Apple tree have knobbly growths on branches greenish colour is this ordinary or a problem looks close to a lichen ? ...
Answers: Your problem sounds like apple scab, which is something you will really involve to take effort of. Apple scab appears as olive green to black spots or lesions on leaves. Heavy infection make leaves turn yellow or brown and drop from the trees. Once the leaves are infected and dropping, there's zilch to spray on the tree.
Here is a site you can check out for preventative maintenance: http://lancaster.unl.edu/hort/articles/2...
Apple scab is a disease of apple trees (genus Malus) cause by the ascomycete fungus Venturia inaequalis. The disease manifests as dull black or grey-brown lesion on the surface of tree leaves, buds or fruits. Lesions may also appear less frequently on the woody tissues of the tree. Fruits and the undersides of leaves are especially susceptible. The disease from time to time kills its host, but can significantly eat up fruit yields and fruit part. Affected fruits are less marketable due to the presence of the black fungal lesion -Wikepedia
If it looks like a lichen it probably is a lichen, and they won't do the tree any impair.