All in all, apple trees can fall victim to approximately two dozen diseases. The twigs are susceptible to several fungal diseases that can ruin the appearance of the tree, destroy a crop or, in the worse cases, destroy the whole tree. Some fungi will be more prominent in certain apple varieties and some will be more prominent in certain parts of the country, but they can strike any tree at any time.
Apple tree twig diseases are fungal. Black pox mostly attacks apple trees in the warmer areas and affects the Rome Beauty and Grimes Golden varieties more than others, but any type can fall victim. Powdery mildew attacks the twigs of apple trees in mild climates. White rot is also known as bot rot and is common in the South.
Black pox is caused by a fungus, Helminthosporium papulosum, that winters over in infected trees and attacks in the spring. Powdery mildew is caused by the Podosphaera leucotricha fungus which winters over in dormant blossoms. It is one fungus that does not need moisture to grow and spread. White rot is caused by the Botryosphaeria dothidea fungus and only affects the wood and fruit, not the leaves.
Black pox symptoms appear on the twigs in the spring as black, shiny, cone-shaped lesions. They will also appear on the fruit and leaves. Powdery mildew appears as a greyish white powder on the twigs. The leaves will also be folded.
With white rot, the limbs and twigs will develop small circular spots and blisters. They will grow larger during the growing season. The fruit will have small sunken in brown spots in light skinned varieties and white or light brown spots in the red ones.
With black pox, the fruit will be small and black and sunken in. The leaves will turn black or purple. Powdery mildew stunts the growth of the twigs. If it is not treated, the blossoms will fall off too early and the growth of the entire tree will be stunted. White rot causes the bark of the tree to become orange and peel away from the trunk. In the most sever cases, the fungus can spread around all the limbs and the trunk. The fruit will also rot and fall off.
Black pox can be treated by cleaning up all of the fallen leaves and fruit at the end of the growing season and dispose of it. Apply a fungicide to fight the disease and stop it from spreading to other trees. Powdery mildew is treated by mildewcide, which is a chemical that slows down the growth of mildew, and by pruning off the white terminal shoots. White rot can be treated with chemicals as well as by pruning away the affected areas every year. Apply a fungicide all the way through the growing season.