Black spots on tomato leaves are not one single problem. University extension research points to four main suspects, and each one leaves a different signature.
Guessing wrong wastes a season. Treating a bacterial problem like a fungal one, or the reverse, does little to slow the actual damage down.
This guide walks through what separates each cause. It uses spot size, pattern, and location to narrow the diagnosis fast.
The short version
Large spots with visible rings inside them point to early blight. Small spots with tiny black dots in the center point to septoria leaf spot.
Small, dark, greasy looking spots with no rings usually mean a bacterial problem. Bacterial speck and bacterial spot both fall into that category.
Location matters too. Early blight and septoria both start on the lower, older leaves first. Bacterial spots can show up anywhere conditions stay wet.
The two questions that narrow it down fast
Size is the first clue. Early blight lesions run a quarter to a half inch across. Septoria spots run much smaller, from a sixteenth to a quarter inch.
Pattern is the second clue. Early blight lesions develop visible concentric rings, often described as a bull’s eye or target pattern. Septoria spots stay uniform without rings, and bacterial spots never show rings at all.
Answer those two questions and the list of suspects shrinks fast.
Early blight: the target pattern
Early blight, caused by the fungus Alternaria solani, is the most common source of black or brown spots on tomato leaves. University of Maryland Extension describes irregular brown lesions with yellow halos. Those lesions enlarge quickly into a bull’s eye pattern.
The size range holds at a quarter to a half inch. Clear concentric rings form inside each lesion, and the surrounding tissue typically turns yellow.
Older, lower leaves show symptoms first. Left unchecked, individual lesions merge and can kill entire leaves.
Early blight does not stop at the leaves either. It can also cause dark, leathery, sunken spots near the stem end of the fruit.
Septoria leaf spot: the tiny black dot giveaway
Septoria leaf spot has one identifying feature nothing else shares. West Virginia University Extension describes dark brown, pimple like structures called pycnidia that develop inside mature spots.
Those pycnidia are the fungus’s fruiting bodies. They are visible with a hand lens, and no other common tomato leaf spot disease produces that specific marking.
The spots themselves start small, typically a sixteenth to an eighth of an inch. They carry tan or gray centers with dark brown margins.
They begin as numerous tiny spots on the lower leaves. From there, the spots spread upward through the plant.
Septoria rarely touches the fruit. And one clean tell: septoria does not affect fruit. That is a useful way to rule it out when spots show up on both leaves and tomatoes.
Bacterial speck and bacterial spot: when it is not a fungus at all
Bacterial problems look different from both fungal diseases described above. Bacterial speck shows as round, dark brown to black spots.
Those spots often develop a light colored ring as they age. They can also merge on young leaves, and the tissue between them tears as the leaf keeps growing.
Bacterial spot looks similar but smaller. It stays under an eighth of an inch, with a greasy, translucent look to the upper surface.
Wet weather makes both worse. Under heavy moisture, bacterial spot symptoms shift from distinct spots to a broader blighted appearance, with water soaked patches between the leaf veins.
Neither bacterial disease produces the concentric rings of early blight or the pycnidia of septoria. That absence is itself a useful clue once the other two have been ruled out.
What to do once you know which one it is
Every one of these problems responds to the same starting moves. That holds true regardless of which fungus or bacteria is actually involved.
Remove and discard affected lower leaves promptly, rather than composting them in the garden. Water at the soil line instead of overhead, since wet foliage is what lets all four of these problems spread.
Space plants for airflow. Rotate where tomatoes grow each season to reduce the pathogens waiting in the soil.
Frequently asked questions
What causes black spots on tomato leaves?
The most common causes are early blight, septoria leaf spot, bacterial speck, and bacterial spot. Spot size, the presence of rings, and whether fruit is also affected help narrow down which one is involved.
How do I tell early blight from septoria leaf spot?
Early blight lesions are larger, a quarter to a half inch, with visible concentric rings and a yellow halo. Septoria spots are smaller, with tan centers and tiny black dots, but no rings.
Do black spots on tomato leaves mean a bacterial infection?
Sometimes. Bacterial speck and bacterial spot both cause small, dark, greasy looking spots without the rings or fruiting bodies that mark the fungal diseases.
Will black spots on tomato leaves spread to the fruit?
It depends on the cause. Early blight can affect fruit with dark, sunken spots near the stem, while septoria leaf spot rarely touches fruit at all.
How do I stop black spots on tomato leaves from spreading?
Remove affected leaves, water at the soil line instead of overhead, and space plants for airflow. Crop rotation each season also reduces how much of the pathogen survives to the next planting.
The bottom line: black spots on tomato leaves usually trace back to one of four problems. Spot size, ring patterns, and fruit involvement separate them quickly.
Early blight brings visible rings and a yellow halo. Septoria leaf spot brings tiny black dots with no rings. The bacterial diseases stay small, dark, and ring free.
For the wider range of leaf symptoms this guide does not cover, see our tomato leaf problems guide. For a full disease overview, see our tomato diseases guide.
SOURCES
University of Maryland Extension: Key to Common Problems of Tomatoes. https://extension.umd.edu/resource/key-common-problems-tomatoes
West Virginia University Extension: Septoria Leaf Spot. https://extension.wvu.edu/lawn-gardening-pests/plant-disease/fruit-vegetable-diseases/septoria-leaf-spot
University of Connecticut, Integrated Pest Management, College of Agriculture, Health and Natural Resources: Bacterial Speck and Spot of Tomato. https://ipm.cahnr.uconn.edu/bacterial-speck-and-spot-of-tomato/
University of Wisconsin Extension, Barron County: Tomato Disorders, Early Blight and Septoria Leaf Spot. https://barron.extension.wisc.edu/files/2023/02/Tomato-Disorder-Early-Blight-and-Septoria-Leaf-Spot.pdf