Q: How can I protect my landscape trees from fungal infection? I’ve heard that a large, seemingly healthy tree can just suddenly drop a major limb or just break in half without any outward sign of heart rot.
Wood-destroying fungi can be beneficial when it breaks down dead wood because it makes nutrients available through the decomposition process. If you come across a fallen tree in its natural environment, you can often see an assortment of mushrooms (fruiting bodies) growing. Some of these are quite beautiful, such as the Turkey Tail mushroom.
For trees that are still standing, fungal infection can become a problem. Occasionally, you will see an unexpected breakage, but upon closer inspection, the center of the tree (or fallen branch) will show evidence of extensive damage due to rot. As an infection progresses, it destroys the cambium, severely weakening the tree. A 10% reduction in tissue can result in a 90% reduction of wood strength, so it’s unsurprising that breakage can occur before an infection is detected.
Older trees are more susceptible to fungal infection, but younger trees can be affected as well. The most common cause of fungal infection is some sort of injury or improper pruning that allows the organism to enter the plant tissue and establish itself.
String trimmer injury is, unfortunately, very common. Trunk guards can protect trees, but they should allow free air circulation and should be checked frequently so they don’t strangle the tree as it grows.
Rabbits, mice, and other animals can seriously damage bark by their relentless gnawing. Placing a cage of ¼-inch or ½-inch hardware cloth can prevent access to your tree. These cages should also be checked and enlarged as the tree grows.
Improper pruning can harm a tree by leaving an open wound that doesn’t heal over. When removing a smaller limb (one smaller than 2 inches in diameter), use sharp pruners or loppers. When you look at the branch, you should be able to see where the branch bark and the main trunk bark meet. There should be a branch collar visible as well – this is the slightly thicker area at the base of the branch. Make your cut just outside the branch collar.
When removing a branch that is 2 inches or larger in diameter, you may want to remove the bulk of the limb in several steps to avoid having to deal with an unwieldy, large branch that may fall where you don’t want it to fall. Once you are ready to make the final pruning cut, first make an undercut about 1 or 2 feet from the trunk on the branch underside. This should only go about 1/3 of the way through the branch. This will prevent the bark from tearing away from the trunk when the branch separates. Then, saw from above several inches out from the undercut to remove the branch. Finally, cut just outside the branch collar. Do not use sealer since the cut will heal naturally.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County
ucceocmghotline@ucanr.edu; http://mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; 951-955-0170; https://ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu