TREES, PLEASE: Should I park under that tree?

By Tim Clancy.

The disease triangle is often used to illustrate the elements necessary for a pathogen to be successful. The three elements of the triangle are the host (the actual tree affected), the pathogen, and the environmental conditions that favor the pathogen over the host. In many cases, humans control the environmental component. Too much water, for example, can kill a plant.

Here in Southern California, our climate is typically dry and Mediterranean, as opposed to southern Florida, which has copious amounts of tropical moisture. As a result, there are lower levels of disease activity here in San Diego than in Miami. However, we still do have our share of climate-based diseases. Some of these are tree killers and some just annoy trees.

Fungal degradation of trees is one example. There are three general categories of wood decay fungi: soft rot, white rot and brown rot. Each rot affects trees differently. Additionally the various fungi typically affect only certain areas of trees. Some work on roots, some at the buttresses and roots, and some only affect the trunk or branches. This makes field identification a little bit easier; we know we will not find honey fungus high up on the trunk, and we will not see sulfur fungus in the roots.

Let's take a closer look at brown rot. Brown rot typically attacks conifers around the world. However, I have seen it on a conifer in San Diego only once in 30 years. I find it more frequently on blue gum eucalyptus (Eucalyptus globulus). Brown rots consume the parts of a tree that give it stability and the strength to remain upright. In the case of sulfur fungus, which causes a brown heart rot, the fungus can do this without causing any symptoms or decline in the canopy.

That is where the problem lies. Lay people often will look at a tree canopy and conclude that the tree is healthy and safe, in spite of the presence of sulfur fungi. Unfortunately, if a tree has sulfur fungus, there is no cure. Once a tree is infected, it is doomed. Such is life. The progress of the brown rot produced by the fungus depends on many factors. However, in the end, if a tree with sulfur fungus is allowed to stand, it will experience a brittle failure, and the whole tree can collapse.

One way to determine the extent of decay caused by fungus is to use some fancy devices that take pictures of the inside of a tree. Analysis of the pictures can reveal the extent of the decay, assisting with a decision on tree retention. If the tree is retained, a follow-up inspection can be scheduled. Of course, this is an expensive process, and the cost must be weighed against the benefit of keeping the tree.

The sulfur fungus can survive the harvest process and then the milling of wood for use. It is one of the only fungi that can survive in both dead and living wood.

I never park my vehicle under or near a tree that I know has sulfur fungus, and you probably should not do so either.