Early Blight vs Late Blight vs Septoria Leaf Spot: How to Tell Them Apart
Three tomato leaf diseases, three different responses. Spot shape, position on the plant and speed of spread tell you which one you have.
Look at the spots and how fast they move. Early blight makes larger brown spots with target-like rings, starting low on the plant. Septoria leaf spot makes many tiny spots with gray centers and black specks, also starting low. Late blight makes water-soaked patches that turn brown or black, can appear on young leaves too, and is the urgent one: bag affected plants and contact your extension office.
Side-by-side comparison
| Feature | Early blight | Septoria leaf spot | Late blight |
|---|---|---|---|
| Caused by | Fungus (Alternaria) | Fungus (Septoria lycopersici) | Water mold (Phytophthora infestans) |
| Where it starts | Older, lower leaves | Older, lower leaves | Young leaves as well as lower ones |
| Spot size | About 1/4 to 1/2 inch | About 1/16 to 1/8 inch | Irregular patches that can spread widely |
| Spot look | Brown to black with concentric rings; often a yellow halo | Gray or beige center, dark border, tiny black specks | Water-soaked at first, then brown or black |
| Number of spots | Often one or two per leaf at first | Numerous | Varies; patches can grow together |
| Favored by | Warm weather (above roughly 75 to 80°F) with rain or high humidity | Rainy weather, about 60 to 80°F | Generally linked to cool, wet spells |
| Fruit | Dark, leathery, sunken spots near the stem, often ringed | Not usually infected, but leaf loss exposes fruit to sunscald | Can be affected |
| Speed | Climbs gradually | Climbs gradually | Can move quickly |
| Response | Prune, mulch, protect new growth | Prune, mulch, protect new growth | Remove and bag; alert extension |
One detail varies by source. University of New Hampshire Extension says septoria spots lack a yellow halo, while University of Wisconsin material says a halo is often present. Do not rely on the halo alone. The black specks in the center of the spot and the small spot size are better clues.
How to check your plant
- Start at the bottom. Turn over the lowest leaves and work upward.
- Use a hand lens or your phone camera to zoom in on a spot. Look for rings or for tiny black specks.
- Note the size. Pencil-eraser spots suggest early blight, and pinhead-to-matchhead spots suggest septoria.
- Check young leaves at the top. Late blight is not limited to the lower plant.
- Check stems and fruit. Early blight can leave dark, sunken cankers on stems and sunken spots at the stem end of fruit.
- Think about weather and speed. Spots that spread through much of a plant within days during wet weather deserve a fast response.
Other diseases can look similar. NC State Extension notes that gray leaf spot and bacterial spot are both look-alikes with smaller lesions. If you are unsure, send a sample or sharp photos to your county extension office.
What to do for early blight and septoria
The two need a similar response, and the goal is to slow them, because neither is easily managed once it has started. Spots will not heal, so you are protecting new growth.
- Remove the worst lower leaves on a dry day and put them in the trash. Avoid stripping the plant.
- Mulch the soil so rain does not splash spores onto the leaves.
- Water at the base. If you must water overhead, do it in the morning so leaves dry by evening. See how often to water vegetables.
- Improve airflow. Keep plants off the ground. Our comparison of tomato cages and stakes covers support options.
- Keep plants fed and watered. Stressed, weak plants tend to suffer more.
- Consider a fungicide only if the disease is a repeat problem. Least toxic options first: NC State lists copper, potassium bicarbonate and several Bacillus-based biofungicides among organic products. Chlorothalonil is a common conventional option. These protect healthy tissue rather than cure spots, so they need repeat applications as new leaves grow. Follow the label for rates, intervals and days to harvest, use one product at a time, never mix products, and store them away from children and pets.
What to do for late blight
Late blight is the one to treat as an emergency, because it can affect tomatoes and potatoes and spread beyond your garden.
- Check nearby tomato and potato plants.
- Pull badly affected plants and seal them in a bag for the trash. NC State Extension advises scouting often and removing infected plants, infected fruit, volunteers and weeds.
- Do not compost them. See what can go in the compost.
- Contact your county extension office. Some areas track local outbreaks, and staff can confirm the diagnosis and tell you which products, if any, are recommended this season.
Fine, pale, fuzzy growth on leaf undersides in humid weather is often described as a sign of late blight. If you see it, treat it as a warning and get the plant checked rather than relying on your eye alone.
Preventing all three next year
- Rotate. Keep tomatoes out of the same spot for two to three years, since early blight and septoria both overwinter in crop debris.
- Clean up. Remove diseased plants and volunteer tomatoes at the end of the season.
- Choose tolerant varieties. No variety is fully resistant to early blight, but some hold up better. Most current varieties are susceptible to septoria.
- Start clean. Use healthy transplants, and clean stakes and cages before reusing them.
- Prune low leaves as plants grow, since they are usually infected first.
Bottom line
Rings mean early blight. Many tiny spots with black specks mean septoria. Water-soaked patches that spread quickly, including on young leaves, call for removing the plant and contacting extension. If yellowing is your main symptom, compare it with our guide to tomato leaves turning yellow.
FAQ
Can a tomato plant recover from early blight?
Damaged leaves will not recover, but plants can keep producing if you slow the spread with pruning, mulch and steady care.
Can I eat tomatoes from a plant with septoria?
Septoria does not usually infect fruit, so sound tomatoes can be used. Discard any fruit with spots or rot, whatever the disease.
Is late blight a fungus?
No. It is caused by a water mold, Phytophthora infestans, though many garden sources call it a fungus.
Should I replant in the same spot next year?
Try not to. Rotate tomatoes and their relatives to another bed for at least two to three years if you can.
