The leaves, twigs, branches, fruit, roots and trunks of an apple tree can feel the effects of a number of fungal diseases. The fungi to blame for these diseases gain access to the apple tree in different ways and cause varying symptoms. These fungi are quite hardy and can survive through the winter months to wreak havoc on the apple tree into the next growing season.
Venturia inaegualis is the scientific name of the fungus guilty of causing apple scab. The symptoms of apple scab include lesions that the fungus eventually encompass, producing a region on a leaf that resemble velvet and is a different shade of green than the rest of the leaf. The leaf tissue dies and the leaf can fall off before it normally would.
These lesions also affect the apples but take much longer to develop, but the result is distorted apples that fall from the tree before their time. The apple scab fungus can live in the leaves at the base of an infected tree and infect the same tree or others around it the following spring by releasing spores that the wind blows onto the trees.
The wind and the rain will spread the Botryospaeria obtusa fungus that can cause a disease in apple trees known as black rot. In the span of one to three weeks after the flower petals of an apple tree come off, the first signs of apple rot may appear. Small purplish areas emerge on the leaves that get larger and acquire a brownish middle. The fruit can have lesions that begin as red spots but then change to purple ones. Dead areas on the branches called cankers can then surface. The cankers may expand to cover several feet of the branch and trunk.
Black rot can stress an apple tree if it strikes the tree year after year, robbing it of leaves and nutrients. The fungus lives through the cold season in the dead bark and the infected apples that stay on the tree. The fungus gets into the apple tree originally via open wounds in the tree from broken branches and cracks in the trunk.
The genus of fungi called Phytopthora brings about a serious apple tree malady called either collar rot, crown rot or root rot, depending on the part of the tree infected. These fungi live in the soil and the cankers they result in can travel down the tree into the root system or upwards into the higher parts of the tree.
The bark of an apple tree with these infections appears brown and when it gets wet from rain, it will feel slimy. The rot that kills the living tissue in an apple tree has the ability to make the tree perish in as little as one year, but some trees make it through several seasons before finally dying.