Divisi
Purple blotch
Alternaria porri · Fungal
Photo: honey-bee from Bangkok, Thailand · CC BY 2.0
Purple blotch (Alternaria porri) is a fungal leaf disease of onions, garlic and shallots. The fungus slips in through wounds — very often the silvery feeding scars left by thrips — during warm, humid, wet-leaf weather. Small spots grow into large purple, ringed patches that girdle and kill leaves and seed stalks, and the same fungus later rots stored bulbs at the neck.
Why it matters
Purple blotch is the disease that turns a green onion crop brown before it fills out. In a warm, wet season it can take up to 60% of your harvest — leaves die back early so the bulbs stay small, and the seed stalks snap before the seed sets. Worse, it follows the bulbs into the store and rots them at the neck weeks after harvest. Once the spots join up across the leaves the fungus is everywhere and sprays only slow it — so the whole game is stopping it early, before the first few spots spread.
When to act
First small water-soaked or purple lesions on older leaves during warm humid weather; start protectant sprays before lesions coalesce
This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.
In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →
The full purple blotch field guide
- Identify it
- Scout for it
- Control it
16 field-guide sections in the app
The full guide to purple blotch — every symptom to check, how and when to walk your field looking for it, and the controls that actually work, ranked. Built into your crop plan, so you are told to look before it costs you a harvest.
Crops it affects
Frequently asked questions
How do I identify purple blotch?
The first thing you see: small water-soaked (wet-looking) flecks or tiny white sunken dots on the older, lower leaves — the leaves furthest from the centre. They often sit right where thrips have rasped silvery scratch-marks.
When should I take action against purple blotch?
First small water-soaked or purple lesions on older leaves during warm humid weather; start protectant sprays before lesions coalesce
How do I control purple blotch?
Prevent it first (cheapest, works best over a season): rotate — do not plant onions, garlic or shallots on the same land two seasons running; give it a 2–3 year break so old crop debris rots away. Plant clean, disease-free seed and sets, and space plants wider so air moves through and leaves dry fast after rain or dew.
Which crops does purple blotch affect?
Among the crops covered here: Garlic, Onion.
Catch it before it costs you a harvest
Divisi builds scouting into your crop plan — it tells you when to walk your field, what to look for at that stage, and what to do when you find it. Free for farmers, works offline.
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