Fruit trees are subject to a number of diseases. Certain bacteria cause some plant diseases, while fungal organisms cause others. You can cure your fruit tree if it comes down with some diseases, but others leave little hope for the tree's survival. One of the most highly recommended methods of treating diseases of fruit trees is to grow only varieties that are bred to resist certain diseases---by doing this, you will prevent the disease from striking. Keeping fallen leaves and fruit cleaned up in the area around your trees is another method of preventing disease.
Treat apple and pear scab by planting resistant varieties of these fruits and by raking up all leaves, twigs, fruit and other tree parts that fall to the ground around your tree. Scab is a fungal disease that manifests itself as rough fruit, premature dropping of leaves and fruit and eventual loss of the tree's vigor and hardiness. Because the fungus lives in fallen leaves during winter, it is important to keep them cleaned up, even in summer when the weather is warm and rainy.
Prune apple and pear twigs and branches that are infected with fire blight to within 12 inches of the dead area. Be sure to wipe your saw or pruning clippers with liquid bleach mixed with water between cuts; 10 percent bleach to 90 percent water is a good mix. Fire blight usually shows up on fruit blossoms, but it can also affect new shoots. Look for twigs that loop downward to spot a common symptom of fire blight.
Spray liquid copper fungicide on citrus trees that develop melanose fungus. Usually striking trees less than 10 years of age, this disease causes fruit rind to develop a scabby appearance. Prune branches that appear dead or unhealthy whenever you notice them.
Control ants that live on your fruit tree, because they bring insects such as aphids, whiteflies and mealybugs and feed from their excretions. The sweet, sticky excretion (called "honeydew") that molds on the leaves of many fruit trees, especially citrus, creates a black coating called "sooty mold fungus." Place ant stakes in the soil surrounding your tree or apply a product called Tree Tanglefoot around the lower trunk, following label instructions. If this fungus takes over, spray your tree with insecticidal soap to kill the insects. Continue spraying until no more insects remain. You can also spray your tree with liquid copper fungicide for severe cases of sooty mold.
Spray peach, nectarine, plum, apricot and nectarine trees that might be the victims of bacterial spot with a copper compound chemical before symptoms occur. This can be difficult, but if your tree has had this disease before, spraying is a good preventative measure. West Virginia University recommends alternating weekly applications of copper spray with an antibiotic, starting when blossoms begin to fall and continuing for three weeks. To prevent this disease, plant varieties of trees that are resistant to bacterial spot.