The fungi causing most fruit diseases can infect plants only when the fruit and foliage are wet, but powdery mildew spores do not require rains or dews to germinate. 50 F. However, newly developing apple leaves are susceptible to infection for only a few days after they unfold. Primary mildew infections develop from buds in which the mildew fungus overwintered. On shoots with primary infections, the fungus grows with the developing shoot and most leaves are completely covered with the powdery white fungus. Spores from these primary infections are blown to other leaves and cause the secondary infections that are most evident this time of year.
Most secondary infections appear as white spots affecting portions of scattered leaves on the trees.
Trees that did not receive adequate mildewcide sprays earlier in the season may have many infected leaves at this time of year. With a few exceptions, however, mildewcide applications are NOT recommended at this time. Apple fruit are susceptible to mildew infection only for a short period around the pink-bud stage, so fruit are no longer at risk. With the dry weather, most trees have already stopped growing or will stop growing very soon. Mildewcides applied now might kill some of the fungus in infections on the older leaves, but the damage to these leaves has already been done.