Apple scab is a disease of apple trees caused by fungus Venturia Inaequalis. In world apple scab is the most economically destructive disease. It is most serious in areas that have cool, wet weather during the spring and may not be economically important in warm or dry climates. The disease can result in severe defoliation of apple trees and also affects the fruit size and quality. Apple scab generally occurs on leaves, fruits and green twigs. Apple scab produces lesions that are observed on leaves and later it can be seen on blossoms, sepals and fruit.
Apple scab overwinters in apple leaves and fruits on the orchard floor. In spring the fungus produces spores onto the air from fallen leaves. These spores are carried out by air to newly developed leaves, flowers or fruits. The spores of the fungus are carried out during rainy periods and carried to long distances by wind. These spores need moisture on the plant surface in order to start new infections. The infections grow into spots or blotches which produces new spores within couple of weeks. These spores spread by wind to the canopy of the tree and the neighboring trees, initiating new infections. So when the moisture is there, the cycle repeats many times in the growing season. Apple scab is severe in those areas where weather remains warm and rainy in the spring season.
Apple scab infections can be found on both side of the leaves, usually developing first on the lower side. The infection appears as spots, circular or irregular in shape. These spots are brown to green in color. Severe infestation can cause defoliation of leaves. Lesions on young fruit are similar to those on leaves, but growing of younger fruit enlarges the spot and become brown and corky. Heavily infected fruit gets deformed and cracked at immature stage. However, if the fruits crack as a result of scab they become prone to fruit rots. Secondary infections that occur in late summer or early fall are often small in size and can not be seen by naked eye at harvest.
Do not run water sprinklers more than 9 hours on trees, which can cause leaf wetness or dew, creating an infection period. Proper air movement helps in reducing the number of infection periods because the foliage dries more quickly. Pruning out stems which are affected, and disposing of fallen leaves and infected fruit will reduce the amount of fungus available to start infections in the next season.
Management of apple scab is done by fungicide sprays. There are number of fungicides available having different mode of action. So the fungicides should be used according to their mode of action. Early infections are controlled by protectant fungicides preventing the spores from emerging to the leaves. The fungicides which are used to control scab fungus inside the leaves and fruits are called post-infection fungicides. They have the capability to penetrate the plant tissue to eliminate the fungus development. The selection of fungicides for apple scab is based on many factors, including the whole fungus spectrum to be managed at the same time.
The fungicides which are used for the management of apple scab are as under: