Septoria leaf spot is the most common leaf spot disease of tomatoes, and it is the reason many tomato plants lose their lower leaves by midsummer and end the season looking bare and tired. It starts as small, circular spots on the oldest leaves, each one with a pale grey or tan centre and a dark border, and each one often holding tiny black dots — the fruiting bodies of the fungus — that are visible if you look closely. The spots multiply, the leaves yellow and drop, and the disease climbs the plant leaf by leaf, until only the top of the plant still has leaves. The fruit is rarely spotted, but a defoliated plant produces less fruit, and the fruit it does produce is exposed to sunscald.

The fungus is Septoria lycopersici, and it is a specialist that attacks tomatoes and, to a lesser extent, other nightshades. It overwinters in fallen leaves and in plant debris, and it spreads in splashing water during wet weather. It needs wet leaves to infect, and it is worst in warm, humid summers and in gardens where tomatoes are crowded, watered from above, and mulched poorly. It has no cure once it is on a leaf, but it is very responsive to the standard leaf-disease controls — dry foliage, airflow, mulch, and autumn cleanup — and a tomato plant that is staked, spaced, and mulched will often stay nearly clean even in a wet year.

What septoria leaf spot looks like: symptoms

  • Small circular spots: small, round spots on the leaves, usually about an eighth of an inch across, with a pale grey or tan centre and a dark brown border.
  • Tiny black dots in the spots: the pale centres of older spots hold tiny black dots — the fungal fruiting bodies — which are the most distinctive sign and are visible with a hand lens or even the naked eye.
  • Spots that multiply: a single leaf can carry dozens or hundreds of spots, and they may merge into larger dead patches as the disease advances.
  • Yellowing and leaf drop: spotted leaves turn yellow and drop, starting with the lowest, oldest leaves, and the disease climbs the plant through the season.
  • Defoliation from the bottom up: a badly hit plant loses most of its lower leaves by midsummer, leaving a bare stem with leaves only at the top.
  • Sunscald on exposed fruit: with the leaves gone, the fruit is exposed to direct sun and can develop pale, sunken, sunscalded patches.
  • Stems that stay clean: unlike some other tomato diseases, septoria rarely spots the stems or the fruit, which helps to distinguish it.

Septoria leaf spot usually appears in early summer, after the first wet weather, with the first spots showing on the lowest leaves where rain splash from the soil is strongest. The disease climbs the plant through the summer, with each rainy spell producing a new wave of spots higher up, and in a wet year a tomato plant can lose most of its leaves by August. In a dry year the same plant may show only a few spots all season. The disease slows in hot, dry weather and stops when the leaves stay dry, which is why the same plant can look terrible in a rainy July and recover with clean new growth in a dry September. The fruit is rarely affected directly, so the loss is in yield and in sunscald, not in rotted fruit.

Which plants get septoria leaf spot

Septoria leaf spot attacks tomatoes above all, and it can also affect potatoes, eggplants, and other members of the nightshade family to a lesser degree. All tomato varieties are susceptible to some degree, though some show more tolerance than others, and cherry tomatoes are as vulnerable as large-fruited types. The disease is worst on tomatoes grown in the same bed year after year, in crowded plantings, and in humid climates. It does not attack cucurbits, brassicas, or roses, so it is a problem for the tomato bed alone.

Conditions that favour septoria leaf spot

Septoria leaf spot needs wet leaves and warm temperatures, roughly 70 to 80 degrees Fahrenheit, with leaves wet for many hours. Rain, overhead watering, and heavy dew all provide the wetness, and crowded plants that dry slowly are hit hardest. The fungus overwinters in fallen leaves, in infected stems, and in the soil near them, so a bed that was not cleaned up starts the season with the disease already present. It is worst in humid climates and in rainy summers, and in beds where tomatoes have been grown year after year. Plants that are stressed by drought or poor nutrition are more badly affected, though vigorous plants can be hit too.

How septoria leaf spot spreads

Septoria leaf spot spreads in splashing water: rain hitting an infected leaf throws spores onto the leaves below and beside it, and overhead watering does the same. It also spreads on hands, tools, and clothing that touch wet plants, and in infected leaves left in the garden. The spores do not travel far on the wind, so the disease moves mainly within a plant and to its immediate neighbours. It overwinters in fallen leaves and debris, so the previous year's leaf litter is the most common source of the next year's outbreak. That is why autumn cleanup and mulching are so effective against it.

How to tell it apart from lookalikes

Septoria leaf spot is often confused with early blight, but early blight spots are larger, show concentric rings like a target, and affect the stems and fruit as well as the leaves, while septoria spots are small, have pale centres with tiny black dots, and stay mostly on the leaves. Bacterial spot makes smaller, angular, water-soaked spots without the pale centres and black dots. Powdery mildew makes a white powdery coating, not spots. Sunscald makes pale, papery patches on the fruit and the leaves without distinct spots. If the spots are small, have pale centres, and hold tiny black dots, septoria is the most likely answer.

