high severity

Onion Purple BlotchIdentification & Treatment Guide 2026

Alternaria porri · Family: Pleosporaceae

Onion purple blotch (Alternaria porri) causes purplish-brown zonate lesions on leaves that girdle and kill foliage, reducing bulb yield 40-60%. Thrips damage worsens the disease. Manage with mancozeb + chlorothalonil sprays every 7-10 days, thrips control, proper spacing for air flow, and avoiding overhead irrigation during bulb development.

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Quick Diagnosis

high

Onion Purple Blotch

Affects: leavesseed stalksbulbs
Peak Season: Warm humid weather (25-30°C), rainy season; heavy dew periods
Spread Risk: Moderate
Spread By: Airborne conidia, rain splash, infected crop debris

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Symptoms: What does Onion Purple Blotch look like?

Visual identification guide — tap each stage for details

Small purplish spots developing on onion leaf from purple blotch — Small purplish-brown spots on older leaf tissue

Zonate concentric purple-brown lesions on onion leaf — Characteristic concentric (zonate) purple-brown lesion with yellow halo

Onion foliage collapse from severe purple blotch infection — Extensive foliage collapse from girdling lesions — reduced bulb fill

ابتدائی مرحلہ

Small white sunken spots that turn purple
Small (2-3 mm) white, sunken, water-soaked spots appear on older leaves. Within days, spots develop a purplish-brown center.

درمیانی مرحلہ

Zonate purplish-brown elliptical lesions
Lesions enlarge into elliptical areas (1-5 cm) with characteristic concentric zonation (target-like rings) in shades of purple and brown. A yellow halo often surrounds the lesion.

آخری مرحلہ

Leaf girdling and foliage collapse
Lesions expand to girdle the entire leaf, causing the portion above to die. Extensive foliage loss during bulb fill reduces bulb size. Seed stalk infection can destroy an entire seed crop.

Causes: Why is my plant getting Onion Purple Blotch?

Environmental triggers and disease cycle

Warm temperatures (25-30°C) with high humidity
Extended dew periods (>12 hours) or overhead irrigation
Thrips damage creating wounds for fungal entry
Dense planting reducing air circulation
Previous onion crop debris harboring fungal inoculum

Disease Lifecycle

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Host Plant

Susceptible crop

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Pathogen

Alternaria

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Conditions

Warm humid weather (25-30°C), rainy season; heavy dew periods

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Infection

Penetrates tissue

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Symptoms

Visible signs

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Spread

Airborne conidia

Often confused with: Cabbage Black Rot. See our comparison guide.

How to Treat Onion Purple Blotch

Tick each step as you complete it — works offline too

🌿 OrganicSafe for all farms

Organic Treatment Steps

4 steps

Apply copper hydroxide (Kocide 2000) at 2 g/L every 10-14 days starting when leaves are 15-20 cm tall.
Control thrips (Thrips tabaci) with spinosad at 0.5 ml/L — thrips wounds provide entry for Alternaria.
Ensure adequate plant spacing for air circulation — at least 10-15 cm between plants in rows.
Use drip irrigation only — avoid overhead watering that extends leaf wetness duration.

🧪 ChemicalUse protective equipment

Chemical Treatment Steps

4 steps

Apply mancozeb (Dithane M-45) at 2.5 g/L as protectant spray every 7-10 days during the wet season.
Alternate with chlorothalonil (Daconil 2787) at 2 g/L for effective rotation.
Use difenoconazole (Score 25EC) at 0.5 ml/L for systemic curative action when disease pressure is high.
Start spray program early when leaves are 15-20 cm tall — do not wait for symptoms.

Prevention & Seasonal Calendar

Year-round protection plan for your crops

Pre-Planting

Start fungicide program early — before disease establishment
Control thrips throughout the season — thrips wounds dramatically worsen purple blotch

Mid-Season

Use drip irrigation exclusively — never overhead sprinklers
Rotate with non-allium crops for 2-3 years

Post-Harvest

Remove crop debris from previous onion crops
Space plants adequately for air movement through the canopy

🛡️ Resistant Varieties

No highly resistant commercial varieties — all onions are susceptibleRed varieties tend to have slightly more tolerance than white/yellow types

View full seasonal calendar for your region →

Frequently Asked Questions

Common questions about Onion Purple Blotch

Why does thrips damage make purple blotch worse?

Thrips (Thrips tabaci) create small feeding wounds (silvery patches) on onion leaves. These wounds serve as direct entry points for Alternaria porri conidia. Research shows that onion leaves with thrips damage develop 3-5× more purple blotch lesions than undamaged leaves. Integrated management of both pests is essential.

Can I grow onions during the rainy season?

Yes, but intensive fungicide management is required. In tropical regions, rainy-season onion production is much more challenging due to purple blotch, Stemphylium, and other diseases. Consider growing in the dry season with irrigation if possible, or choose the least wet months for planting.

How do I distinguish purple blotch from Stemphylium leaf blight?

Purple blotch lesions are elliptical with concentric zonation (target pattern) and purple-brown color. Stemphylium lesions are more irregular, olive-brown, and lack distinct zonation. In practice, both diseases often occur together on the same plant and are managed with the same fungicide program.

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Related Disease

Cabbage Black Rot

Xanthomonas campestris pv. campestris

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Affected Plant

Onion

Allium cepa

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Treatment Guide

Full Treatment Protocol

Detailed spray schedule & dosage guide

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