Tomato late blight: spotting the first outbreak
Oily patches on the edge of a leaflet come two days before the canopy collapses. Here is how to read the signs while there is still something to save.
By Nabatic agronomy team
A late blight outbreak gives no warning. It is visible, two days before the plot turns, provided you know where to look.
What happens before the spot
Phytophthora infestans needs three things: free water on the leaf for at least four hours, a temperature between 15 and 22 °C, and a source of inoculum. Under glass or plastic, those conditions come together almost every night once condensation settles on the film.
That is why the first outbreak nearly always shows up in the same place: on the edge rows, where the temperature gap with the wall is widest.
The three signs, in order
The oily patch. A dark green area, translucent when held to the light, two to three centimetres across, usually on the edge of a lower leaflet. It is not brown yet. At this stage a contact treatment is enough.
The white felt. On the underside of the leaf, along the edge of the patch, a greyish-white down appears in the early morning. That is sporulation. The outbreak is now producing inoculum for the whole greenhouse.
Necrosis and smell. The patch turns brown, the tissue collapses, and a distinctive smell develops. By then the outbreak is two or three days old, and it has already spread.
The confusion trap
Early blight gives concentric spots, brown from the outset, on older leaves — with no felt underneath. Bacterial speck gives angular spots bounded by the veins. Sunscald burns the exposed edge, symmetrically, without spreading.
If you are unsure, the rule is simple: turn the leaf over. White felt on the underside does not lie.
What to do within twenty-four hours
- Remove affected leaves and take them out of the greenhouse in a closed bag — not onto the walkway.
- Ventilate at first light to break the condensation.
- Treat before 9 a.m., on dry foliage, with a product approved on tomato.
- Mark the row: that is where the outbreak will start again.
One photo of the affected leaflet, sent from the plot, is enough for an agronomist to decide. That is twenty-four hours gained on the outbreak, and often a season saved.
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