Fruit infection starts when fruit are small, but russeting is only visible when fruit size increases and fruit begins to colour. Powdery mildew infects new buds when they begin to form until they are mature. Once the terminal buds have set, they are no longer susceptible. Russeted fruit is downgraded to juice. On young trees, on in heavily infested trees, vigour and productivity are reduced.
Infected buds are more sensitive to cold temperature injury than healthy buds.
Powdery mildew can only survive on live tissue, so that infected winter-killed buds actually reduce mildew inoculum. Less susceptible varieties should not be inter planted with mildew-susceptible varieties to facilitate spray applications where extra fungicides are required on more susceptible blocks. However, powdery mildew infects during periods of hot, humid weather when trees are growing rapidly. Sufficient fungicide residues from a scab control program may not be present or may not have redistributed to new growth under such growing conditions. Also, not all scab fungicides are highly effective on powdery mildew.