Series: Agfact H4.AB.8 Edition: Second edition Last updated: 18 Jun 1998
The fungus Podosphaera leucotricha.
These become partly or entirely covered with the white powdery fungus, which eventually kills them. The edges of infected leaves may become wavy, and often roll upwards andinwards, exposing the lower surfaces. Infected leaves fall prematurely.
When heavily infected, shoots become covered with the fungus and may bear totally infected leaves. Such shoots either die or only grow weakly.
Infected buds are smaller than healthy ones, have a pinched appearance, and open later.
These are usually only infected when flower clusters are produced from infected buds. Usually all blossoms and leaves produced from an infected bud are affected and fail to setfruit.
Fruit may become infected soon after petal-fall. The affected areas become russeted and, where growth is restricted, cracks may develop. The russet occurs as a network offine lines.
Powdery mildew, if not controlled, gradually reduces the vigour of infected trees until they become seriously debilitated and yield is substantially reduced. Infected fruit is downgraded because of russet.
The fungus enters the buds while they are being formed on new growth in spring and summer, and survives from one season to the next in dormant, infected buds. Mildewinfection appears on the growth produced from these buds soon after they burst the following spring. Spores are produced in quantity on the surface of infected parts and these set up new infections. The disease is most active in spring and early summer, and again in autumn if new growth occurs. Because of the mild winters in New South Wales, the fungus can also overwinter as surface growth on infected shoots and spurs.
Of the main cultivars grown in New South Wales, the most susceptible are Pink Lady, Braeburn and Granny Smith.
The spores are spread by wind.
Powdery mildew is favoured by humid weather. The surface mycelium can withstand very hot dry weather and will subsequently produce more spores when favourable conditionsreturn. However, the spores themselves are quickly killed by high temperatures.
Prune out infected shoots in winter. On infected trees of highly susceptible cultivars such as Jonathan, the terminal buds are likely to be infected. Shorten all laterals to remove these buds. Prune out infected shoots as they appear in spring. Burn all prunings.
A complete program for the control of powdery mildew and other diseases and pests of apples is given in the Orchard Plant Protection Guide, which commercial growers can obtain from their local district horticulturist. The program for powdery mildew control is required on susceptible cultivars.
Author: Les Penrose