04Is this Armillaria mellea, the dreaded oak root rot fungus? I really do not know. All the elements are here. The stump is that of a coast live oak. Bellow the stump there are the remains of roots. Those necrotic roots are undoubtedly decomposing as a result of rot. That rot is undoubtedly associated with this fungus. Furthermore, it fits the description of oak root rot fungus. The toasted spots were probably caused by weathering as the mushrooms started to develop while the weather was still warm and dry.

Now that the soil and rotting wood are damp from rain, this fungus is really proliferating. The individual mushrooms within the soccer ball sized mass were only about as big as those at the lower left margin of the picture just prior to the rain. They do not last long, and might become gooey black slop after only a few sunny days. The stump may continue to rot for a few more years. It is out of the way, so there is not need to get rid of it.

It annoys me when landscape professionals tell me that a particular spot in the region has oak root rot fungus. Of course it does. It is everywhere. It is quite natural here.

Oak root rot fungus is justifiably dreaded. It can easily kill the most majestic oaks that have survived for centuries.

However, most of the problems with oak root rot fungus in landscape situations are caused by supplemental irrigation and other gardening techniques. Oaks that have survived on natural seasonal rainfall for centuries are much more susceptible to rot if their roots are kept continually moist by the irrigation needed to sustain other plants added around them. Roots that get severed for the installation of pavement or building foundations are likewise much more susceptible to rot, particularly in spots where drainage from roofs or paved surfaces enhances soil moisture.

For this particular pathogen, gardening is more often the problem.

16 thoughts on “The Humongous Fungus Among Us

  1. We call it Honey Fungus and it is the bane of my life. The rosaceae family is particularly vulnerable and part of my garden is an ancient orchard. I have it cropping up everywhere and there is no way to confine or cure it. The only defence is to keep things watered, well fed and free from stress. Still I tremble every time I plant a new rare and expensive tree.

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    1. That is another name for it. In our region, it kills ancient oaks after the area around them gets developed and landscaped . . . and irrigated. Valley oaks and coast live oaks can tolerate watering if they grow up with it, but not if they survived for more than a century without it.


  2. I was wondering if they were edible as well. The thing is, with mushrooms, even experts can be wrong. I have fond childhood memories of eating puffballs and morels, but have not foraged for them as an adult.

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    1. By the time the mushrooms are evident, the roots are too decayed for the affected tree to survive for long. However, some lightweight trees can live with it for many years. The problem with big trees is that they become dangerously destabilized by the decay. The decay can spread into the trunks of multi trunked trees and compromise structural integrity. Such trees often get cut down before falling or dropping huge limbs. The stump in the picture is that of a tree that was cut down years ago.


    1. Well, it is common everywhere here. Those in the picture are on a stump of a tree that was cut down years ago. All trees eventually die. This pathogen typically prefers trees that are already deteriorating. It is only a problem because we live among trees that do what is natural to them, and our activity accelerates some of the processes. It is always hard to lose the biggest and most majestic of the oaks, and I have lived with a few when I lived in town, but I can blame neither the oaks nor the pathogen for their natural behavior.


    1. It is natural though. Before landscaping moved in around the massive valley oaks in the Santa Clara Valley, the big oaks lived with the disease for decades. It only becomes a problem when the disease proliferates in irrigated environments. A slight bit of extra moisture accelerates the process and kills the oaks sooner. Those in the wild still live with the disease like they have always done, and eventually succumb in a natural way.

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