Mildew on apples is caused by the fungus Podosphaera leucotricha. It can also colonise pear and quince trees. The fungus develops on fruit and leaf buds in spring, and these colonies produce spores, or conidia, that cause secondary infections on new leaves in spring and summer, and on fruit, on which it creates a powdery white coating. Mildew develops best in warm, dry conditions.
How to deal with mildew on apples
To control mildew on apples, when winter pruning, cut out any misshapen or silvery-coloured shoots or buds, cutting back to several buds below the affected area. Later in the season, observe the terminal buds carefully. Infected leaves and blossoms are covered in the white powdery spores, and leaves tend to curl upwards. Infected blossoms will not produce fruit.
Prompt action is essential because mildew spreads by means of wind-blown spores. Cut out infected buds and shoots in early spring or as soon as you see them. These prunings have to be definitively destroyed or removed from the orchard or garden, as they are a source of infectious spores. Disinfect the secateurs when you have finished.
It may be necessary to use low-toxicity fungicides. Apply early in the season, before the blossom opens. We recommend expert help in selecting the product used and its frequency of application. The use of Bordeaux mixture is no longer possible.
Mildew is less likely to form if your trees are planted in good sunlight, with good air circulation through the canopy, made possible by pruning apple trees effectively. Reduce stress on the tree by ensuring that it is sufficiently watered in dry spells. Application of a layer of mulch for a diameter of about a metre around the tree from April on will help keep the soil moist.