Many common tree problems can be addressed by adjustments to our treatments to trees.  Starting with unhealthy trees, improper planting, improper mulching, improper pruning and over fertilization often lead to tree problems.  Too often the secondary organisms get the blame and our attention and efforts and time are spent (wasted) getting rid of the latter.  When we treat trees and effect their chemistry in ways that reduce the capacity to manufacture and store energy (starch), we reduce the trees defense system.  In a sense we place the tree in a predisposition to receive secondaries.  In advanced tree biology we call the secondaries the clean-up crew.  When adjustments are made, these treatments can be of great value.  E.g., I cannot emphasize enough the value of proper mulching!  Trees with wounds or trunks, many codominant leaders with included bark, trees without roots (woody and non-woody) would be best if left behind in the nursery.  Some nurseries have what is called a cull pile for such trees.  I would like to provide some links for sound organic tree care suggestions.