Purple blotch (onion & garlic)
White-centred spots that enlarge into purplish, zonate blotches, girdling and toppling the leaves and seed stalks of onion and garlic.
- Pathogen
- Fungus (Alternaria porri / Stemphylium)
- Spreads by
- Air- & rain-splashed spores; debris & seed
- Favoured by
- Warm, humid, wet, dewy weather; dense stands
- Main risk
- Leaves & seed stalks girdled; small bulbs
Overview
Purple blotch (Alternaria porri, often together with Stemphylium leaf blight) is the most important foliar disease of onion and garlic. It begins as small, water-soaked spots with a whitish centre on the leaves and seed stalks; these enlarge into elongated, sunken blotches with a characteristic purplish to brownish colour and concentric (zonate) rings, often surrounded by a yellow zone that extends above and below the spot.
As the blotches enlarge and merge they girdle the leaf or stalk, so the leaves yellow, break and topple, and the seed stalk can snap at the blotch — a serious loss in onion seed crops. Loss of leaf area reduces bulb size and yield and hurts storage life. It is favoured by warm, humid, wet weather with dew and by dense stands, and it carries over on debris, volunteers and seed. Management combines spacing and rotation, clean seed and debris removal, and protective fungicide sprays (with a sticker, because onion leaves are waxy).
How to identify it
- Small, water-soaked spots with a whitish centre on the leaves and seed stalks.
- Spots enlarging into elongated, sunken, purplish-to-brownish blotches with concentric (zonate) rings and a yellow surround.
- Blotches girdling the leaf/stalk so leaves yellow, break and topple, and seed stalks can snap.
- Worst in warm, humid, wet, dewy weather and in dense stands; also often with Stemphylium leaf blight.
What causes it and when
- The fungus Alternaria porri (often with Stemphylium), spread by air- and rain-splashed spores.
- Favoured by warm, humid, wet weather with dew and long leaf wetness, and by dense stands.
- Carries over on infected debris, volunteers and seed to infect the next crop.
- Builds through the season and is especially damaging to onion seed crops when it snaps the seed stalks.
Cultural & organic control
- Use recommended spacing and good drainage so the canopy dries; avoid over-dense, lush stands.
- Rotate away from onion/garlic and remove and destroy infected debris and volunteers.
- Use clean, healthy (treated) seed and healthy sets/bulbs, and balanced nutrition.
- Keep fields weed-free for airflow and avoid overhead irrigation that keeps leaves wet.
Chemical control
- Spray a CIB&RC-registered fungicide (e.g. mancozeb, and recommended systemics) protectively from the first spots, rotating groups.
- Add a sticker/spreader because onion and garlic leaves are waxy and shed spray otherwise.
- Repeat on the recommended schedule through warm, humid spells, especially in seed crops to protect the stalks.
- Confirm the current product, dose and schedule for your area with your local KVK.
Prevent it coming back
- Make spacing, drainage, rotation and clean seed the routine, and remove debris and volunteers.
- Plan protective sprays (with a sticker) for warm, humid spells, especially in onion seed crops.
- Avoid overhead irrigation and dense, lush stands that keep the leaves wet.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My onion/garlic leaves have purplish spots with rings and are breaking and toppling. What is it?
Those white-centred spots that enlarge into purplish, zonate (ringed) blotches with a yellow surround, girdling the leaves so they break and topple, are purple blotch (Alternaria porri), often with Stemphylium. It shrinks bulb size and can snap seed stalks. Use recommended spacing and rotation, clean seed, and protective fungicide sprays from the first spots — with a sticker, since onion leaves are waxy and shed spray.
Why do I need a sticker when spraying onion for purple blotch?
Because onion and garlic leaves are narrow, upright and coated with a waxy bloom, so plain fungicide spray tends to run off and not stick, leaving the leaf poorly protected. Adding a recommended sticker/spreader helps the spray spread over and cling to the waxy surface, giving proper cover. Without it, even a good fungicide can fail on onion. Combine the sprays with spacing, rotation and clean seed for reliable control.
Why is purple blotch especially serious in an onion seed crop?
Because in a seed crop the disease attacks the tall seed stalks as well as the leaves, and a blotch that girdles a stalk can snap it — dropping the developing seed head and causing direct seed loss. Combined with the loss of leaf area, this makes purple blotch very damaging to onion seed production. So seed crops need particularly careful protection: spacing and rotation, clean seed, and timely protective sprays (with a sticker) to keep the stalks intact.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.