A layer of moisture on the leaf is necessary for infection, and infection is determined by the number of hours the leaf is wet and by the temperatures. The number of hours of leaf wetness at various temperatures has been worked out and is used as a forecasting tool. Secondary infection continues through the season as environmental conditions develop that are conducive to fungus development. The fungus can also build up on leaves after fruit harvest. Management Resistant varieties are available in various apple development programs.
Sanitation is a valuable tool.
Cleaning up the fallen leaves and eliminating them through composting or destruction will decrease the initial infections. Leaf destruction can also be hastened by applying nitrogen to the leaves on the ground in the fall. Other cultural practices, such as pruning to open up the tree to allow more air movement and removal of any obstruction that prevents air movement, will help in mitigating the impact of the fungus. Fungicides are used in many orchard programs, and weather monitoring models have been developed to time fungicide applications in commercial orchards. While the number of fungicides available to the home owner is limited, commercial growers have a wide range of materials available.