Armillaria Root Disease
Collectively (and in many cases individually), Armillaria species have a huge host range (®). Many conifers and hardwoods, and even some herbaceous plants, are susceptible.
Age of the host may influence the disease. With conifers, killing of young, vigorous trees is fairly common, especially in plantations. Older trees can tolerate infections much better and they survive much longer with infection. The older trees tend to get butt rot if they are infected.
Several species in the genus Armillaria, commonly called oak fungus or honey mushroom, cause similar diseases. These fungi, in the Agaricales, form mushrooms. The characters of the mushrooms for field identification are:
We used to call everything in this group Armillaria mellea. Now we know that there are over 30 distinct entities there. They are difficult to distinguish morphologically. In most cases, however, we now recognize them as distinct species because: (a) they are intersterile; (b) there are some differences among them in morphology and molecular-genetic characters; (c) there are differences in distribution, host range, and virulence, and; (d) it is a very important genus, both ecologically and economically. Following are some of the most studied species (®®):
Stress may play a role in some cases. The fungus may behave like a saprobe or cause a nonlethal butt rot until the trees are stressed (e.g., gypsy moth defoliation of oaks). Then, host physiology is altered and fungus can successfully attack and kill vital tissues. This tends to be the case more often in hardwoods. In conifers, killing of vigorous trees is more often observed. However, killing of hardwoods can be seen without apparent stress too.
This disease is so difficult to deal with partly because it uses as a food base not only the trees it kills, but also stumps of trees killed by felling or other means. Thus, any management we do, if it involves felling, stands a chance of backfiring and making the situation worse.
While an infected tree is still alive or long after it has died, the pathogen has three means of dispersal to to other trees:
The first two means of dispersal, via mycelium at root contacts and rhizomorphs, are short-range. This local spread is very important and is generally the dominant source of infection. The fungus can move by these means from an old root system, perhaps from a previous forest, to plants currently growing on the site. This leads to the appearance of disease in a previously uninfected population. This has been so common in California on sites formerly occupied by oak and cleared for orchard production that the pathogen there is commonly called the "oak root fungus." Because the pathogen may survive for 50 years or more in stumps (®), it can wait until the new generation provides a large target for infection. The fungus can also move this way from a diseased tree to a neighboring healthy tree, leading to expanding areas of disease and mortality, usually called root disease centers.
Root disease centers, as well as saprobic growth of the fungus, involve indeterminate growth through the forest. Resulting clones can cover many hectares, perhaps even miles, and be thousands of years old. Armillaria is the famous humongous fungus that was in all the news some years ago (more on this below).
Rhizomorphs are macroscopic, 1-5 mm diam., reddish brown to black, bundles of organized hyphae with an organized apically growing tip. Rhizomorphs grow through soil, produce branches, and look very much like roots (rhizo-morph) or shoestrings (thus a common name for the disease, "shoestring root rot"). They use energy from a stump or killed tree to grow and infect a nearby tree. They can grow many meters through the soil.
The third means of dispersal, basidiospores, seems to be the least common. Although mushrooms and basidiospores are common enough, the spores don't seem to get established very easily. There is indirect evidence that they do occasionally colonize stumps or wounds, especially in moist climates.
Persistance in dead roots and stumps, the saprobic phase, may be dominant for some species that subsist by colonizing dead trees but rarely seem to kill them. Armillaria species may be abundant in the forest without a lot of obvious, damaging disease in some situations. Other species decay dead trees and stumps and build up energy to attack neighboring trees.
External, above-ground symptoms on individual trees are variable and not specific to this disease or even to root diseases in general. It is not uncommon for the disease to be very advanced but showing no obvious symptoms. Symptoms include:
Internally, the disease may develop as butt rot in some situations and as cambial killing in others. The difference may be related to stress and host differences. If the host is resistant, a major wound may be required for infection, and the fungus would be restricted to inner, inactive wood (butt rot). If the host becomes stressed, the fungus is then able to attack cambial regions, even of unwounded trees.
Clear mortality centers, from a few trees to several hectares, may be seen in some cases and not in others.
The decay is a spongy, often wet, white rot. Zone lines can usually be found in the decayed wood. In hardwoods the decay often has a gelatin in it.
This pathogen is very generous in providing oodles of signs to allow diagnosis. They all require some experience to identify to the genus Armillaria with certainty. They are:
Armillaria species cause root disease throughout most temperate regions of the world. See the list of species under Pathogen, above, for details.
Here's what doesn't work:
Here's what often does work:
Here are management guidelines for PNW conifers:
Here's something that might work: biological control. We found a fungus that is similar to Armillaria and apparently competes with it. We investigated the degree to which they compete and how they are affected by silvicultural treatments in stands threatened by gypsy moth and subsequent Armillaria root rot (®). Others have investigated biological control as well. This may be the future for management for this disease, as silvicultural approaches are often quite limited.
Small fields like mycology and forest pathology can be turned upside down when something strikes the media's fancy on a slow news day. Such was the case in 1992 when M. Smith, J. Bruhn and J. Anderson published an article in Nature (®) highlighting the ability of Armillaria gallica to form very large clones. All it took was for the media to link that concept with the phrase, 'humongous fungus', and the hounds were released! An article in The New York Times, a feature on ABC News with Peter Jennings, and David Letterman's Top Ten followed in quick succession. Find more on this amusing story. Uhaul has a great web page on it, as it is one of the featured supergraphics on their rental vans.
Probably the coolest thing about Armillaria species really has nothing to do with pathology. Its mycelium glows in the dark! The mushrooms do not glow much, if at all, but if you open a piece of wood with advanced decay caused by Armillaria, and view it in the dark, you stand a good chance of seeing the luminescence. It helps to let your eyes adjust to the dark. The amount of light varies greatly from different pieces of decaying wood. It's not likely to blind you, but some pieces are quite bright. Other fungi glow, including in some cases the fruitbodies, but Armillaria is certainly the most common and widespread luminescent fungus.
It certainly is a wondrous thing to see in the night, bringing a strange mix of delight and spookiness. So it is easy to imagine strange and magical things behind it. Glowing wood has indeed found its way into folklore and mythology. It has been termed "fairy fire" but is more commonly called "foxfire" in modern times (is that derived from "faux," or false, fire?). Aristotle called it a "cold fire". It made a brief appearance to lend an eerie air of dread to a scene in the oldest surviving piece of English literature, Beowulf (see right).
It is fun to speculate what selective advantage luminescence may confer. Some have suggested it may attract animals that serve the fungus in some way. Others have suggested some physiological role in metabolizing damaging waste products. Also, keep in mind that not every character has a 'purpose'. If it is selectively neutral, or a byproduct of some advantageous trait, it may persist in a population of fungi indefinitely.