Tomato blight: how to spot it and act before the first spot
Two days at 10 °C minimum and at least six hours of humidity a day open the blight window. Learn to read it so you can act before the first spot.
You spot a mark on a leaf. You hesitate. Three days later, the plant is black. Tomato blight attacks the leaf, the stem, the flower and the fruit. It moves faster than your diagnosis. The reflex is to spray as soon as you see a spot. But copper only works before infection. Once the spot is visible, it comes too late. Yet you can see blight coming. It mostly thrives on two mild, humid days in a row, which you can spot on a weather forecast. If the spots are already there, go straight to the last section, "When it is already there".
Recognise it in 30 seconds
Look at the leaves, then the stem, then the fruit.
- On the leaf: water-soaked, olive-green spots of irregular shape. They turn brown and die off quickly.
- Under the leaf: a white fuzz, very visible in the morning. It is the pathogen producing spores.
- On the stem: large, irregular brown patches that often go all the way round.
- On the fruit, green or ripe: irregular brown patches. The fruit becomes lumpy.
The culprit is called Phytophthora infestans. It is an oomycete, an organism close to fungi. It also attacks potatoes. If you grow both, watch both.
Late blight or early blight?
A brown spot on a tomato leaf is not necessarily late blight. And what you do changes with the disease.
| Late blight | Early blight | |
|---|---|---|
| The spots | Water-soaked, olive-green, irregular, they turn black fast | Brown, with concentric rings and often a yellow halo |
| Where it starts | All the aerial parts: leaves, stems, flowers and fruit | At the bottom of the plant, then it climbs |
| Favoured by | Long-lasting humidity (rain, fog, dew, wet foliage), from 2 to 25 °C | Dense foliage, leaves that stay wet a long time |
| What to do | Remove the affected parts, act before the next window | Mulch, space the plants, thin the foliage, destroy the debris |
The white fuzz under the leaf in the morning points to late blight. Concentric rings with a yellow halo point to early blight. Two other problems can fool you. A white fuzz that quickly turns olive-brown under the leaf, with yellowish spots on top, is leaf mould, another fungus. A well-defined brown patch at the tip of the fruit is blossom end rot. This physiological disorder is not an infection at all.
Without a microscope, you may still hesitate. In that case, remove the affected parts right away. It costs nothing and it helps in both cases.
The Hutton criteria: two mild, humid days
Blight thrives when the air stays humid for a long time and the nights are not cold. Researchers at the James Hutton Institute, in Scotland, drew a rule from this. In 2017, the AHDB made it the national potato blight warning system for Great Britain. The AHDB is the British agriculture and horticulture development board.
| Condition | Value |
|---|---|
| Duration | Two days in a row |
| Temperature | Not below 10 °C during those two days |
| Humidity | At least 6 hours at 90 % relative humidity or more, each day |
The old British rule, the Smith period, required 11 hours of humidity a day. The researchers tested the Hutton criteria on more than 2,000 historical reports of potato blight. Six hours matches real outbreaks better.
These criteria have two limits. They were calibrated on potatoes, in Great Britain. And a window does not guarantee the disease. It only says that if the pathogen is there, it has everything it needs to infect. Blight can also take hold in cooler weather. The DRAAF Centre-Val de Loire (a French regional agriculture service) gives it as active from 2 to 25 °C. The Hutton window targets the conditions that suit it best.
In Belgium, humidity decides
In midsummer, the temperature condition is easy to meet. At Uccle, the average minimum temperature of a summer is 13.4 °C (1991-2020 normal from the Royal Meteorological Institute, the RMI). The coolest night of a normal summer still drops to 6.8 °C. So there are nights under 10 °C, but the average of the minimums stays well above. In 2023, the thermometer did not drop below 10 °C all summer, with a minimum of 10.1 °C. That had never happened since measurements began in 1892. The three summer months have 42.6 days of precipitation on average.
So humidity makes the difference. In Wallonia, the CARAH advises farmers and publishes potato blight warnings. Its first spraying notice came on 25 May in 2020 and on 8 June in 2021. Watch your tomatoes from the end of May.
How to read the window
Open an hourly forecast that shows relative humidity. For each day, count the hours at 90 % or more and find the lowest temperature. Take a made-up example. Monday, 11 °C at the lowest and 8 hours at 93 %. Tuesday, 12 °C at the lowest and 7 hours at 91 %. These two days make a window. With only 4 hours at 90 % on Tuesday, or a night at 9 °C, there would be none.
The Vegiwise app does this calculation on the forecast for your address and warns you when the window is approaching.
Moves that cost nothing
These moves help the leaf dry fast and cut the spores in the garden.
- Water at the base, in the morning, without wetting the foliage.
- Strip leaves from the bottom up, bit by bit, without going above the first truss. Also remove the side shoots so air can move.
- Space your plants at least 30 cm apart, more for varieties that grow tall.
- Pick a sunny, dry spot. Avoid too much nitrogen, which favours blight.
- Rotate your crops. The following year, no tomato, aubergine, pepper or potato in the same place. For outdoor tomatoes, the RHS, the British horticultural society, ideally recommends a four-year rotation.
- Clear the debris. The pathogen survives in tomato and potato remains left on the ground. Also pull out volunteer potatoes.
