Purple-Brown Lesions on My Leek Leaves

Distinct, oval or irregular brown-purple lesions with a slightly darker border on leek leaves are the signature of purple blotch, caused by the fungal pathogen Alternaria porri. The same disease affects onions and garlic. In humid conditions, a dark, sooty-looking sporulation may develop within the older lesions. Purple blotch is a mid to late season disease, most common from July through to harvest, and is favoured by warm, humid conditions with frequent leaf wetness.

Identifying purple blotch

Purple blotch lesions start as small, water-soaked pale spots that enlarge into oval, pale brown lesions with a distinct purple or dark brown border. They are most commonly found on the upper surface of mature, outer leaves. In humid conditions, the surface of older lesions develops a dark, olivaceous (dark green-black) mould — this is the spore mass of Alternaria. The lesions are distinct from rust (which produces raised orange pustules) and from downy mildew (which produces a diffuse grey coating). The surrounding tissue around lesions often turns yellow.

How serious is it?

In a typical home garden, purple blotch causes cosmetic damage to outer leaves but rarely causes serious harm to the edible shank. The disease affects the older, outer leaf tissue; the inner leaves and shank usually remain clean. In a prolonged, warm, humid summer it can spread more aggressively and affect a higher proportion of the leaves, potentially weakening the plant before harvest. In commercial production, purple blotch can cause significant losses — but home garden leeks, growing more widely spaced with better airflow, generally suffer less.

Cultural management

Improve air circulation between plants by ensuring adequate spacing (at least 15 cm between plants, 30 cm between rows). Avoid overhead watering, particularly in the evening — wet leaves through the night are the primary condition needed for Alternaria spore germination. Remove and dispose off-site any leaves with heavy lesion coverage to reduce the spore load in the bed. Copper-based fungicides (approved for organic use) can slow progression if applied at the first sign of infection. Clear all crop debris at the end of the season.

Eating affected leeks

Leeks with purple blotch on outer leaves are perfectly edible. Strip the outer damaged leaves — the inner shank is clean. Inspect the inner leaf sheaths for any lesions that may have progressed inward; in most cases the shank is unaffected. Use as normal.