Apple scab is a fungal disease present in apple trees. According to the New York State Agricultural Experiment Station, apple scab is considered the most destructive disease of apples in the world. This destruction causes an economic impact for apple growers. Destruction to the tree increases the tree's susceptibility to winter damage and insect infestations. Apple trees grown on a commercial level must follow a control and prevention plan.
The first symptoms of apple scab occur in spring when lesions form on the leaf underside. The lesions are small and green in color and become darker and more defined as the infection ages. Leaves will become deformed and drop from a tree with a severe infection of apple scab. The fruit forms the same lesions that become scabby in appearance with age.
Apple scab is a Venturia inaequalis fungus infection. The fungus will invade over winter in leaves that have fallen to the ground and start producing new spores in spring when the weather is cool and damp. Wind carries the fungus spores to new leaf growth, apple buds and young fruit. Germination of fungus occurs on the tree parts when water is present, resulting in penetration of the area. Symptoms of an apple scab infection show in approximately 9 to 17 days. Apple scab continues to spread by spores moving through splashing rain. A wet spring makes the disease more prevalent.
Planting resistant apple tree cultivars reduces the chance of an apple scab infection. The variety of available cultivars varies by growing zone location. Dispose of fallen leaves in the fall season to remove the fungus carrier. Shredding leaves or applying nitrogen to the fallen leaves increases the decomposition period. Prune trees to increase air circulation. Spores will not spread as quickly when the leaves dry quickly. Do not use irrigation systems that spray water on the leaves.
Apply fungicide in early spring to protect tree growth and control the fungus causing apple scab. Application of a fungicide should begin when the tree's tips turn green if the fungus was present the previous year. Fungicide gives best results when applied at each stage of new tree growth in spring. Cluster bud breaking is the most important period for fungicide application. Use of one fungicide type will cause scab resistance. Alternate types of fungicides for best results with disease control.
Several varieties of apple cultivars are more resistant to the apple scab disease. The cultivars include Liberty, Jonafree, Enterprise, Prima, Priscilla, Sir Prize, Freedom, Williams' Pride, Novamac, Goldrush, Redfree, Nova Easygro, and Pristine. Verify the growing zones are compatible for your area prior to purchase. Cultivars that are highly susceptible to apple scab are Rome Beauty, McIntosh, Red Delicious, and Cortland.