Apple trees (Malus domestica), with their attractive shape, ease of care and tasty, nutritious fruit, are a favorite with many home gardeners. Unfortunately, apple tree diseases can greatly reduce the quality of blooms in the tree, causing poor-quality, malformed fruit, an overall reduction of the amount of fruit on the tree, or both. Most apple tree diseases can be avoided by the use of proper culture and preventative chemical sprays, or by planting resistant varieties.
Rust diseases are caused by fungi. The specific symptoms vary depending on the particular fungus infecting the tree, but all of them manifest as spots on the leaves or fruit. The spots can be orange or green, and are usually sunken. In some cases, the spots will spread and merge together, and the affected leaves may drop from the tree prematurely. The fungi that cause rust diseases are transported on water, such as rain blown by the wind, and often are more serious during exceptionally wet, warm springs.
Apple scab is a very common disease of apple trees. This disease is caused by a fungus--Venturia inaequalis--and is recognizable as olive green spots that appear on the leaves in early spring. The leaves may become curled up, brown, or drop from the tree before summer even arrives. As the fruit ripens, it also develops symptoms of the fungal infection, which manifest as bumpy black spots. The fungus can overwinter on the ground in the dropped leaves and fruit. Like rust diseases, this fungus spreads on rain.
Fire blight is a serious disease that can cause major damage to an apple tree. This disease is caused by a bacteria. The bacteria spreads on rain and attacks the tender buds and shoots of the tree in early spring, causing them to become malformed or killing them outright. The bacteria kills the tree from the outermost parts in, causing the tree to look as if it was scorched by fire. During the winter, the bacteria lies dormant between the dead bark and live bark, rising up again when spring arrives to attack the tree anew.
Powdery mildew is a common but not serious disease of apple trees. This disease is caused by a fungus that spreads on water (rain) and lies dormant in the buds of the tree during the winter. As the leaves emerge in the spring, they become covered with the white spores of the fungus, giving them a powdery appearance. of apple is caused by a fungus that overwinters in the dormant buds. When the buds open, the fungus spreads to nearby leaves and even other trees.