Plant guide:ðŋ Basil
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â CriticalYellow, vein-following patches on top of the leaf with fuzzy grey mould underneath ïŋ― the hallmark of basil downy mildew.
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Fungal
Downy Mildew on Basil
Commonly affects: Ocimum basilicum (Basil)
Yellow, angular patches appear on the upper surface of leaves, following the leaf veins, while a fuzzy grey-purple mould develops on the underside directly beneath the yellowed area. Affected leaves eventually turn brown and collapse.
Potential Causes
- 1Peronospora belbahrii, an airborne water mould that spreads via wind-blown spores
- 2High humidity and extended leaf wetness, especially overnight
- 3Poor air circulation in dense plantings or greenhouse conditions
- 4Infected seed, a common source of the pathogen in commercial basil seed
Solution Steps
- 1Remove and destroy infected leaves and, in severe cases, entire plants, as soon as symptoms appear.
- 2Improve air circulation by spacing plants further apart and avoiding overhead watering.
- 3Apply a copper-based fungicide preventively if downy mildew is a recurring problem in your area.
- 4Buy resistant basil varieties bred specifically for downy mildew tolerance if the disease returns each season.
Prevention Tip
Downy mildew spreads through humid, still air overnight ïŋ― water at the base in the morning rather than evening, and choose resistant varieties if the disease has affected your basil before.
