Alternaria Blight — Permaculture Database

Overview

Alternaria blight is a common fungal disease complex caused by several Alternaria species that attack leaves, stems, and fruit of carrots, brassicas, solanaceous crops, and many ornamentals. The fungi survive on crop residue and seed, then splash or blow onto new tissue when weather turns warm and damp. Concentric "target" rings inside lesions are the classic field clue. In mixed plantings the disease rarely kills overnight, but repeated defoliation and fruit scarring cut yield and quality. Permaculture systems reduce pressure by diversifying host plantings, mulching to limit soil splash, and never letting diseased debris linger through winter.

Symptoms

Dark brown to black leaf spots with distinct concentric rings develop first on older foliage, then move upward. Spots may merge until large areas of leaf turn necrotic and drop. Seedlings can damp off at the soil line. Stems show elongated dark lesions that may girdle young plants. Fruit develops sunken, dark, leathery patches, often near the stem end on tomatoes. Severely hit crops look ragged, thin-canopied, and produce smaller, blemished harvests.

Environmental Conditions

Humid

Prevention

Buy or save clean seed; hot-water treat carrot and brassica seed when the pathogen is seed-borne. Rotate host families at least three years out of the same bed. Mulch early to stop soil splash. Space plants for morning sun and drying breezes. Choose cultivars with documented Alternaria tolerance when available. Keep nitrogen balanced so soft, lush growth does not invite infection. Clean tools and trellis gear between plantings.

Management

Remove and destroy heavily spotted leaves as soon as they appear; do not compost them if infection is active. Improve airflow by thinning crowded plantings and staking or trellising sprawling crops. Switch to drip or soaker irrigation so foliage stays dry. Organic copper or potassium bicarbonate sprays can slow spread when timed before wet weather; neem oil and sulfur have limited but useful activity on some hosts. After harvest, pull vines and bury or hot-compost residue thoroughly. Solarize small beds in hot climates if Alternaria returns yearly.