I have two questions. When is a good time to prune Parrotia persica?
What can I do about a canker on the trunk of a Southern Magnolia?
According to the American Horticultural Society’s Pruning and Training (edited by Christopher Brickell; DK Publishing, 1996), Parrotia persica should not need a great deal of pruning, but if you do prune, it should be from fall to early spring.
If you are growing it as a shrub-like shape, you should not thin or shorten laterals, as this will cause congested growth. If you are growing it in a tree-like form, the trunk can be cleared to about 5 feet, allowing the crown to branch. If needed, you can shorten pendulous tips to give clearance for walking beneath the tree. Once established, this tree should not be pruned. If the tree was a grafted specimen, remove any suckers.
This information from University of Florida discusses Southern magnolia (Magnolia grandiflora), and says that cankers may kill branches, but the affected branches may be pruned.
Excerpt:
“Canker diseases will kill branches. Cankers on branches can be pruned out. Keep trees healthy with regular fertilization and by watering in dry weather.”
Magnolias by Rosemary Barrett (Firefly Books, 2002) says that
“various cankers, such as nectria canker, dieback and trunk decay can all be dealt with by cutting out the dead or diseased wood. Rarely will any of these diseases cause the death of the plant.”