Fruit trees are susceptible to diseases just like any other living thing. Each fruit tree will have diseases that attack only that one fruit, and then there are diseases that attack many different fruit trees. Fruit trees need to be monitored for signs, not left on their own.
Alternaria blotch causes small, round, black or purple lesions to appear on leaves in late spring or early summer. The leaves can turn yellow and up to 50 percent can be lost. Mites are to blame and controlling them can prevent infection. Delicious apples are the most susceptible, but it can strike any variety of apple.
Prunus stem pitting is caused by tomato ringspot virus. Leaves look like they have been in a drought and turn yellow in late summer, before the normal time for leaves to change. The tree will produce a lot of fruit, but they will be small, ripen early and fall off. The next year, the tree will be weak and show stunted growth. Eventually, it will die. This disease will attack not only the peach, but all other stone fruit trees, such as plums and nectarines.
Prunus necrotic ringspot is a virus that can cause the leaves of both sweet and sour cherry trees to become tattered, and the plant can produce up to 50 percent less fruit than it would otherwise. It takes more than 1 year for the virus to do all the damage. In the first year, only a few branches may be involved. The next year, those branches may be fine, but others will be infected. There can also be years when there is no damage. Green fruit can develop rings and the bark can be killed. It can also attack peaches and plums
Bacterial spot can defoliate a tree as well as reducing the yield. It can also severely affect the quality of the fruit. It strikes most in areas that get in excess of 20 inches of rain a year. The fruit, leaves and the new twigs are all vulnerable. It will appear as water-soaked spots on smooth-skinned fruits such as the plum. The skin will break open and the fruit directly under the spot will become sunken in. Eventually, the part of the leaf with the spot will fall out, leaving holes and a tattered appearance. All it takes for a leaf to fall off is two or three spots. In the worse cases, only the young leaves will remain. It can also attack peach, nectarine, apricot and almond trees.