Notes on Apple Diseases
Powdery mildew
Identification
Powdery mildew is a white, felt-like fungus that covers the leaves and
stems of apple terminals. Infected leaves become narrow, folded, discoloured
and brittle. In summer these leaves often turn down and die. A net-like
russeting on fruit is sometimes caused by early mildew infection. Powdery
mildew can also cause blind wood on apple shoots because infected buds
are more susceptible to winter injury.
Period of activity
Powdery mildew first appears on leaves at silver tip and the disease
continues to spread until the terminal buds have set. Powdery mildew tends
to be more of a problem during high humidity and temperatures between
18°C-27°C.
Monitoring and Thresholds
Use fungicides to prevent powdery mildew before symptoms appear. In orchards
with a history of powdery mildew problems, begin a fungicide program as
early as half-inch green. Otherwise, sprays for the management of powdery
mildew are not required until tight cluster. Certain varieties are far
more susceptible than others to powdery mildew.
Management notes
Powdery mildew is present in orchards throughout the growing season,
but new infections do not occur after shoot growth stops and terminal
buds are set. Be aware of susceptible varieties. See Examples of Apple
Cultivars Susceptible to Key Diseases for more information.
Learn more...
