Tomato blight: how to prevent it, and why you cannot cure it

Every summer, the same scene. A week of warm rain, then brown patches on the lower leaves, climbing within days, reaching the stems, then the fruit. The gardener starts looking for a spray, and by then it is already too late: late blight is not cured, it is prevented.

That sounds fatalistic. It is not. It follows directly from the biology of the organism involved, and once you understand that biology, the list of useful actions becomes short, coherent, and something you apply on planting day.

It is not a fungus, and that changes everything

Phytophthora infestans belongs to the oomycetes. They were filed alongside fungi for a long time because they look the part: threads, spores, a parasitic way of life. Modern biology has separated them out — they are closer to certain algae than to true fungi.

The practical consequence fits into one image: this organism behaves like something that came from water. Its spores, once they land on a leaf, need liquid water to germinate and get into the tissue. Not humid air, not a muggy afternoon: water, as a continuous film on the leaf surface, for several hours at a stretch. A leaf that is wet for twenty minutes is not the same problem as a leaf that stays wet all night.

The second condition is a moderate temperature band. Blight likes neither cold nor real heat. The figure usually quoted in general gardening literature sits somewhere in the mid-teens to low twenties Celsius — roughly the high fifties to low seventies Fahrenheit — and it should be read as a rough marker, not a measured threshold. The leaf wetness duration involved is likewise usually described as a matter of hours rather than minutes. These orders of magnitude are not a forecasting tool; they explain why outbreaks follow spells of mild rain and cool nights rather than dry heatwaves.

The third decisive trait is speed. Once established, the organism produces fresh spores within days, and those spores travel on water and wind to the neighbouring leaves. You are not facing a linear decline but an acceleration. That is why a bed can look healthy on Monday and be finished by Friday.

Spotting the first signs, and not mistaking them

On a leaf, the first sign is a greenish-brown patch with poorly defined edges, often described as greasy or water-soaked because it looks soaked rather than scorched. It spreads without respecting the veins. In damp weather, turning a leaf over early in the morning sometimes reveals a fine whitish down at the edge of the patch: that is the organism fruiting, and it is the most distinctive sign of all.

On stems and leaf stalks, it shows as brown to blackish-brown areas, often elongated. A stem lesion low down condemns everything above it. On the fruit, you get irregular brown mottling, usually on the shoulder, with a firm, bumpy texture. The fruit stays hard rather than collapsing into soft rot, which is what confuses people most.

The commonest mix-up is with early blight. Where late blight makes diffuse, greasy patches, early blight produces sharper brown spots, often rounded, marked with concentric rings that give them a target-like look. It tends to start at the bottom of a plant that is late in its cycle or short of vigour, and it moves far more slowly. A plant shedding its lower leaves gradually all summer probably does not have late blight; a plant that turns in three days after a wet week very probably does.

Doubt is reasonable, and other leaf spots exist. If the distinction decides the fate of a crop, the useful move is a diagnosis from a local extension service, plant clinic or garden advisory service, not a photo comparison online.

Everything that works is preventive

Since the organism needs liquid water on the foliage for several hours, there is a single objective with several expressions: cut the time your leaves spend wet.

Spacing comes first, because it governs the rest. Crowded plants form a block of foliage that air does not move through. Dew lingers inside it, rain cannot escape it, and the interior microclimate becomes exactly what blight asks for. Generous spacing costs you plants, and it is the best investment of the season.

Watering follows the same logic. Water at the base, at soil level, never over the leaves, and preferably in the morning so that anything wetted dries during the day instead of sitting damp overnight. An evening sprinkler recreates the conditions of a rainy night all by itself.

Cover, where you can manage it, is the strongest lever of all, because it attacks the water source: it keeps rain off the foliage. But it only helps if it is ventilated. A closed greenhouse or polytunnel over dense foliage relocates the problem rather than solving it, trapping the moisture the plants transpire. Ventilate generously, open early, close late: unventilated cover can be worse than none.

Removing the lowest leaves completes the set. Taking off the foliage nearest the soil keeps leaves clear of rain splash and opens up the base of the plant. Do it in dry weather, without overdoing it, keeping enough leaf to feed the plant and shade the fruit.

Then the structural measures: rotation, so that you are not putting tomatoes and their relatives back in the same ground year after year; distance from potatoes, which are susceptible to the same organism and act as a nearby source of inoculum; and clearing crop debris at the end of the season, including volunteer tubers left in the soil.

Copper, honestly

Copper comes up in every conversation about blight and deserves a straight account. It is a contact material with preventive action: it works on spores landing on an already-treated surface. It does not move into the plant and does nothing about an infection already inside the tissue. Applied to a diseased plant, it will not cure it.

It is also not harmless. Copper does not break down: it accumulates in soil across repeated applications, with documented long-term effects on soil life. That is why its use is restricted, and why those restrictions differ from one country to another along with what is approved for sale at all.

In practice: doses, concentrations, conditions of use and application intervals are set by the product label and by the plant protection regulations where you live, which are not the same in the UK, the EU, the US or Canada. That is where to look, not in a blog post. No home-made spray recipe is recommended here: the ones that circulate are neither assessed nor regulated.

"Resistant" does not mean "immune"

Varieties with improved blight resistance exist, and they are a genuine advance for anyone growing outdoors in a wet climate. They delay infection, slow its progress, and often let you bring in a crop where a susceptible variety would have been lost.

What they do not do is make a plant immune. Under heavy pressure — several wet weeks, an infected garden next door, foliage that never dries — a resistant variety can still go down. The organism also exists as varied populations, so resistance that holds up well in one place may hold up less well elsewhere or later on. It is one more lever to stack on the others, not a licence to skip spacing, base watering or rotation.

What Marmanda does

Marmanda takes account of the space you actually have and the orientation of the plot when it places crops, so that the distance between plants is a decision made at planting rather than something you notice in August. The log keeps what you observed and where, season after season, which makes it visible when the same bed gives you the same trouble two years running. Marmanda does not forecast weather and does not detect disease: what it keeps is your own record.

What to take away

Blight is a problem of water and time. It needs foliage wet for several hours, a temperature that is neither cold nor baking, and a source of inoculum nearby. Everything you can act on comes down to removing one of those conditions, and all of it has to happen before the first patch appears.

On the day it does appear, the only decision left is to remove the affected growth, keep it off the ground, and see what the crop can still give you. That is an unsatisfying answer. It is also why the spacing you decide in May is worth more than any sprayer in July.