What Early Blight Is
Early blight is a serious fungal disease that strikes tomatoes and potatoes, caused by two closely related species: Alternaria tomatophila and Alternaria solani. 1 Both pathogens can also infect peppers and several weeds in the Solanaceae family, including black nightshade and hairy nightshade. 1
While tomatoes and potatoes bear the worst of it, peppers and eggplants are rarely affected to the same degree. 4
How to Spot It
The most recognisable sign of early blight is a distinctive bulls-eye pattern on leaves and stems. 5 These lesions can appear on all above-ground plant parts, and the disease can also reach potato tubers. 5
Early blight produces a wide range of symptoms depending on how far the infection has progressed. 5 Early blight produces a wide range of symptoms depending on how far the infection has progressed. 5
Conditions That Drive Infection
The disease develops at moderate to warm temperatures, between 59 and 80°F, with an optimum range of 82 to 86°F. 1 Wet weather, heavy dew, or relative humidity at 90% or greater are the conditions most likely to spread the pathogen. 1
That combination of warmth and moisture is common during the growing season, which is why early blight recurs year after year in affected plots.
Why Soil and Sun Matter
Fertile, rich soil and plenty of sunlight can help tomatoes, potatoes, peppers, and eggplants stay productive even when early blight begins to appear. 2 Adding compost supports plant health and continued fruiting without the setbacks that stressed plants suffer. 2
A strong, well-fed plant can tolerate infection better than one grown in poor conditions. Fertile, rich soil and compost support plant health and continued fruiting even as infection progresses. 2
What to Watch Next
Monitor plants closely during warm, humid spells — the conditions where Alternaria solani thrives most. 1 Monitor for the characteristic bulls-eye lesions that can appear on all above-ground plant parts. 5
Remove infected leaves promptly and avoid overhead watering where possible. Supporting plant vigour through good soil and sun remains one of the most effective long-term strategies. 2
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