A young leaf is covered in pores called stomata that can open and shut. When they are open gases are exchanged (carbon dioxide in – oxygen out) and water can be transpired. During the open period ingress is possible by fungi, bacteria and viral pathogens into the leaf tissue. In this case the first or dominant ‘invader’ appears to have been a white mold. In time this may kill the leaf and it is discarded by the parent tree or shrub.