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Basil downy mildew
Peronospora belbahrii
Fungal diseaseHigh impact
A highly destructive disease specific to basil that can devastate a planting rapidly under warm, humid conditions. Unlike many downy mildews, it can spread quickly even in summer warmth when humidity is high.
Currently in peak season - extra vigilance worthwhile.
Key facts
- Type
- Fungal disease
- Affects
- Basil
- Active season
- Jun - Sep
- Peak season
- Jul - Aug
- Vulnerable stages
- Growing · Harvesting
- Spread rate
- fast
- If confirmed
- Act now
Signs & symptoms
- -yellowing of upper leaf surfaces, often between leaf veins giving a patchy appearance
- -grey-purple fuzzy growth on the underside of affected leaves
- -rapid browning and collapse of leaves in humid conditions
- -distorted or blackened young growth
Management
Do now
- -Remove heavily affected plants or leaves immediately
- -Move remaining plants to a well-ventilated position outdoors if possible
- -Avoid all overhead watering - water at the base only
- -Space plants to allow maximum airflow
Prevention
- -Grow basil outdoors wherever possible - glasshouse and windowsill conditions favour disease
- -Avoid overcrowding plants
- -Avoid evening watering which leaves foliage wet overnight
- -Remove any infected plants promptly to prevent spread to neighbouring plants
Further reading
- -Basil downy mildew is specific to basil (Ocimum spp.) and does not affect other crops
- -It can spread quickly even in summer - unlike most downy mildews it does not strictly require cold conditions
- -Once basil is infected there is no cure; management is about protecting uninfected plants