After three years, this remains a mystery. No one is complaining.
Phytophthora ramorum is the pathogen that initiates Sudden Oak Death Syndrome, which is known simply as SODS. Monarthrum scutellare, which are known as ambrosia beetles, and are the secondary pathogen associated with the syndrome, infest and kill tanoak and coast live oak that are infested with Phytophthora ramorum, about as quickly as symptoms are observable.
Hypoxylon thouarsianum is a tertiary but merely opportunistic pathogen associated with the syndrome. By the time it gets established within galleries excavated by the ambrosia beetles, the affected trees are almost completely necrotic. That first ‘S’ in SODS is there for a reason. It is an efficient process. Death occurs too suddenly for affected trees to drop any of their leaves!
Each of these three pathogens causes distinct symptoms. Phytophthora ramorum causes trees to bleed black tar-like fluid, and causes tanoak to exhibit foliar flagging as seen in the picture above. Monarthrum scutellare expels finely textured frass from the galleries it excavates into infected trees. Hypoxylon thouarsianum produces distinct small and black fruiting structures.
In the past several years though, Sudden Oak Death Syndrome has often been a bit less than sudden. There are a few tanoaks here that have exhibited foliar flagging for a few consecutive years, without any bleeding from the trunk or infestation by ambrosia beetle. Some coast live oaks have exhibited minor bleeding, but likewise have not become infested by ambrosia beetle.
It is as if the ambrosia beetle is no longer proliferating as it had been. It actually seems to be rather scarce. Trees that were expected to succumb suddenly to ambrosia beetle infestation are succumbing slower to infestation of only Phytophthora ramorum. The process is variable, so might have potential to kill some trees rather suddenly, but may take a few years to kill others.
Could some possibly survive?
Is 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.
Remember our concern about the mistletoe? (https://wordpress.com/view/tonytomeo.wordpress.com) You might think that everyone would be pleased to see it gone. Yet, there is the concern that whatever killed the mistletoe might kill something else. That is what happened when the SODS killed so many coast live oaks after being ignored for a long time in the tan oaks. We were aware that the tan oaks were dying, but because the trees were so unpopular, we were not too concerned about it.
Now the box elders are dying around Felton. It is hard to say how widespread the problem is because it has not been investigated yet. Like tan oaks and mistletoe, the native box elders are not exactly popular trees. They do not even make good firewood. We only use them as firewood to get rid of them. However, when so many are die, it leaves big holes in the forest canopy. This is not really a problem, since the forest will have no problem filling the holes in, but it certainly gets our attention. Will the disease or insect pathogen that killed the box elders kill something else next?
The trees seemed healthy as they defoliated last autumn. They were bare through winter when the San Lorenzo River came up higher than it had since 1982. A few box elders got taken away by the River, along with all sorts of other riparian trees from the flood zone, but that is to be expected. Then, after the River receded, many box elders that were in the flood zone did not foliate in the spring. A few foliated, only to have their new foliage shrivel and die shortly afterward. Some of the dead trees became infested with boring beetles. All of the dead trees deteriorated rapidly through summer, and some have dropped big limbs or fallen over.
What is happening with the box elders could be completely normal, and caused by an endemic pathogen; but it makes one wonder. There are so many new insects and diseases being brought in with plants arriving from all over the world, without any regulations to limit the spread of such insects and diseases.