Powdery mildew: identify, prevent, control

Worst in late summer and autumn with warm days and humid nights, January to April.

Powdery mildew is a fungal disease that coats leaves in a white powdery film, common on cucurbits, peas and many other crops. It thrives in warm days with cool humid nights and poor air flow, and while it rarely kills plants outright it cuts yields and shortens the season.

How to identify it

• White powdery patches on the upper surface of leaves, spreading to cover them
• Yellowing and browning of leaves as the disease advances
• Distorted, stunted new growth in bad cases
• Worst on older, shaded and crowded leaves first

How to prevent it

• Space plants well and prune for good air movement
• Water at the base in the morning and keep foliage dry
• Grow mildew-resistant varieties of cucurbits, peas and pumpkins where available
• Avoid heavy nitrogen feeding that produces dense soft growth

Organic control, step by step

• Remove and bin the worst-affected leaves at the first sign
• Spray a milk solution of 1 part milk to 9 parts water weekly in dry weather
• Spray potassium bicarbonate or eco-fungicide, covering both leaf surfaces, every 7 to 14 days
• Use a wettable sulphur spray on susceptible crops, but only under 28 degrees and not on sulphur-sensitive plants
• Clear away infected crop debris at the end of the season
• Improve airflow by thinning crowded plantings

Plants it attacks