Powdery mildew
A fungal disease that coats leaves, buds and young shoots with a white, flour-like dust, weakening the plant and spoiling flowers and fruit.
Also known as: White mould, Oidium
Symptoms and identification
White or grey powdery patches on the upper surface of leaves, later spreading to stems, buds and flowers. Affected leaves curl, turn yellow and dry out. On mango, flower clusters turn brown and drop before fruit sets. Roses show distorted new shoots and buds that fail to open.
Causes
Several related fungi cause powdery mildew. Unlike most fungal diseases, it spreads fastest in warm days with cool, humid nights and dry leaf surfaces. Crowded planting, shade, poor airflow and soft growth from heavy nitrogen feeding make plants more vulnerable.
Organic treatment
Pick off infected leaves. Spray the whole plant with a solution of 1 teaspoon of baking soda and a few drops of liquid soap per litre of water, or with diluted neem oil, every 7–10 days. A spray of 1 part milk to 9 parts water in sunny weather also slows the fungus. Always test on a few leaves first.
Prevention
Grow susceptible plants in full sun with space between them for airflow. Water at the base in the morning, avoid overfeeding with nitrogen, and prune crowded growth. Remove and bin affected leaves at the first sign, and choose resistant varieties where available.
Chemical treatment (use with care)
For severe or recurring outbreaks, use a fungicide labelled for powdery mildew, such as wettable sulphur or a hexaconazole-based product, following the label. Do not apply sulphur in temperatures above 32 °C, and respect the waiting period on fruit and vegetables.