Tomato blight strikes fear into every vegetable gardener. Watching healthy, productive tomato plants rapidly yellow, develop dark lesions, and collapse is one of the most frustrating experiences in the garden. Blight is the collective term for several fungal and oomycete diseases that attack tomato foliage and fruit, and understanding which specific disease you are dealing with is essential for effective treatment.
The three most common tomato blight diseases — early blight, late blight, and Septoria leaf spot — affect tomatoes in nearly every growing region. Together, they are responsible for more tomato plant losses than all other diseases combined. The encouraging news is that with proper identification and management, you can successfully grow healthy tomatoes even in blight-prone regions.
Key Takeaways
- Early blight shows concentric ring patterns (target-shaped spots) on lower leaves first and progresses upward — manageable and rarely kills plants
- Late blight produces large, water-soaked gray-green patches that spread explosively in cool, wet weather — can destroy plants within days
- Septoria leaf spot creates numerous tiny dark-bordered spots causing progressive defoliation from the bottom up
- Prevention through resistant varieties, mulching, proper spacing, and avoiding wet foliage is far more effective than treating active infections
- Crop rotation (3 to 4 year cycles) and thorough fall cleanup are the most important long-term management practices
Early Blight (Alternaria solani)
Identification
Early blight is the most common tomato disease. Look for dark brown to black spots on lower leaves that display distinctive concentric rings — the spots resemble a target or bullseye pattern. This target pattern is the key identifier. Spots are typically a quarter-inch to half-inch in diameter surrounded by a yellow halo. The disease progresses from bottom to top as spores splash upward during rain and watering.
As the disease advances, affected leaves yellow, dry up, and drop. Severe defoliation exposes fruit to sunscald and reduces the plant’s ability to support fruit development. Early blight can also cause dark, sunken, leathery spots on fruit at the stem end.
Conditions That Favor Early Blight
Warm (75 to 85 degree) humid conditions with frequent rain or overhead irrigation. The fungus overwinters in infected plant debris and on garden tools. Alternating wet and dry periods accelerate disease development.
Organic Treatment
Remove and destroy affected lower leaves immediately. Copper-based fungicide sprays provide moderate control when applied preventively or at first signs. Apply every 7 to 10 days during favorable conditions. Bacillus subtilis (Serenade) provides preventive biological protection.
Late Blight (Phytophthora infestans)
Identification
Late blight is the devastating disease that caused the Irish Potato Famine. Unlike early blight’s gradual progression, late blight can destroy entire plantings within days.
Symptoms begin as large (1 to 2 inch) water-soaked, gray-green patches on leaves, often starting at leaf edges. In humid conditions, white fuzzy growth appears on the underside of affected areas — this white fuzz is the definitive identifier. Affected areas quickly turn dark brown to black and become papery. Stems develop dark brown lesions. Fruit develops large, firm, brown blotches.
Late blight spreads by wind-borne spores that can travel miles. It is an oomycete (water mold), not a true fungus — some fungicides effective against fungi are ineffective against it.
Conditions That Favor Late Blight
Cool (50 to 70 degree), wet conditions with high humidity — the opposite of early blight. Extended leaf wetness from rain, fog, heavy dew, or overhead irrigation is essential for infection.
Organic Treatment
Remove and bag (do not compost) all affected tissue immediately. If more than one-third is affected, remove the entire plant to protect neighboring gardens. Copper fungicide applied preventively provides moderate protection but cannot cure active infections.
Septoria Leaf Spot (Septoria lycopersici)
Identification
Septoria produces numerous very small spots (one-sixteenth to one-eighth inch) with dark brown borders and light gray centers. Tiny black dots (pycnidia) may be visible in spot centers with a hand lens. Spots are far more numerous than early blight lesions. Like early blight, it causes progressive defoliation from bottom up but primarily affects foliage — fruit infection is uncommon.
Organic Treatment
Same approach as early blight: remove affected lower leaves, apply copper-based fungicide preventively, maintain air circulation, and avoid wetting foliage. Septoria is typically less aggressive and responds well to cultural management.
Prevention: The Real Solution
Crop Rotation
Rotate tomatoes and all Solanaceae (peppers, eggplant, potatoes) to different locations on a 3 to 4-year cycle. In small gardens, raised beds with replaced or heavily amended soil provide a partial alternative.
Resistant Varieties
Mountain Merit, Defiant, Iron Lady, and Jasper (cherry) are notable resistant varieties with excellent flavor alongside disease tolerance. For container growing, choose compact resistant varieties for the best chance of blight-free harvests.
Mulching
A 3 to 4-inch layer of organic mulch prevents soil splash — the primary mechanism for transferring blight spores to lower leaves. Mulching alone reduces early blight and Septoria by 50 percent or more. Our mulching guide covers materials and techniques.
Pruning and Spacing
Remove lower branches up to 12 to 18 inches above ground. Provide adequate spacing (24 to 36 inches) for air circulation. Stake or cage plants to keep foliage off the ground.
Watering Practices
Never use overhead sprinklers on tomatoes. Water at the base using drip irrigation, soaker hoses, or careful hand watering. Wet foliage is the single most controllable risk factor. Morning watering allows any incidental moisture to dry quickly.
Fall Cleanup
Remove all tomato plant debris after the season. Do not compost blighted material. Clean and sanitize cages, stakes, and tools using a 10 percent bleach solution.
Frequently Asked Questions
Can I eat tomatoes from blighted plants?
Fruit without visible disease symptoms is safe to eat. Blight pathogens are plant-specific and do not cause human illness. Fruit with visible lesions should be discarded for quality reasons.
Is blight contagious to other garden plants?
Early blight and Septoria primarily affect tomatoes, potatoes, and occasionally eggplant and peppers. Late blight devastates tomatoes and potatoes but does not spread to unrelated plants.
Can I save seeds from blighted tomato plants?
Yes, if the fruit itself is clean. Blight pathogens are not seed-borne. Allow healthy fruit to ripen fully, save seeds using the fermentation method, and dry thoroughly.
Should I spray copper preventively even without blight?
In regions with high blight pressure, preventive copper every 10 to 14 days significantly reduces disease. In drier climates, cultural practices alone may suffice. Copper accumulates in soil with repeated use — use the minimum effective rate.
Can I plant tomatoes in the same spot after blight?
Wait 3 to 4 years before planting tomatoes, peppers, potatoes, or eggplant in the same location. If space is limited, replace the top 6 to 8 inches of soil, apply fresh compost, and choose resistant varieties.
