Well, OK, maybe that's a little much. Admire a forest pathogen? But bear with me.

Armillaria is the genus name for a group of fungi that are found in forests worldwide. Some species of Armillaria are virulent tree-killers, while others mostly play a role in forest recycling by decaying dead wood. In coastal California, the most common species of Armillaria can produce bleeding symptoms on hardwood trees that look exactly like the symptoms caused by P. ramorum.

Armillaria (white) beneath bark, with typical bleeding symptoms above (photo courtesy of Jack Marshall)

So what is there to admire? Well, for one thing, Armillaria is native to California. Researchers from UC Davis sampled Armillaria species throughout California and found four species. The overwhelming majority of their samples, though, consisted of only two of these species: A. mellea and A. gallica.

A. mellea is primarily found in developed areas. It causes a root rot that kills trees, although it rarely kills large numbers of trees at a time. A. gallica is found throughout coastal forests and is the species of Armillaria that I most commonly come across. (At least, I think that it is the species I most commonly come across. Definite identification of Armillaria to the species level requires DNA analysis and/or sophisticated morphological diagnostic skills that I don't have.)

In any case, A. gallica causes a heart rot that slowly develops inside the tree, causes bleeding lesions that show up on the bark, and eventually produces a more or less open column of rot within the tree. It can also decay tree roots and butts, but not as swiftly or commonly as A. mellea.

Since it is a native fungus, A. gallica behaves much as we would expect something to behave when it has evolved over thousands of years in the same environment with its hosts. It is ubiquitous in the forest environment, surviving on what dead woody material it can and picking off the occasional small tree that is stressed because of having to grow under a dense canopy with insufficient light. Healthy, mature hardwood trees apparently resist it easily.

In other words, A. gallica is one of nature's cull and decay agents. Not like P. ramorum at all! Since P. ramorum is not native, its host trees have not evolved defenses adequate to resist infection. This enables the rapid epidemic spread of P. ramorum in our coastal forests.

In other areas with different forest stresses, Armillaria species, even though native, can cause far greater problems. A. ostoyae, for example, kills large numbers of trees in the high-elevation forests on the east side of the Cascades and up into Canada. In these areas, trees have to compete for much more limited water resources and are much more susceptible to a variety of insect and disease problems. Our coastal forests have it easy in comparison.

So far, I have admired Armillaria for (1) being a native member of our Western forests and (2) for being a versatile survivor that plays an important role in forest nutrient cycling. I also admire the fact that the mycelium (main fungal body) of many Armillaria species glows in the dark--i.e., it's "bioluminescent." Pretty cool, huh? And there are plenty of other nifty things about this fungus--material for another blog entry someday.

California black oak with Armillaria mycelium (below left); symptoms at top right could be caused by Armillaria or P. ramorum (photo courtesy of Jack Marshall)