Dwight Sipler from Stow, MA, USA / Wikimedia Commons — source (CC BY 2.0)
Early blight alternaria solani
Alternaria solani
Early blight is a fungal disease caused by Alternaria solani, attacking tomatoes, potatoes and related crops, identified by dark spots with bull's-eye concentric rings on lower leaves. Remove and destroy infected plant parts immediately to limit spread. Improve air circulation and avoid overhead watering to prevent outbreaks.
Early blight, caused by the fungus Alternaria solani, is a common foliar disease in UK gardens, particularly affecting tomatoes and potatoes. It thrives in warm, wet conditions, starting on lower leaves and potentially spreading to stems and fruit. While it can reduce yields significantly if unchecked, you can manage it effectively through cultural practices, hygiene and targeted treatments. Stressed or poorly nourished plants are most vulnerable, so healthy growing conditions are your first defence.
Active from spring onwards, peaking in warm, humid summer months.
- •Spot bull's-eye rings on lower leaves early.
- •Prune and destroy infected parts at once.
- •Rotate crops and mulch to prevent.
- •Keep foliage dry and plants vigorous.
- •Use bicarb spray organically.
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How to identify
Look for small, dark brown spots on the older, lower leaves, typically 6-12mm across, with distinctive concentric rings resembling a bull's eye or target. These spots have a leathery texture and yellow halos around them. As the disease advances, spots merge, causing leaves to yellow, wither and drop. On stems, you may see similar dark lesions; on fruit, sunken brown patches often appear near the stem end. Distinguish it from other leaf spots by the target pattern—Septoria spots lack rings and start smaller without yellowing.
Damage and symptoms
Michal Maňas / Wikimedia Commons — source (CC BY 4.0)
Affected lower leaves turn yellow then brown, shrivelling and falling off, exposing fruit to sunscald. Defoliation weakens the plant, reducing photosynthesis and yield; stems may develop cankers, leading to wilting. On tomatoes and peppers, fruit develops dark, leathery spots, often at the calyx end, rendering them unmarketable. Severe infections can kill young plants or cut mature yields by half, especially in prolonged humid weather. Potato tubers rarely show symptoms but plants suffer blighted foliage.
Lifecycle
The fungus overwinters in infected plant debris, soil or on seed/tubers. Spores (conidia) spread by wind, rain splash, tools or insects, germinating in wet conditions (as little as 2 hours leaf wetness). New lesions produce more spores within days, accelerating in warm (24-30°C), humid spells. It favours moderate UK summers but slows in hot, dry weather. Interrupted wetting (dew then dry) boosts spore production.
Most affected crops
Prevention
Crop rotation
Avoid planting tomatoes, potatoes or related crops in the same spot for at least three years. This breaks the fungus's lifecycle by allowing debris to decompose fully. Incorporate rotated beds with non-hosts like beans, onions or brassicas to reduce soil inoculum.
Site and spacing
Choose sunny, well-drained sites with good airflow. Space plants adequately—60cm for tomatoes, 45cm for potatoes—to prevent crowding and humidity buildup. Stake or cage tomatoes to keep foliage off the ground and improve circulation.
Mulching
Apply a 5-10cm layer of organic mulch like straw or compost around stems after planting. This suppresses soil splash, which carries spores to lower leaves during rain.
Healthy plants
Feed with balanced fertiliser, avoiding excess nitrogen. Water consistently at the base to keep foliage dry. Remove lower leaves touching soil as plants grow.
Organic control
Hygiene first
Inspect plants weekly from lower leaves up. Remove and bin (don't compost) any spotted foliage promptly. Clean tools with alcohol between plants and at season's end. Clear all debris in autumn and dig in lightly to speed breakdown.
Baking soda spray
Mix 1 tsp bicarbonate of soda, 1 tsp horticultural oil and 1 litre water. Spray weekly on dry mornings, covering undersides. It raises leaf pH, inhibiting fungal growth. Test on one leaf first to avoid burn.
Compost tea or garlic spray
Brew aerated compost tea or blend garlic cloves with water, strain and spray. These boost beneficial microbes and act as mild fungistats. Apply preventively in humid weather, reapplying after rain.
Chemical control
Approved fungicides
Use products containing copper oxychloride or mancozeb, permitted under UK home garden rules. Apply at first signs, repeating every 7-10 days in wet weather, up to label limits. Alternate types to avoid resistance; stop use before harvest per instructions.
Companion plants and deterrents
Frequently asked
How do I get rid of early blight on my tomatoes?
Prune off infected lower leaves and destroy them immediately. Improve airflow by spacing and staking. Spray with bicarbonate of soda mix weekly, and mulch to block soil splash. In severe cases, apply copper-based fungicide early. Remove plant debris in autumn to prevent overwintering.
Is early blight harmful to potatoes?
Yes, it causes similar leaf blight on potatoes, reducing tuber yield by defoliating plants early. Spots start on lower leaves; manage with rotation, hygiene and avoiding wet foliage. It rarely affects tubers directly but stressed plants produce smaller crops.
Why does early blight keep coming back?
It survives in old debris, soil or seed. Break the cycle with strict rotation, thorough autumn cleanup and disease-free seed. Wet, humid conditions and stressed plants invite repeat attacks—focus on vigour and dry foliage.
Can I grow peppers near tomatoes with early blight?
Peppers are susceptible too, so separate them by 2m or more, or rotate separately. Plant in different beds and monitor peppers closely. Resistant varieties help, but hygiene applies to all solanaceous crops.
What organic spray works best on early blight?
Bicarbonate of soda with horticultural oil is effective and simple—spray weekly preventively. Garlic or compost tea sprays also deter spores. Combine with pruning for best results; they work by altering leaf conditions unfavourably for the fungus.
Does early blight affect the fruit?
Yes, it can spot fruit near the stem end with sunken, bull's-eye lesions, especially on ripe tomatoes. Prevent by controlling leaf infection early—healthy foliage protects fruit. Harvest promptly and discard affected ones.
How do I prevent early blight next season?
Rotate crops, mulch bases, water at soil level and space for airflow. Start with certified seed, feed balanced and prune lower leaves. Autumn cleanup is key—hot compost or bin all debris.
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