Early Blight
Alternaria solani, Alternaria tomatophila
Overview
Early blight, caused mainly by Alternaria solani and related species, is a familiar fungal disease of tomatoes and potatoes. Concentric target-spot lesions start on lower leaves and climb the canopy, often joined by stem and fruit lesions near the calyx. The fungus overwinters in debris and soil, then splash-infects after warm rains. Mulch, pruning for airflow, resistant cultivars, and residue cleanup form the backbone of organic management in gardens and market plantings.
Symptoms
Dark brown lesions with concentric rings appear first on older, lower leaves. Surrounding tissue yellows and leaflets drop, exposing fruit to sunscald. Stems may show elongated dark lesions. Tomato fruit develops dark, leathery, sunken spots near the stem end. Potato foliage browns prematurely and tubers can show surface lesions in severe cases. Canopies look "burned" from the bottom up as the season progresses.
Environmental Conditions
Humid
Prevention
Rotate solanaceous crops three or more years. Start with clean seed and transplants. Mulch after soil warms to block splash. Choose cultivars with early blight tolerance. Avoid excess nitrogen and severe water stress. Space rows for morning sun. Clean stakes and cages between seasons.
Management
Strip infected lower leaves early and dispose of them off-site or in hot compost that reaches pathogen-killing temperatures. Stake or cage plants and prune suckers for drying breezes. Water at the soil line. Organic copper, Bacillus-based biofungicides, or potassium bicarbonate can slow spread when applied before forecast wet weather. Harvest ripe fruit promptly. After frost or final harvest, remove vines completely rather than leaving them to overwinter.