- Ventilate the shelter if you grow under a tunnel or in a greenhouse. Outdoor tomatoes are more exposed.
Varieties that help
No tomato is immune. So-called resistant varieties rarely are completely. They can still produce when the others give up. Several seed companies write that early varieties and cherry tomatoes cope better. We found no independent Belgian trial that confirms it, so take it as a hint. An early variety has at least one advantage. It carries more ripe fruit when blight arrives. Sativa, a Swiss organic seed company, describes the Rouge précoce Matina in the catalogue as early, light-leaved and fairly tolerant of leaf diseases.
Copper goes on in dry weather, before the window
Copper stops spores from germinating. It only works before infection. Spraying a spot that is already there is therefore pointless. A leaflet from the Alsace Chamber of Agriculture, written for grapevines, puts it this way. "Copper treatments must be applied before an infecting rain; after the rain, it is too late."
Rain also washes it off. On grapevines, the same leaflet measures 25 to 40 % of the product gone from 2 mm of rain. After 5 mm, more than half has disappeared. The leaflet advises renewing after 15 mm of cumulative rain. So spray on the last dry day before the Hutton window, as close as possible to the forecast rain.
Authorised products in Belgium
In Belgium, an amateur gardener is only entitled to products authorised for non-professional use. Their authorisation number ends in G/B or G/P (G for garden, B for Belgium). A number ending in P/B means a product reserved for professionals. Fytoweb, the website of the Belgian federal public health service, lists the authorised products and offers a filter for amateur users.
The range of copper products is shrinking. Since the European renewal of 2019, copper compounds are among the candidates for substitution. Belgium has withdrawn several authorisations of copper products, mostly for professionals (Fiwap, 2025). Applied repeatedly, copper also builds up in the soil (INRAE). Before you buy, check on Fytoweb that a product is authorised on tomato for amateurs. Then follow its label for the dose, the number of applications and the delay before harvest.
If no product suits, you still have the moves that cost nothing, above.
When it is already there
No treatment cures an affected leaf. Remove the affected parts right away. It slows the disease a little without stopping it. In a severe attack, pull out the whole plant. Put the debris in a closed bag, with household waste or at the recycling centre, never in the compost.
Think about the harvest. An affected plant often carries infected fruit, even without a visible spot. Eat the well-coloured ones first. Green fruit ripens poorly indoors and a good share risks rotting. Cook it right away, in chutney or in a sauce. A spotted or soft fruit goes in the bin.
Then clean stakes and tools. If you grow in containers, do not reuse the potting mix for tomatoes.
Further reading
- The collection: Tomate Lovers
- The Start page: Tell us which seeds you have, and we tell you when to sow each one where you live
- Three other guides: Saving your own tomato seeds, The Triomphe de Liège tomato and Sowing calendar for Belgium
Photo: Scot Nelson, Wikimedia Commons, licence CC0
Sources
- AHDB, Development and implementation of a new national warning system for potato late blight in Great Britain (Hutton Criteria), projet mené par le James Hutton Institute, 2014 à 2017
- AA Farmer, Timely transformation for Potato Late Blight risk criteria (période de Smith : 11 heures, critères de Hutton : 6 heures, plus de 2 000 déclarations historiques testées)
- DRAAF Centre-Val de Loire et Chambre régionale d'agriculture, fiche Mildiou de la tomate (symptômes, conditions, gestes de prévention), août 2015
- RHS (Royal Horticultural Society), Tomato blight (symptômes, fruits, élimination des débris, variétés résistantes, pas de fongicide pour jardiniers au Royaume-Uni)
- University of Wisconsin Extension (Horticulture), Early blight (alternariose : symptômes, conditions, gestion)
- IRM, Bilan climatique saisonnier, été 2023 (Uccle : température minimale moyenne d'été, normale 1991-2020, record de 2023, jours de précipitations)
- CARAH asbl, Avertissements VigiMAP, retour sur 3 ans (présentation Fiwap, 13 décembre 2023 : premiers avis de traitement du mildiou de la pomme de terre en 2020 et 2021)
- Chambre d'agriculture d'Alsace et FiBL, Cuivre et mildiou : les enseignements 2021 (cuivre produit de contact, uniquement préventif, lessivage, essais sur la vigne)
- INRAE, Peut-on se passer du cuivre en protection des cultures biologiques ? (expertise scientifique collective, synthèse)
- SPF Santé publique (Fytoweb), Dans son jardin (produits autorisés pour l'usage non professionnel, numéros se terminant par G/B)
- Fiwap-Info n° 186 (avril à juin 2025), Dossier Cuivre : où en est-on ? (renouvellement européen de 2019, retraits d'autorisations de produits cuivrés en Belgique)
- Règlement d'exécution (UE) 2018/1981 du 13 décembre 2018 renouvelant l'approbation des composés du cuivre comme substances candidates à la substitution (fiche AIDA, INERIS)
- Sativa Rheinau, fiche de la tomate Matina (précoce, feuillage léger, assez bonne résistance aux maladies du feuillage)
He gardens in Belgium and tries out in his own vegetable patch what the app recommends. His articles always start from a source you can check and a step you can take this weekend.
