SYMPLASTLESS BRANCH, INSIDE VIEW
When symplastless branches are removed promptly, decay-causing fungi will be resisted by the branch protection zone at position A. When the symplastless branch persists or when the invading microorganisms are aggressive, the corewood [B] of the branch may be infected. When symplastless branches remain for a long time, and the infecting microorganisms are very aggressive, the wood in the trunk present at the time of branch symplast death may be infected [C]. Then a barrier zone [D] will separate the infected wood from the sound wood that formed after the death of the symplast of the branch. It is not well understood how C occurs and how D forms. The basic compartmented structure of the tree may be the primary answer to the limitations of the infection. If the pathogens are going to spread, it would be many-fold easier for them to spread vertically than outward in a radial direction. E is where the trunk pith and branch pith come close together but do not meet. Diagram from Modern Arboriculture
Left: Experiments on branch development and pruning on 12 species of trees for 8 years showed that trees form strong protection boundaries at the base of dying branches. Pruning cuts that remove the boundaries lead to rapid trunk decay. The photograph shows the proper cut (YES line). The arrows at the branch base show the branch protection zone. The arrows along the branch core show the compacted xylem. The arrows below about the small branch show the results of boundaries. that walled off the decayed stub.
Protection zones form within the bases of branches. The zones are made up of chemicals that come from stored energy reserves - starch, oil- in living wood cells. The zones resist spread of organisms from the branch into the trunk. The chemicals are phenol-based in hardwoods and terpene-based in conifers.
Suggested book on "Pruning".
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