Powdery mildew

Powdery mildew (caused by Podosphaera leucotricha) is one of the most common diseases in apples. It can severely affect tree growth, particularly in young trees.

Disease identification

Leaves and shoots are most susceptible to powdery mildew in the first few days after opening. The first indication of powdery mildew is pale patches on the upper leaf surface. As the disease advances, the patches become powdery white and will cover both sides of the leaf. Leaves infected with mildew will have crinkled and cupped edges, giving them a narrow appearance.

Damage

Infected leaves can often fall during summer, reducing the photosynthetic rate. Powdery mildew can severely affect tree growth, particularly on young trees. Infected floral buds might abort or produce small, stunted fruit, resulting in reduced yield and pack-out. Infections during flowering and early fruit development can damage the skin of apples, resulting in a russet that makes the fruit unmarketable.

Monitoring

Powdery mildew outbreaks are most likely in spring and early summer, and with new growth in autumn. Relatively humid, mild conditions (10–25 °C) without rain favour this disease, although it can withstand hot, dry conditions and produce spores. These spores are then spread by wind but are killed in high temperatures. Monitor powdery mildew during winter and throughout the growing season. Growers can choose designated monitoring trees and assess 10 extension shoots on these trees. Inspect the top 5 unfolded leaves per shoot for the signs of powdery mildew, recording any incidences. Frequent monitoring of these shoots will increase the opportunity to intervene at the correct time, as powdery mildew can develop rapidly.

Management

Cultural and physical: powdery mildew can be controlled by removing infected buds as this reduces the source of spores for infecting new leaves and buds. Winter pruning is a good time to remove infected shoot tips. Look for the white stems of infected one-year-old shoots and cut them back to uninfected wood. Removing infected prunings from the orchard will help reduce the potential for the disease to spread.

Pruning to open the canopy can also help reduce infection, as it encourages airflow through the canopy. Modifying the environment around the trees can also reduce the incidence of powdery mildew. Windbreaks, netting and tree planting should be managed to optimise airflow, particularly in regions where powdery mildew is problematic.

Biological: there is currently no biological control available for powdery mildew in apples.

Chemical: refer to the Orchard plant protection guide for chemical suggestions.