The diseases and insect pests of apples occur at different times during the growing season, depending on environmental and biological factors (see Table 4.4). Many strategies should be used to control these pests, and pesticides should be applied only when no other control strategy is available. Cultural controls are listed below, and further information on disease control strategies is included in the previous section under specific insects and diseases. Table 4.5, Table 4.6, Table 4.7, Table 4.8, and Table 4.9 provide information about pesticides and their use on pome fruit.
Many insect and disease problems can be prevented before they occur. Once present, diseases and insects can be very difficult to control. See basic cultural guidelines under Pest Management.
Plant scab-resistant apple trees. If susceptible varieties are being grown, remove leaves soon after they have fallen to reduce the carryover of disease from one season to the next. Never allow fruit to remain on the ground from one season to the next. Prune the trees annually to improve air circulation and rapid drying.
Apple Powdery Mildew
Plant mildew-resistant apple trees. If susceptible varieties are being grown, prune off white infected terminals as they appear from bloom to mid-June. Remove prunings from the orchard.
Cedar Apple Rust
Plant rust-resistant apple trees. If susceptible varieties are being grown, remove red cedar trees from the orchard area if possible. If it is not possible to remove the red cedar trees, then prune the galls off the cedar trees each spring before the apples bloom.
Apple Summer Rots
No variety resistance to the summer rots is known. Prune trees annually to improve air circulation and reduce the potential for disease. Remove rotted fruit from the tree or the ground as it occurs. Be careful to prevent wounding of the fruit since the rot fungi readily infect the fruit at wounds. Prune out dead branches and remove prunings from the orchard area. The apple rotting fungi survive well on dead wood and can spread to fruit on the tree. Cool fruit after picking to retard the development of the disease.
Sooty Blotch and Flyspeck of Apple
No variety resistance to these diseases is known. Prune trees annually to improve air circulation and reduce the potential for disease. Cool fruit after picking to retard the development of the disease.
Fire Blight of Apple and Pear
Apple varieties vary in their susceptibility to fire blight. All pear varieties are susceptible to fire blight. If apple or pear trees are overfertilized and are too vigorous, fire blight will be a more severe problem. It is important to maintain a balanced fertilization program to control this disease; refer to the section on fertilization for more information. Crabapple trees also are susceptible to fire blight, and the disease might spread from crabapple to apple trees in the area. Removing blighted shoots when they appear (about 2 weeks after bloom to late summer) is an important control strategy to keep the disease from spreading within the tree or to nearby trees. When pruning out blighted shoots, breaking out the shoots by hand is better than pruning them off since the bacteria that cause fire blight can be spread on pruning tools. Break out shoots 8 to 12 inches below the obviously blighted or killed wood to ensure the removal of all of the diseased wood. If pruning tools must be used to remove larger wood, disinfect the tools between each cut with a 10 percent chlorine liquid bleach solution. Wash the tools well after use since bleach is highly corrosive to the metal. Insects might spread the disease once it is established in an area. Good pest control measures must also be followed in the summer, if the spread of fire blight is to be limited.
Pear Scab and Leaf Spot
No variety resistance to pear scab is known. Remove leaves soon after they have fallen to reduce the carryover of disease from one season to the next. Never allow fruit to remain on the ground from one season to the next. Prune the trees annually to improve air circulation and rapid drying.