Leaf Spot Diseases: Septoria, Alternaria, Bacterial
title: "Leaf Spot Diseases: Septoria, Alternaria, and Bacterial Leaf Spot"
—- title: "Leaf Spot Diseases: Septoria, Alternaria, and Bacterial Leaf Spot" slug: leaf-spot-diseases hub: problems category: Problem description: "Leaf spot on tomato, vegetables, and ornamentals: how to distinguish Septoria leaf spot, Alternaria blight, and bacterial leaf spot, and how to manage each." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-
"Leaf spot" describes the symptom, not the disease. Multiple pathogens — fungal and bacterial — cause spots on leaves, and the management differs depending on which organism is responsible. Treating bacterial leaf spot with a fungicide won't help. Treating septoria leaf spot with a bactericide is similarly pointless.
This guide covers the most common leaf spot diseases on outdoor vegetable crops and ornamentals in the Northeast, with diagnostic features for each and management protocols sourced from university Extension publications.
Septoria leaf spot on tomato
Pathogen and appearance
Septoria lycopersici is a fungal pathogen specific to tomato. Per Penn State Extension, it is the most common and destructive tomato leaf disease in the northeastern United States.
Identification:
- Small, circular spots, 1/8 to 1/4 inch in diameter
- White or gray centers with dark brown borders
- Dark specks visible in the center of spots (pycnidia — spore-producing bodies)
- Spots appear first on lower, older leaves
- Severe infections cause complete yellowing and defoliation from the bottom up
- Fruit is not directly infected
Per Cornell Cooperative Extension, septoria leaf spot first appears in the Northeast after plants are set out in warm, wet weather — typically June in Long Island gardens. By August, unmanaged plants may be nearly defoliated. The defoliation reduces fruit size and quality because exposed fruit sunscalds.
Conditions favoring infection: Per Penn State Extension, the disease is most active when temperatures are 60–80°F and leaf wetness events (rain, dew, overhead irrigation) occur frequently. New England and Middle Atlantic states have summer weather that consistently meets these criteria.
Alternaria blight of tomato
Alternaria solani causes early blight on tomato, despite the confusing name — it is not limited to early in the season.
Per Rutgers NJAES, Alternaria blight symptoms on tomato:
- Larger, irregular brown spots with dark concentric rings (bullseye pattern) — diagnostic
- Yellow halo around spots
- Lesions on stems as well as leaves, often at the soil line
- Fruit can also be infected (black, leathery lesion at stem end)
- Lower leaves affected first, progressing upward
The concentric ring pattern (bullseye) distinguishes Alternaria blight from septoria leaf spot, which has uniform circular spots with lighter centers.
Bacterial leaf spot of tomato and pepper
Xanthomonas vesicatoria (and related Xanthomonas species) cause bacterial leaf spot, which is distinct from fungal spots.
Per Penn State Extension:
- Water-soaked, greasy-looking spots initially; turn brown or black
- Irregular, angular spots often bounded by leaf veins
- Yellow halo around spots
- Spots may have ragged edges
- Fruit lesions — raised, scabby spots on green fruit
- Spreads rapidly in wet weather; enters through wounds and natural openings
The key distinction: Bacterial leaf spot lesions often appear water-soaked and angular (following leaf veins); fungal spots are more uniform and circular with defined borders.
Leaf spot on ornamentals
Rose black spot vs. leaf spot
Per Missouri Botanical Garden, Diplocarpon rosae causes black spot on rose — circular black spots with fringed (feathery) edges on upper leaf surfaces, followed by yellowing and defoliation. This is technically a leaf spot disease though commonly listed separately.
Bacterial leaf scorch
Per Clemson HGIC, bacterial leaf scorch of trees and shrubs (Xylella fastidiosa) causes brown scorch patterns on leaf margins that resemble drought stress. This is a systemic bacterial disease distinct from spot-pattern leaf diseases.
General ornamental leaf spots
Per Clemson HGIC, ornamental plants can develop leaf spots from a wide variety of fungal pathogens (Cercospora, Phyllosticta, Marssonina, Entomosporium, and others). Management principles are similar across hosts:
- Remove and dispose of infected leaves promptly
- Avoid overhead watering
- Improve air circulation
- Apply fungicide preventively in high-risk periods
- Fall cleanup to remove overwintering spore source
Diagnostic comparison table
| Feature | Septoria Leaf Spot | Alternaria Blight | Bacterial Leaf Spot |
|---|---|---|---|
| Pathogen type | Fungus (Septoria lycopersici) | Fungus (Alternaria solani) | Bacterium (Xanthomonas spp.) |
| Spot shape | Small, circular, uniform | Larger, irregular, bullseye rings | Angular, irregular, water-soaked |
| Spot color | Gray-white center, dark border | Brown with concentric rings | Brown or black, water-soaked initially |
| Yellow halo | Occasional | Common | Common |
| Spore bodies visible | Yes (dark specks in center) | No | No |
| Fruit affected? | No | Yes (stem end) | Yes (raised scabby spots) |
| Starts on | Lower/older leaves | Lower leaves, stems | Lower leaves |
| Treatment | Chlorothalonil, copper, mancozeb | Same fungicides | Copper bactericide |
Management
Cultural controls
Avoid overhead watering. Per Penn State Extension, wet foliage is required for septoria spore germination. Drip irrigation or a soaker hose keeps foliage dry and significantly reduces leaf spot infection.
