The Sudden Oak Death pathogen

Hi folks,

Below is some information The SMMHOA Board asked me to distribute to the Community regarding this topic of community interest. Please feel free to comment/post your thoughts and additional ideas or information regarding this topic. Thanks, Rev. Joe

The Sudden Oak Death pathogen, Phytophthora ramorum has been identified on well over 100 diverse species of plants; hosts include hardwood and conifer trees, shrubs, herbaceous plants and ferns. On most plant species, symptoms are limited to leaf spots and shoot dieback and are not lethal. However, since the mid-1990s, Phytophthora ramorum has killed millions of tanoak trees and several oak tree species (coast live oak, California black oak, Shreve oak, and canyon live oak), and caused twig and foliar diseases in numerous other plant species, including California bay laurel, Douglas-fir, and coast redwood.

Phytophthora species are water molds that are well known plant pathogens. They are water-loving and produce plentiful spores in moist, humid conditions. While most foliar hosts do not die from the disease, they do play a key role in the spread of P. ramorum, acting as breeding grounds for inoculum, which may then be spread through wind-driven rain, water, plant material, or human activity. Trunk hosts such as oaks are considered terminal hosts – the pathogen does not readily spread from intact bark cankers – and they become infected only when exposed to spores produced on the leaves of neighboring plants.

An official list of host plants is maintained by the USDA APHIS as part of the federal quarantine. For regulatory purposes, hosts are divided into “regulated hosts” or “associated hosts” depending on the amount of testing that has been conducted to demonstrate that the plant species is a host for the pathogen. Additionally some plant species have been confirmed as P. ramorum positive hosts but have not yet been added to the USDA host or associated host list. In all of these cases, symptom photos and regulatory status is noted on the Plant Symptom Photos page. For wildland plants, see the list of Regulated Forest Hosts.

Infections by Phytophthora ramorum on oaks were originally called “Sudden Oak Death” because of the apparent rapid (2 to 4 weeks) browning of leaves throughout the entire crown without a prolonged period of visible decline. While this sudden browning may occur, death of the tree due to P. ramorum infection usually takes place after an extended period of infection, estimated as more than two years. During that time, external symptoms may not be present or may be limited to slight, intermittent oozing that can be difficult to detect. A number of other organisms tend to colonize oaks with P. ramorum infections, including ambrosia beetles (Monarthrum scutellare and M. dentiger), bark beetles (Pseudopityophthorus pubipennis), and a decay fungus, Annulohypoxylon thouarsianum. Though these attacks are secondary to the original P. ramorum infection, they act to further weaken the trunk and may hasten tree mortality.

Tanoak is the most susceptible of the known hosts to P. ramorum. All sizes and ages of tanoaks (seedlings, saplings, mature trees) can be infected and killed. Trunks, branches, twigs, leaves, and leaf petioles may become infected and the pathogen can sporulate (reproduce) on tanoak twigs and leaves. Tanoak trees may be infected without showing cankers or bleeding symptoms, which complicates diagnosis. Trunk cankers are similar in appearance to those on species in the red oak group. As with the red oaks, death can occur with a sudden browning of leaves throughout the entire crown, or more slowly with gradual leaf loss. P. ramorum infection in twigs can lead to shoot tip dieback or a shepherd’s crook. Secondary organisms are likely to attack weakened and dying trees.

See additional symptoms on tanoak on this handout from the Garbelotto lab at UC Berkeley.

Multimedia overviews available on UCCE Sonoma website and radio interview

Learn about Sudden Oak Death in Ecology of Tree Diseases. Also read more in the Frequently Asked Questions (FAQs) and connect to external information about P. ramorum and our partners on the Links page. For the complete disease chronology, visit our Library. Source: http://www.suddenoakdeath.org/about-sudden-oak-death/