Over the last few months, I've noticed lots of dry leaves and dieback of the top branches of one of my favorite red maples. The tree was here when we bought the property about eight years ago, so I don't know the exact cultivar or how it was planted.
There are a couple of common issues that might be to blame for the recent dieback. A normal, healthy root system grows away from a tree's stem, rather like spokes on a wheel. Trees grown in containers can become pot bound, forcing roots to grow in a circular pattern around the root ball, rather like a ball of string. These girdling roots cause compression of the stem and sapwood. If the roots aren't unwound, straightened out and properly trimmed when planted, the roots will become tighter and tighter as the tree grows.
This can also occur when trees are planted too deep, as adventitious (growing sideways from the stem) roots grow against the stem and squeeze the sapwood. This compression severely slows or stops the flow of water, nutrients, and food. Over time, it will choke the life from the tree.
If detected early, it is possible to cut away the girdling roots, allowing the tree to recover and thrive. But I dug down several inches to inspect the root ball and found no evidence of girdling roots.
Another common cause of dieback is verticillium wilt, a soil-borne fungal disease that enters the plant through the roots and shuts down the tree's ability to receive water. The tree will attempt to compartmentalize the fungus to keep it from spreading. You may see flagging, which is partial or total defoliation on one side of the tree. The tree will survive if it is successful in containing the fungus, but often the fungus will move throughout the tree and saving it won't be possible.
Signs of infection include reduced vigor, undersized, discolored, curling and drying leaves, and branch dieback. Peeling back or slicing into the infected bark often reveals a discolored, darkened area, which my tree doesn't have. However, the only way to be completely sure it isn't verticillium wilt is to have a sample tested at a diagnostic lab.
Unfortunately, another common cause of dying maples is simply lack of water. Japanese maples prefer the climate of their native homeland, where they commonly receive year-round rain, fog, and moisture. Although my tree survived the recent, lengthy drought, it appears that it may have thoroughly stressed the tree.
An arborist helped me inspect and diagnose my maple, and decide on the best course of action: Increase the amount of water it's getting, wait until cooler weather sets in to prune away damaged branches, and keep our fingers crossed.
By UC Master Gardener Rebecca Jepsen
This article first appeared in the September 1, 2019 print issue of the San Jose Mercury News.