Pathogen Information
purple blotch is caused by Alternaria porri. Spreading it spores in the wind common symptoms are:
- small, purplish-tan lesions
- watery lesions on leaves
- Yellow halos surrounding the lesions
- Bulb rot in more severe cases
Environmental Conditions
- Temperature: 70-80°F
- Humidity: periods of extended wetness
- Most effected species: onions, garlic, and leeks
Disease Progression
Early Stage
small purple blotches appear on leaves, leaves can be watery
Mid Stage
yellow halos form around the blotches
Late Stage
bulbs will begin to rot underground
Prevention Methods
- Avoid damaging the leaves while watering
- Maintain proper plant spacing
- Remove and destroy infected plant debris
- Use disease-free seeds and resistant varieties
- Rotate crops with non-susceptible plants
- Keep leaves dry and improve air circulation
Treatment Options
- mancozeb fungicides
- Chlorothalonil-based fungicides
- Neem oil treatments
- Copper-based fungicides
