🧅 Onion How hard it is to control: ★★★☆☆ 3/5

Purple blotch on onion

A fungal disease (Alternaria porri) producing oval leaf spots with a purple centre and a yellow halo. It usually moves into tissue already weakened by downy mildew or damage.

Symptom photos

Purple blotch — Leaf — lesions with a purplish-brown centre and a paler halo — Leaf — lesions with a purplish-brown centre and a paler halo Photo: Jochen Kreiselmaier, Dienstleistungszentrum Ländlicher Raum Rheinpfalz, licence CC BY-SA 4.0

Purple blotch — Lesion close-up — concentric zoning and sunken, dying tissue — Lesion close-up — concentric zoning and sunken, dying tissue Photo: Jochen Kreiselmaier, Dienstleistungszentrum Ländlicher Raum Rheinpfalz, licence CC BY-SA 4.0

Symptoms

Oval spots with a purple-brown centre and a yellow halo
Spots zoned into concentric rings
A dark, velvety bloom of spores on the spots
Leaves drying and snapping over at the spots
Spots starting at the tips of the older leaves

What it does to the crop

Leaves drying off early and a smaller crop
Bulbs rotting later through the infected neck
Poorer keeping quality
Further weakening of plants already hit by other diseases

What encourages it

Warm, humid weather at 21-30°C
Water staying on the leaves for long periods
Leaf damage and earlier infection by downy mildew
Too much nitrogen

Prevention

Rotate with a 3-year break for alliums
Clear crop debris after harvest
Water in the morning and avoid overhead watering in the evening
Control downy mildew, which opens the way for purple blotch
Feed in a balanced way with adequate potassium

⚠️ Before you reach for a spray

This atlas is informational and still growing — different diseases can look almost identical. Confirm the diagnosis and read the product label before applying any plant protection product. If you are unsure, ask your local extension or advisory service.

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