The most common reason for poor yield from apple trees is apple tree diseases like rust, fire blight, apple scab and powdery mildew. Nearly all the well apple varieties are susceptible to either one or more of these diseases. Some form of organic or inorganic pest and disease control methods have to be employed in order to keep the apple trees entirely free from disease. If the diseases are not checked in time they lead to damaged fruit and weak trees. In addition to these diseases, common disease of apple tree trunk and branches include apple union necrosis or tomato ringspot virus, armillaria root rot, European canker, sappy bark, and crown gall.
Apple union necrosis
Apple union necrosis is transmitted by the dagger nematode Xiphinema americanum and is caused by the tomato ringspot virus. Grafting is a common method by which the virus is transmitted. The virus is most common in trees of fruit-bearing age and cause delays in bud break in spring, small and sparse leave growth, pale and dull leaf colour, reduced growth of the terminal shoot and smaller, highly coloured fruit. The trunk is often swollen over the union of the graft. When the bark is removed from below and above the graft union it reveals abnormally spongy, thick and orange-coloured bark. If not treated in time, apple union necrosis causes the death of the entire tree.
The fruit trees that are most susceptible to European canker include pears, apples, other hardwood tree species. European canker is also referred to as Nectaria galligena. The infection starts with leaf scars and wounds after the first rains during autumn. The symptoms do not appear until the following spring and are characterised with reddish brown lesions around the infected areas on the young spurs and shoots. As they enlarge, the lesions turn into cankers with concentrated ridges outside the elliptical sunken areas of the dying tree tissue. The bark covering the diseased areas cracks and takes on a paper-like appearance. European cankers destroy the fruit wood and reduce the crop yield.
Sappy bark or Trametes versicolor is also called papery bark and it is a fungal disease of apple trees that occurs on the older trees. The fungus enters the tree through the pruning cuts. The infection causes the bark and wood tissues to decay whereas the wood becomes discoloured and spongy. Affected areas start to peel frequently and expose the decay underneath. The bark appears spongy during damp weather and papery when it is dry. The diseased areas often exude a dark sap. As the disease progresses, it creates bracket like structures along the edges of the affected areas which produce spores of the disease. When sappy bark appears on the trunk it can girdle and kill off the entire tree. It is possible to control sappy bark by removing the cankers before they can cause major damage.