Recently, much of my work has involved the epidemic of “sudden death” of oaks. Although the name is simple, the disease is rather complex and still not completely understood by those who have been studying it since it first appeared in Marin County several years ago. When I first wrote about this disease, there were several theories about how and why it is so easily transmitted. One theory has been getting more attentions from entomologists and pathologists, but still has not suggested a cure or means with which to slow the spread of the disease.

Sudden death of oaks is caused by ambrosia fungus, which is spread by the oak ambrosia beetle. The beetles bore into the trunks of mature oaks and inoculate their galleries (bored tunnels) with spores of the fungus. The fungus grows profusely in the galleries, providing sustenance to the beetles. The beetles actually eat only (the mycelia of) the fungus, and not the wood into which they bore. Besides providing an ideal environment for the fungus, the galleries are also where the beetles lay the eggs for the next generation. When second generation beetles leave their parents’ galleries, they grab some spores on their way out to take to their new home in another oak tree, starting the process all over again.

Back at the parent beetles’ galleries, the fungus continues to proliferate, eventually clogging the vascular system of the tree and killing it. Beetles do not normally live in small populations, but establish entire subdivisions in a single tree so that there are hundreds or thousands of galleries functioning at the same time. With so much ambrosia fungus, death is so sudden that the leaves don’t have time to fall. Trees that die in this manner are distinct from trees which die a normal, slow death, losing their leaves in the process. Unfortunately, the beetles are also very fast and a new generation has already moved on before infected trees die.

Infestation by oak ambrosia beetle is usually, but not always preceded by infestation of oak bark beetle. These beetles can kill trees as they excavate around the trunk just under the bark. If they manage to get all the way around the trunk, the tree dies; but normally, trees survive repeated attacks. Unfortunately, the stress caused by an attack is just the sort of thing which attracts the oak ambrosia beetles.

Initially, it was thought that the oak bark beetle was the primary pathogen, followed by the ambrosia beetle. However, the attack of so many seemingly healthy trees could not be explained. Now the primary pathogen is believed to be a (phytophthora) root rot fungus which has yet to be isolated and identified. The stress caused by this fungus is not obvious to people, but very attractive to the beetles. Infection of root rot would also explain why shoots which sprout from the roots of few of the killed trees die immediately after they emerge.

Anyone with coast live oaks, tanbark oaks or black oaks in their garden should be aware of the sudden death of oak syndrome and how to recognize the symptoms. It is also important to know what to do about it and how to minimize the risk of infection. These topics will be discussed next week. If the suspense is too great, keep in mind that the beetles are relatively inactive; at least for now.