Mulch. Per Cornell Cooperative Extension, mulching soil prevents rain splash from spreading spores from infected soil and plant debris onto lower leaves. A 3-inch layer of straw, wood chips, or shredded leaves under tomatoes is one of the most effective management practices.
Remove infected leaves. Per Penn State Extension, when leaf spots appear, remove and destroy infected leaves immediately. Do not compost diseased foliage. This reduces the spore load available to spread to upper leaves.
Space plants adequately. Tomatoes crowded together dry more slowly after rain or irrigation. Per Rutgers NJAES, standard spacing for indeterminate tomatoes is 24—36 inches in-row; larger spacing improves air circulation and reduces disease severity.
Fall cleanup. Per Cornell Cooperative Extension, septoria overwinters on infected plant debris in the soil. Remove all tomato plant material in fall — stems, leaves, fruit — and dispose of it. This reduces the spore load for the next season.
Crop rotation. Per Penn State Extension, rotate tomatoes to a different bed each year. The fungus does not survive multiple years in the absence of tomato tissue, so a 2- to 3-year rotation significantly reduces disease pressure.
Fungicide options (fungal diseases)
Per Penn State Extension, for septoria and Alternaria blight:
- Chlorothalonil (Daconil) — broad-spectrum contact fungicide; most effective in university trials; apply every 7—10 days during wet weather
- Mancozeb — broad-spectrum; similar efficacy to chlorothalonil
- Copper fungicide — lower efficacy than chlorothalonil but OMRI-listed for organic production
- Neem oil — some preventive activity; less effective than chlorothalonil in trials
Begin applications at transplanting or when weather conditions favor disease, before symptoms appear. Per Cornell Cooperative Extension, "fungicides are only effective as preventives."
For bacterial leaf spot
Copper-based bactericides (copper hydroxide, copper octanoate) are the primary treatment. Per Penn State Extension, copper applications are most effective when initiated early and repeated at 7-day intervals during wet weather. Copper does not cure existing lesions but reduces spread to healthy tissue.
Note that conventional fungicides (chlorothalonil, mancozeb) do not affect bacterial diseases. Use copper for bacterial leaf spot.
Common problems table
| Symptom | Cause | Fix |
|---|---|---|
| Small circular spots with gray centers on tomato lower leaves | Septoria leaf spot | Remove infected leaves; mulch; apply chlorothalonil preventively |
| Brown bullseye spots on tomato leaves and stems | Alternaria blight | Same cultural and chemical management as septoria |
| Angular water-soaked spots on tomato or pepper | Bacterial leaf spot | Copper bactericide; avoid overhead watering; remove infected tissue |
| Leaf spots despite fungicide application | Existing spots cannot be cured | Continue protection of healthy tissue; focus on fall cleanup |
| Rapid defoliation from bottom up | Severe septoria | Stake plants for airflow; remove all lower infected leaves; fungicide program |
Frequently asked
Will my tomatoes die from septoria leaf spot?
Usually not, but severe defoliation reduces yield and exposes fruit to sunscald. Per Penn State Extension, the plant typically survives the season with reduced production. Tomatoes are most productive when they retain a full canopy of healthy leaves. Losing the lower two-thirds of the plant's leaves by August, which is common without management, produces undersized fruit.
Is it safe to eat tomatoes from plants with leaf spot?
Yes. Septoria leaf spot and Alternaria blight do not contaminate the fruit, though Alternaria can cause a stem-end rot on fruit in severe infections. Bacterial leaf spot can produce raised scabby lesions on green fruit; the lesions do not penetrate deeply and the rest of the fruit is safe to eat.
How do I tell septoria from early blight (Alternaria)?
The spot pattern is the key. Per Rutgers NJAES, septoria spots are small (1/8—1/4 inch), circular, and have a gray-white center with a dark brown border and visible tiny dark specks in the center. Alternaria spots are larger, irregular, and display the distinctive concentric ring (bullseye) pattern. Both start on lower leaves and progress upward.
Can I prevent leaf spot entirely?
No — not in a typical Northeast summer with warm, wet weather. Per Cornell Cooperative Extension, the combination of mulching, drip irrigation, crop rotation, and resistant varieties (where available) can dramatically reduce disease severity, but in a wet June and July on Long Island, some septoria infection is the normal baseline.
Recommended gear: Best Soaker Hose for Vegetable Gardens (2026) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.
Sources
- Penn State Extension — <a href="https://extension.psu.edu/septoria-leaf-spot-of-tomato">Septoria Leaf Spot of Tomato</a>
- Penn State Extension — <a href="https://extension.psu.edu/bacterial-leaf-spot-of-tomato-and-pepper">Bacterial Leaf Spot of Tomato and Pepper</a>
- Cornell Cooperative Extension — <a href="https://cals.cornell.edu/new-york-state-integrated-pest-management/outreach-education/whats-wrong-my-plant/vegetables-herbs-fruits/septoria-leaf-spot">Septoria Leaf Spot</a>
- Rutgers NJAES — <a href="https://njaes.rutgers.edu/plantdisease/">Plant Disease Profiles</a>
- Missouri Botanical Garden — <a href="https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/problems/rose-black-spot">Rose Black Spot</a>
- Clemson HGIC — <a href="https://hgic.clemson.edu/factsheet/leaf-spots/">Leaf Spots</a>