A blight-risk day is logged when two consecutive days each have a minimum temperature of 10°C and at least six hours of humidity at 90% or above. When your region records a Hutton period in July or August, that is the week to keep leaves dry, water only at the base early in the day, and avoid leaving the greenhouse shut and steamy overnight. Check blightwatch.co.uk for your area; the service is free.
What to do the moment you find it
Nothing cures an infected plant, so the job turns to containment, and it cannot wait. Remove affected plants whole rather than picking off bad leaves and hoping. Bag them on the spot so you are not carrying spores past healthy plants. The RHS draws a firm line on disposal: infected material goes deeply buried, into the council green waste collection, or onto a bonfire, never onto your compost heap, because the resting spores can survive there to reinfect next year. Clean your hands and any tools before you touch a healthy plant.
Can you still eat the fruit?
Sound green fruit from a plant you have just pulled repays using, though not on the windowsill. Once the foliage is badly hit, many fruit are already infected even when they look clean, so the usual trick of ripening green tomatoes indoors tends to end in a bowl of brown mush. Use them straight away instead: green tomato chutney and cooked sauces both make good use of an unripe crop, and a few hours in the pan settles any doubt about what was lurking.
Keeping it out next year
Two habits and one shopping decision cover most of it. Water at the base of the plant in the morning so leaves dry by evening, never over the foliage at dusk. Keep the greenhouse moving air on humid days rather than shutting it up warm and wet. Then, when you order seed in winter, choose a blight-resistant variety: 'Crimson Crush' and 'Mountain Magic' both performed strongly in the RHS trial at Wisley in 2021, the year blight was everywhere. They are not immune, but in a bad blight summer they are the difference between a crop and a clear-out.
From Waldenhaus: dry leaves form your best defence, and that comes down to airflow. Our NORDIC wooden greenhouses include a rear opening window opposite the door as standard, so you get a through-draught that dries the air without leaving the whole greenhouse open to drifting spores. The advice above works for any greenhouse you own.
Tomato blight questions
Will blight come back next year?
It does not survive in the soil from tomato debris the way some diseases do, but it overwinters on living potato material, including stray tubers left in the ground. Clear those, never compost infected plants, and start the new season with a blight-resistant variety to take the pressure off.
Can blight spread between my tomatoes and potatoes?
It moves freely between the two. Late blight is one organism, Phytophthora infestans, that infects both crops, so an outdoor potato outbreak is a direct warning for your greenhouse tomatoes. Keep the two apart where you can.
Is blight worse under glass or outdoors?
Outdoors is where it usually strikes first, because rain and wind carry the spores onto open crops. Under glass you are better protected, until warm humid air through open doors lets it in, at which point the greenhouse humidity speeds it up. Dry leaves and moving air are your defence.
What to do this week
Walk into the greenhouse and look at the lower leaves: brown, fast-spreading, watery patches mean act within the hour, while slow yellowing from the base usually means feed and water instead. Set a reminder to check blightwatch.co.uk through July and August, switch to morning base-watering now, and note 'Crimson Crush' for next year's seed order. Caught in the first day, blight costs you a plant or two. Left a week in an August greenhouse, it takes the lot. For the wider picture on heat and humidity under glass, see our guide to keeping a greenhouse cool, and the RHS tomato blight profile for the full detail.
