Powdery Mildew

The clearest sign of powdery mildew is a white or greyish, powdery or felt-like growth on the upper or lower surfaces of leaves, on stems, and sometimes on flowers or fruit. Affected leaves may yellow, distort, or drop prematurely as infection progresses.

Powdery mildew fungi are obligate parasites, meaning they require living host tissue to grow. They colonise the plant surface and draw nutrients through specialised structures that penetrate epidermal cells. Spores produced on the powdery growth are carried by air currents to new tissue and other plants.

Unusually among fungal diseases, many powdery mildews are favoured by high humidity but do not require free water on the leaf to infect, which allows them to develop under drier conditions than many other foliar pathogens.

Powdery mildews affect a wide range of agricultural and horticultural plants, including cereals such as wheat and barley, many vegetables and cucurbits, grapevine, and numerous ornamentals. Because the fungi are largely host-specific, the presence of powdery mildew on one crop does not necessarily threaten unrelated crops nearby.

• Moderate temperatures combined with high humidity
• Dense canopies and poor air circulation
• Shaded, sheltered conditions that keep humidity high
• Susceptible varieties and excessive, lush growth

Because powdery mildews are widespread and spread readily, management emphasises prevention and early detection rather than eradication.

• Choose resistant or tolerant varieties where available
• Improve air movement through appropriate spacing and canopy management
• Avoid conditions that produce dense, humid canopies
• Scout regularly so that any response can be timely and targeted