Leaf Spot Diseases: Septoria, Alternaria, Bacterial
title: "Leaf Spot Diseases: Septoria, Alternaria, and Bacterial Leaf Spot"
"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.
This article contains affiliate links. As an Amazon Associate, we earn from qualifying purchases - at no extra cost to you. We only recommend products we have personally tested or that are the universal first recommendation from university Extension publications. See our full disclosure.
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.
Stop fungal disease early
Neem oil suppresses powdery mildew, black spot, and rust when sprayed at first sign. Cheaper than copper-based fungicides and safe on veg. Read our full pick.
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.