Hosts and Symptoms
In terms of regulations, Regulated Hosts are those plants naturally infected by P. ramorum, and have had Koch’s postulates completed, documented, reviewed, and accepted. Associated Hosts are also found naturally infected, and P. ramorum has been cultured and/or detected using PCR, but Koch’s postulates have not been completed or documented and reviewed. Additional plants 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, we make symptoms photos and regulatory status available on the Plant Symptom Photos page. For the most current information, see the full USDA-APHIS list of current host & associated plants. with further details at http://www.aphis.usda.gov/plant_health/plant_pest_info/pram/index.shtml. If you work in wildlands, see the list of Regulated Forest Hosts.
Biologically, there are two categories of hosts for Phytophthora ramorum: bark canker hosts and foliar hosts. The bark canker hosts are tankoaks and oaks that become infected on the woody portions of a tree. Cankers on the trunk of these trees are the most damaging, and often lead to death. Additionally, diseased oak and tanoak trees are often attacked by other organisms once they are weakened by P. ramorum. These secondary invaders can also kill the tree, and include such organisms as Hypoxylon thourasianum (a fungus that decays sapwood), and bark beetles. In foliar and twig hosts, symptoms can range from leaf spots to twig dieback, but these hosts rarely die from the infection. This page describes symptoms on more common California hosts, but follow the links below for more detailed information.
Only oaks in the red or intermediate groups have been found infected – oaks from the white group are not thought to be susceptible. Only larger (> 4 in dbh) adult plants show symptoms and infections of smaller saplings have never been seen in nature. The most obvious and useful symptom to look for on oaks is a canker on the trunk. Cankers have red-brown to black discoloration and seep dark black to red or amber sap. They usually develop 1 to 2 m off of the ground, although they can be at soil level, or as high as 4 m or greater; they are not thought to extend below the soil line. Bleeding sap initially appears on intact bark, absent any obvious holes or wounds, though, in later stages of the disease, the bark may split.
Infections by Phytophthora ramorum on oaks were originally called “Sudden Oak Death” because of the rapid (2 to 4 weeks) browning of leaves without an apparent 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 disease and perhaps more than two years from the onset of infection. 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, Hypoxylon thouarsianum. Though these attacks are secondary to the original P. ramorum infection, they act to further weaken the trunk and may hasten the death of the tree.
Tanoak (Notholithocarpus densiflorus)
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 by the disease. P. ramorum also infects trunks, branches, twigs, leaves, and leaf petioles of adult trees. Experiments on tanoak trees revealed that they could be infected without showing cankers or bleeding symptoms; diagnosis of tanoak can be difficult because of this. Trunk cankers are similar to those of the red oak group. As with the red oaks, death can occur with a sudden browning of the leaves, or over time with gradual leaf loss. P. ramorum infection in twigs can lead to shoot tip dieback and a Shepherd’s crook. Leaf flagging is a particularly good indicator of disease. Secondary organisms are likely to attack weakend and dying trees.
See additional symptoms on tanoak on this handout from the Garbelotto lab at UC Berkeley.
California bay laurel (Oregon myrtle) (Umbellularia californica)
On bay laurel, Phytophthora ramorum causes leaf spots, usually brown tips surrounded by a halo of yellow. Lesions are typically found where water collects on the leaf – generally the leaf tip, though on leaves that are flat or turned sideways, a leaf spot can develop elsewhere on the leaf. We have not seen any cases of bay laurel dying due to a P. ramorum infection, but these trees are likely important reservoirs of inoculum for the pathogen and may play a role in spreading disease to other plants.
See additional symptoms on bay laurel on this handout from the Garbelotto lab at UC Berkeley.
Camellia symptoms are limited to leaf spots, which can vary in size from a half a centimeter in diameter to covering nearly half the leaf, depending on environmental conditions. Lesions are usually on the leaf tip or leaf edge, and can be surrounded by diffuse margins or thick black zone lines. Plants will drop their infected leaves, and the lower part of the plant can defoliate. Tip dieback or small branch cankers have not been observed on Camellia species.
Leaf spots are the main symptom, though dieback of small branches is seen on plants in Europe. Lesions penetrate through the plant tissue so that spots are identical both on the top and bottom of the leaf. They can be triangular in shape and extend along the leaf mid-vein, or can be found where water collects on the leaf surface (along edges, near the petiole, and at the leaf tip). Leaf spots have diffuse margins and the appearance of watersoaking.