Septoria Leaf Spot
TreatableFungal disease · spreads to other plants
Septoria is early blight's more numerous cousin and often shows up on the same plant. Where early blight makes a few large ringed spots, septoria makes dozens of small pale-centred ones. It does not infect the fruit, but it defoliates a plant quickly enough to ruin the crop indirectly.
What it looks like
Many small round spots, a sixteenth to a quarter of an inch across, with dark brown borders and pale tan or grey centres. Under a hand lens the centres show tiny black pinprick dots. Leaves yellow and drop from the bottom up.
How to confirm it
- Very many small spots rather than a few large ones.
- Pale grey or tan centres with dark edges, and no concentric rings.
- Minute black specks in the centre of each spot under magnification.
- Attacks leaves only — the fruit stays clean.
What to do
- 1Strip and bin the worst-affected lower leaves. Do not remove more than about a third of the foliage at once.
- 2Mulch to stop rain splash, and water at the base.
- 3Copper or chlorothalonil protects healthy new growth if applied before the disease reaches it.
What it gets confused with
One observation settles each of these.
Early blight spots are larger, fewer, and show concentric rings. Septoria spots are small, numerous and pale-centred.
Bacterial spots look greasy and water-soaked when wet, and often have angular edges bounded by leaf veins.
Preventing it next time
- Rotate out of the nightshade family for two to three years.
- Stake or cage plants so no leaf touches the soil.
- Remove volunteer tomato seedlings and nightshade weeds nearby.
Plants affected
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.