Downy mildew is a common fungal disease affecting many plants, including broccoli. It is caused by oomycetes, specifically the species Peronospora parasitica.
Downy mildew thrives in cool, wet, and humid conditions and can spread rapidly through the air, water, and infected plant debris. The disease appears as yellow patches on the upper surface of leaves, eventually turning brown, while the underside may show a fuzzy white or grey growth.
Severe infections can lead to leaf drop, reduced growth, and even plant death.
Prevention
To prevent downy mildew in your garden, follow these proactive steps:
- Choose resistant varieties: Opt for broccoli varieties resistant to downy mildew, found in seed catalogues or at local nurseries.
- Proper spacing: Allow adequate space between plants to ensure proper air circulation, which helps reduce humidity around the leaves.
- Water wisely: Water your plants early in the day, at the base of the plant, to minimise wetting the leaves. This allows the foliage to dry quickly and reduces the chances of fungal growth.
- Crop rotation: Rotate your crops every 2-3 years, especially if you’ve experienced downy mildew. This helps reduce the buildup of disease-causing organisms in the soil.
- Garden hygiene: Remove and destroy infected plant debris, and keep your garden weed-free. Weeds can act as alternative hosts for the disease.
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Treatment
If you’ve detected downy mildew in your garden, take action with the following treatments:
- Remove infected leaves: Prune away affected leaves as soon as you notice symptoms. Throw them in the trash, not the compost pile, to prevent the disease from spreading.
- Improve air circulation: Thin out overcrowded plants to allow better air circulation and reduce humidity.
- Fungicides: Apply a suitable fungicide, such as copper-based fungicides or products containing potassium bicarbonate, following the label instructions for application rates and timing. These can help control the disease if applied early and regularly.
- Biological control: Some commercially available beneficial microorganisms, like Bacillus subtilis or Bacillus amyloliquefaciens, can help suppress downy mildew.