Rust Disease on Green Onions: Identification and Control in Hawaii

Container-grown green onions in Hawaii developed small, dark orange, dusty bumps on leaves after heavy rains. The bumps remain when rubbed but release orange powder. The plants otherwise appear healthy.

Confirming Rust Disease

The description perfectly matches allium rust (Puccinia allii or Puccinia porri), a common fungal disease on onions, garlic, and leeks. Key diagnostic features:

• Pustules: Pinhead-sized, raised, orange-brown bumps on leaves
• Spore release: Rubbing releases orange powder (urediniospores)
• Tissue test: Wiping leaves with white tissue leaves orange-yellow smudge
• Conditions: Favored by high humidity, rain, and moderate temperatures

Management Strategies

Immediate Actions

1. Remove severely affected leaves to reduce spore load
2. Improve air circulation around plants
3. Avoid overhead watering—water at soil level
4. Apply fungicide labeled for edible alliums (copper-based or sulfur products work well if applied early)

Long-term Prevention

• Crop rotation: Don't replant alliums in same container/soil
• Clean starts: Use disease-free sets or seeds, not transplants with leaves
• Sanitation: Remove and destroy all plant debris after harvest
• Resistant varieties: Some cultivars show better rust tolerance

Can You Eat Affected Onions?

Yes—rust only affects leaves. Bulbs and unaffected green portions are safe to eat. Wash thoroughly before use.

In Hawaii's humid climate, rust may be a recurring challenge. Preventive copper sprays during wet periods and good cultural practices offer the best control.