Early blight
A fungal disease of tomatoes and potatoes that causes brown spots with target-like rings on the lower leaves.
Also known as: Target spot, Alternaria blight
Symptoms and identification
Brown spots with concentric rings, often surrounded by a yellow halo, appear first on the oldest leaves. Leaves yellow and fall, and the disease moves up the plant. Dark, sunken spots may form on stems and near the stalk end of fruit.
Causes
The fungus Alternaria survives on plant debris and in soil, and spreads by water splash and wind. Warm, humid weather, wet leaves and stressed or poorly fed plants make it worse.
Organic treatment
Remove and bin affected leaves. Spray with a copper-based fungicide (allowed in organic growing) or a Bacillus subtilis product every 7–10 days in humid weather.
Prevention
Mulch to stop soil splashing onto leaves, remove the lowest leaves once plants are established, water at the base and rotate crops. Keep plants well fed.
Chemical treatment (use with care)
Use a fungicide labelled for early blight on tomatoes, following the label and the waiting period before harvest.