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Downy mildew
Peronospora spp.
Fungal diseaseMedium impact
A fungal-like disease that causes yellowing on upper leaf surfaces with grey or purple fuzzy growth underneath. Favours cool, damp, humid conditions and spreads rapidly in still, wet weather. Unlike powdery mildew, it requires wet or very humid conditions to spread.
Key facts
- Type
- Fungal disease
- Active season
- Apr - Oct
- Peak season
- Apr - Oct
- Vulnerable stages
- Growing · Harvesting
- Spread rate
- moderate
- If confirmed
- Act soon
Signs & symptoms
- -angular yellow patches on the upper surface of leaves
- -grey, purple, or white fuzzy growth on the underside directly beneath the yellow patches
- -affected leaves curling, dying back, or collapsing
- -rapid spread through a planting in cool humid weather
Management
Do now
- -Remove and bin affected leaves immediately - do not compost
- -Improve airflow by reducing plant crowding
- -Avoid all overhead watering - switch to base watering
- -Ventilate polytunnels or cold frames to reduce humidity
Prevention
- -Grow in an open, well-ventilated position
- -Space plants at recommended distances to allow airflow
- -Avoid growing susceptible crops in dense plantings or under low cover in cool, wet periods
- -Water at the base rather than over the foliage
Further reading
- -Downy mildew requires high humidity and cool temperatures to spread - unlike powdery mildew, dry conditions limit it
- -Spores spread by wind but only infect when foliage is wet or humidity is very high
- -Improving airflow and reducing humidity is the most effective cultural control