Apple powdery mildew occurs throughout the year on apple trees. The fungal pathogen overwinters inside the bud tissue of shoots and produces the primary infections. Spores are produced in warm weather concurrent with new shoot elongation. Secondary leaf infections spread quickly after petal fall and can also infect pear leaves. The pathogen, however, does not overwinter on pear trees. Leaf infections decline as the weather becomes hot and shoots set terminal buds. Some highly-susceptible cultivars, such as ‘Honeycrisp’ can be infected throughout the spring and summer.
