Cause A physiological problem that occurs when a cool, rainy period of 4 to 5 days precedes several hot, sunny days. A hormonal imbalance may be the reason. The disorder is restricted to 'Golden Delicious' and its sports.
Symptoms Irregular necrotic blotches occur on mature leaves within 12 to 24 hours. The shape of the necrotic area is restricted by larger veins, which are not affected. Blotches may vary in number from one to six or more per leaf. Affected leaves generally turn yellow and drop within 4 to 7 days of symptom development. Leaf symptoms and defoliation occur in two to four waves during the growing season. Young, vigorously growing trees are less affected than older, slower growing trees.
Cultural control
- Incidence of this problem is less when grown on M9 or M26 rootstocks.
- Zinc oxide applied in 3 to 4 cover sprays can reduce symptoms.
- Two foliar applications of manganese can reduce symptoms.
Chemical control Although this is not a fungal problem, some symptom relief has been obtained by using EBDC fungicides such as mancozeb during cover sprays. In most years, control is not needed.
Reference Sutton, T.B., Aldwinkle, H.S., Agnello, A.M., and Walgenbach, J.F. 2014. Compendium of Apple and Pear Diseases and Pests, 2nd ed. St. Paul, MN: APS Press.