Septoria leaf spot

Septoria lycopersici

also called Septoria blight, celery leaf spot

Many small round spots with pale gray centers and dark rims, starting on the lowest leaves and creeping upward until the plant is bare from the ground.

Fungus Serious Rain splashSpread in waterSpread on tools & hands Spreads to neighbors

What you'll see

Small circular spots, about 2–4mm, with tan or gray centers and a darker brown margin.
Tiny black dots (the fruiting bodies) speckle the pale centers — a hand lens shows them clearly.
Damage begins on the oldest, lowest leaves and works steadily up the plant.
Badly spotted leaves yellow, then brown and drop, leaving stems and fruit exposed.
Fruit is rarely spotted, but sunscald follows once the canopy is stripped.

Why it happens

A leaf-spotting fungus that survives winter on old tomato debris and on weeds in the nightshade family. Spores are rain-splashed and carried on wet hands and tools onto the lower leaves, where they need only a film of moisture to take hold.

Warm, wet, humid spells favor it, especially where plants are crowded and the lower leaves stay damp. Overhead watering and splashing rain move it up the plant fastest.

Treating it

Pick off spotted leaves as soon as you see them and bin or burn them, never compost.
Clear the lowest leaves to lift the canopy off the wet soil and improve airflow.
Water at the base in the morning so foliage dries quickly, and mulch to stop soil splash.
Disinfect secateurs and stakes between plants to avoid carrying spores along the row.
On a valued crop an approved fungicide or a copper-based spray can slow its climb.

Keeping it away

Rotate tomatoes and relatives so they don’t return to the same ground for three years.
Clear and destroy all haulm at the end of the season and keep nightshade weeds down.
Space and stake plants generously, and prune for an open, quick-drying canopy.
Mulch under plants to break the splash of rain and irrigation off the soil.
Start with clean plants and avoid working among them while the leaves are wet.

Who gets it

Chiefly tomatoes, with celery and celeriac and other nightshades such as potato and aubergine also at risk.