Prevention: cultural steps that work

  • Mulch under tomatoes to stop soil and spores from splashing onto the lower leaves — this is the single most effective step against septoria.
  • Space plants generously, stake them, and prune for airflow so leaves dry quickly after rain or watering.
  • Water at soil level, in the morning, and avoid wetting the foliage.
  • Rotate tomatoes so they do not return to the same bed for at least two to three years.
  • Remove the lowest leaves as the plant grows, so no leaf touches the soil or the mulch, and air can move under the plant.
  • Choose healthy transplants from reputable sources, and avoid bringing in plants from a garden with septoria.
  • In autumn, pull up the whole plant, roots and all, and clear the bed completely, so the fungus does not overwinter in debris.

Treatment step by step

  1. Pick off spotted leaves as soon as you see them, starting with the lowest leaves, and dispose of them with household waste. Do this in dry weather so you do not spread spores on wet foliage.
  2. Mulch the bed if you have not already, so rain does not splash soil and spores onto the leaves.
  3. Stop overhead watering and switch to watering at soil level, in the morning, so the foliage stays dry.
  4. Prune out crowded growth and remove the lowest leaves, so air moves under and through the plant.
  5. If you choose to spray, a fungicide labelled for septoria leaf spot on tomatoes can protect new growth, but it will not cure spots that are already there. Apply it to clean new foliage exactly as the label directs, and start early in the season.
  6. Feed the plant moderately so it can replace lost leaves, but avoid heavy nitrogen that pushes soft, vulnerable growth.
  7. At the end of the season, pull up the plants and clear all debris from the bed, so the disease does not overwinter there.

What recovery looks like

Recovery from septoria leaf spot looks like clean new growth: once the weather dries, the mulch is down, and the spotted leaves are removed, the new leaves at the top of the plant should come through without spots, and the plant can go on to produce a reasonable crop from its upper growth. The spotted leaves will not heal — they will yellow and drop — and the lower stem may stay bare for the rest of the season. A plant that keeps its upper leaves and sets fruit after the outbreak is recovering, even if it looks thin at the bottom. Full recovery is a next-season matter: a bed that is cleaned up in autumn, rotated, and mulched in spring will usually show only a few spots the following year, rather than a full outbreak.

When a plant is beyond saving

A plant is beyond saving from septoria leaf spot when it has lost most of its leaves, when new growth keeps coming through spotted despite dry weather, mulch, and cleanup, and when it has stopped growing and fruiting. At that point the plant is spending more energy fighting the disease than fruiting, and removing it protects the neighbouring plants. A plant with a few spotted lower leaves and a clean, growing top is not beyond saving — prune the worst leaves, mulch, dry the foliage out, and let it carry on.

Frequently asked questions

Can I eat tomatoes from a plant with septoria leaf spot?

Yes, generally — septoria rarely affects the fruit itself, so the tomatoes are usually fine, though they may be smaller and fewer if the plant is badly defoliated. Discard any fruit that is sun-scalded, cracked, or rotting, and wash the fruit before eating it.

Will septoria leaf spot kill my tomato plant?

Rarely on its own, but severe defoliation weakens the plant, cuts the yield, and exposes the fruit to sunscald. A plant that loses most of its leaves will produce little, and it is often better to replace it with a fresh planting.

What are the tiny black dots in the spots?

They are the fruiting bodies of the septoria fungus, and they are the most distinctive sign of the disease. If you can see tiny black dots in the pale centre of a small leaf spot, you are almost certainly looking at septoria.

Can I compost tomato leaves with septoria?

No. Home compost rarely gets hot enough to kill the fungus, so composting infected leaves returns the disease to the garden. Bag the leaves and put them with household waste.

Why does septoria hit my tomatoes every year?

The fungus overwinters in fallen leaves and debris, and it returns in the first wet weather of summer. Thorough autumn cleanup, rotation, mulching, and staking are the ways to break the yearly cycle — and check your local extension service for the tomato varieties that hold up best in your area.

Getting help and going further

If you are unsure which disease you are dealing with, take clear photos of the symptoms and check your local extension service — they can confirm a diagnosis from a sample and advise on the varieties and the timing that work best in your area. You can also try our plant disease checker to narrow down the problem, and browse the guides hub for related disease guides. For growing advice, see growing tomatoes. For growing advice, see growing roma tomatoes. For growing advice, see growing cherry tomatoes. Related guides: early blight tomatoes, leaf spot disease, bacterial spot. Whenever you remove diseased material, dispose of it with household waste — do not compost diseased material, since home compost rarely gets hot enough to kill the organisms that cause these diseases. Good record-keeping helps too: note the date, the weather, and the variety when a problem first appears, so you can spot the pattern next year and act earlier.