Powdery Mildew: Why It Appears and What to Do
Owen Petrakis · Sep 9, 2026 · 5 min read
Quick answer
That white dusting on courgettes, roses, and phlox is a fungus. It loves dry roots and humid air.
Powdery mildew looks as though someone has dusted the leaves with flour. It starts as small white spots and spreads until whole leaves are coated, then turn yellow and die back.
It rarely kills a plant outright, but it weakens it and reduces harvests.
1 Plants most often affected
- ✓Courgettes, squash, pumpkins, and cucumbers
- ✓Peas, especially later sowings
- ✓Roses, phlox, asters, and monarda
- ✓Apples and gooseberries
2 What encourages it
Unlike many fungal diseases, powdery mildew does not need wet leaves. It is worst when plants are dry at the roots but the air around them is humid and still — typical of late summer, crowded beds, and plants against walls.
3 Managing an outbreak
- ✓Remove the worst-affected leaves and put them in the bin, not the compost
- ✓Water at the base in the morning, deeply, so roots do not dry out
- ✓Thin crowded plants to improve airflow
- ✓A dilute milk spray (one part milk to nine parts water) has some evidence behind it for courgettes
- ✓Approved fungicides can help, used according to the label
4 Prevention next year
Choose varieties described as mildew-resistant, which are widely available for courgettes, peas, and many ornamentals. Space plants generously, mulch to keep soil moisture even, and avoid heavy nitrogen feeds that produce soft, susceptible growth.
This is general gardening information. Results depend on your climate, soil, and conditions. Follow label directions for any fertiliser or treatment, and check whether plants are toxic to pets or children before bringing them home.