Early blight is a fungal disease that causes target-like brown spots with concentric rings on tomato and potato leaves, typically appearing mid to late summer and progressing upward from lower foliage. The disease can defoliate plants entirely and reduce yields by 50% if untreated. Control it through crop rotation, wider plant spacing, removing lower leaves, and applying copper or sulfur fungicides at first sign of infection.
Early blight is a common fungal disease caused by Alternaria solani that affects tomatoes, potatoes, and occasionally eggplants. It typically appears mid to late summer as circular brown lesions with concentric rings on lower leaves, progressing upward. Left unchecked, it can completely defoliate plants and reduce harvests by up to 50%. The fungus overwinters in soil and plant debris, spreading via wind, water splash, and contaminated tools.
Identification
Early blight starts on the lowest, oldest leaves as small circular brown spots. As lesions enlarge, they develop distinctive concentric rings like a target board, often surrounded by a yellow halo. Affected leaves eventually turn yellow and brown before dropping. On tomato stems and fruit, you'll see dry, leathery cracks or sunken spots, typically near the stem end. Symptoms usually appear mid to late July in the UK, though the disease can develop whenever humidity is high and foliage is dense.
Damage & symptoms
Early blight causes progressive defoliation starting from the base of plants and working upward. Individual lesions merge on heavily infected leaves, destroying large areas of leaf tissue. Severe infection can lead to total loss of lower foliage and, in extreme cases, plant death. On tomatoes, fruit develops cracks or sunken spots near the stem, making them unmarketable. Potatoes rarely show tuber infection but leaves are heavily affected. Uncontrolled early blight can cut harvest short by weeks.
Lifecycle
Alternaria solani overwinters in infected plant debris and soil, surviving for at least one season. Spores germinate when temperatures reach 68–86°F (20–30°C) with leaf wetness from rain, irrigation, fog, or dew. Lesions form 2–3 days after infection. Once established, the fungus produces spores continuously throughout the growing season, spreading via wind and water splash. Multiple infection cycles occur within a single season, with rapid disease expansion from mid to late August onward.
Prevention — Crop rotation and debris removal
Rotate tomatoes, potatoes, and eggplants off the same patch for at least 2–3 years. Thoroughly turn under all crop residue in autumn, as the fungus survives in decomposing plant material. Remove volunteer plants and weeds from the solanaceae family. Clean up fallen leaves immediately during the growing season.
Prevention — Spacing and airflow
Space plants widely to promote air circulation and reduce humidity around foliage. Avoid overcrowding, which creates the warm, moist conditions early blight thrives in. Prune lower branches to improve airflow, especially once plants are established. Good ventilation keeps leaves dry longer, reducing infection risk.
Prevention — Water management
Water plants at soil level using drip irrigation or soaker hoses rather than overhead sprinklers. Overhead watering splashes spores from soil onto leaves and prolongs leaf wetness. Water early in the day so any wet foliage dries quickly. Avoid handling wet plants, as this spreads spores between plants.
Prevention — Tool hygiene
Clean and disinfect pruning tools between plants using rubbing alcohol or a 10% bleach solution. Contaminated tools readily transfer the fungus from infected to healthy plants. Wash hands after handling diseased foliage. This simple practice significantly reduces secondary spread.
Prevention — Vigorous varieties
Choose vigorous tomato varieties, particularly cherry tomatoes, which can sometimes outgrow early blight pressure. Select disease-resistant cultivars where available. Healthy, well-nourished plants are less susceptible than stressed or poorly fed specimens.
Organic control — Copper fungicides
Apply copper-based fungicides such as Bonide Liquid Copper Fungicide every 7–14 days, starting in early summer before symptoms appear. Spray thoroughly, including leaf undersides. Copper is most effective as a preventative; once symptoms are visible, control becomes difficult. Ensure 12 hours of dry weather after application for best results.
Organic control — Removal of infected foliage
Remove lower branches and affected leaves as soon as symptoms appear. This reduces fungal spore production and improves airflow. Disinfect pruning tools with rubbing alcohol between cuts. Dispose of removed material in sealed bags or burn it; do not compost infected plant parts.
Organic control — Sulphur-based treatments
Sulphur dusts and sprays can help suppress early blight if applied regularly from early summer onward. Follow label instructions carefully. Sulphur works best in cooler conditions and may cause leaf burn in very hot weather. Combine with good cultural practices for best results.
Frequently asked questions
Why is it called 'early' blight if it appears in mid-summer?
Early blight is named for its appearance early in the season relative to late blight, another tomato disease. However, symptoms typically don't show until mid to late July in the UK, once plants have developed a full canopy and humidity is high. The disease progresses most rapidly from mid-August onward. Confusingly, early blight is often more common and damaging than late blight in UK gardens.
Can I save a plant once early blight appears?
Once symptoms develop, control is difficult but not impossible. Remove lower infected branches immediately, disinfecting tools between cuts. Apply copper fungicides every 7–14 days on a strict schedule, spraying thoroughly including leaf undersides. Improve airflow by further pruning. The goal is to slow disease spread enough to harvest fruit before the plant is too damaged. Vigorous varieties may recover better than weak ones.
Does early blight affect potatoes the same way as tomatoes?
Early blight is serious on both crops, causing the same target-like leaf lesions and progressive defoliation. On potatoes, the disease rarely infects tubers but heavily damages foliage, reducing yield. Potato tubers can develop circular, slightly sunken lesions if severely infected. Crop rotation between tomatoes and potatoes is essential, as the fungus affects both and overwinters in soil.
Can I compost infected leaves and stems?
No. The fungus survives in plant debris for at least one season and possibly longer. Dispose of infected material in sealed bags in household waste or burn it if local regulations allow. Do not add to garden compost, as compost piles rarely reach temperatures high enough to kill Alternaria spores. This is one of the most important steps in breaking the disease cycle.
What's the best time to start fungicide sprays?
Start spraying in early summer, ideally June, before symptoms appear. Copper and other protectant fungicides work best as preventatives applied on a regular 7–10 day schedule. Once lesions are visible, the disease is harder to control. Early, consistent spraying throughout July and August gives the best results, especially in wet years.
Should I remove all lower leaves as a preventative?
Removing lower leaves once plants are established improves airflow and removes the most vulnerable foliage where early blight typically starts. However, avoid over-pruning, which stresses the plant and exposes fruit to sunscald. Prune selectively to improve ventilation, then remove additional leaves only if symptoms appear.