Septoria leaf spot: how to recognise and treat it

Disease

Symptoms

Many small round spots, under 3 mm, with a grey or tan centre and a dark brown edge, sometimes with a yellow halo; they start on the lowest, oldest leaves.
Tiny black dots (pycnidia) in the centre of the spots, visible with a hand lens.
Spotted leaves turn yellow and die, and the disease moves up the plant; spots also appear on stems and leaf stalks, rarely on fruit.
Tomato, celery, parsley and chrysanthemum each have their own Septoria species, which do not pass between these crops.

Remove spotted leaves as soon as you see them, and all infected plant parts at the end of the season.
A fixed copper fungicide certified for organic use, started early when spots first appear; it slows the spread but does not cure infected leaves.

Chlorothalonil where it is approved for home gardens (for example in the US; it is not approved in the EU or UK); it only protects healthy leaves, so start before the disease spreads and follow the label.
On chrysanthemums and other ornamentals — a fungicide with tebuconazole + trifloxystrobin labelled for leaf spot; follow the label.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Do not grow the same crop in the same spot for at least 2 years, and clear crop debris and nightshade-family weeds that carry the fungus.
Stake and space plants, prune off the lowest leaves of tomatoes once fruit sets, mulch against splash and water at the base.
Do not handle plants while the leaves are wet — workers and tools spread the spores.
Celery — the fungus is seed-borne; use seed treated in hot water (48 °C for 30 minutes) and remove old celery debris.

Figures checked against these sources.