Why Do My Tomato Leaves Have Brown Spots?

Brown spots are the most common disease symptom on tomatoes, and the moment you spot them the clock starts ticking, because the two usual culprits both spread upward and can strip a plant bare if ignored. The good news is that you can identify which disease you have by looking closely at a single spotted leaf, and the treatment is broadly the same. Let me show you how to read the spots and act before it climbs.

Early blight: brown spots with target rings

If the brown spots are fairly large, roughly the size of a coin, and have concentric rings inside them like a tiny archery target or tree stump, you are looking at early blight. It almost always starts on the lowest, oldest leaves and works its way up. The tissue around each spot often yellows, and badly affected leaves brown off entirely and drop. Early blight loves warm, humid weather and thrives when spores splash up from the soil onto the lower leaves.

Septoria leaf spot: many small spots with pale centres

If instead you see lots of small, round spots with grey or tan centres and dark brown margins, often peppered with tiny black dots in the middle, that is septoria leaf spot. The spots are smaller and far more numerous than early blight, sometimes dozens on a single leaf. It also begins low and climbs, and like early blight it explodes in warm, wet conditions. The two often appear together.

The treatment is the same for both

Whichever one you have, the response is identical and you should start today. First, remove every affected leaf and bin it — never compost diseased foliage, because the spores survive. Strip the lowest leaves so none touch the soil, where the spores live. Second, change how you water: always water at the base of the plant, never over the leaves, and do it in the morning so any splash dries quickly. Wet leaves are how these fungi spread.

Third, improve airflow by spacing plants generously and pruning out congested growth so leaves dry fast after rain or dew. If the disease is advancing despite this, a copper-based or biological fungicide applied to the remaining healthy leaves can slow it down, but it protects, it does not heal — so apply it before the fungus reaches the top.

Stopping it coming back

Both diseases overwinter in the soil and on old plant debris, which is why they so often return to the same bed year after year. Clear away all tomato debris at the end of the season, rotate your tomatoes to a different spot for a couple of years, and lay down a clean mulch each spring to act as a barrier between the soil and the lowest leaves. A thick straw mulch alone prevents a surprising amount of splash-borne infection. Catch the spots early, keep the leaves dry, and your plant will keep producing right through the season.