Once established, the fungus continues to grow on shoots, regardless of moisture or humidity. Apple tree heavily infested with powdery mildew. New leaves and shoots are more susceptible to infections than older leaves, especially when conditions are good and shoot growth continues. Blossom infection is less common than leaf and shoot infection, but it is important because diseased blossoms do not set fruit. 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.