Disease name: Sudden oak death, ramorum blight
Pathogen name: Phytophthora ramorum. (water mold or oomycete).
Hosts: Foliar hosts for the pathogen Phytophthora ramorum include rhododendrons and azaleas (Rhododendron species), viburnums (Viburnum species), lilacs (Syringa species), camellias (Camellia species), and andromeda (Pieris species). USDA APHIS maintains a more extensive and official list of host plants. When these foliar hosts are infected, they develop a non-fatal disease referred to as ramorum blight.
Oaks species in the red oak group are major hosts for the pathogen Phytophthora ramorum. When they become infected by this pathogen, they develop the disease referred to as sudden oak death.
Oak species in the white oak group are not susceptible.
Sudden oak death is currently found in parts of California and Oregon.
This fatal disease is federally regulated to prevent spread to other parts of the country.
Symptoms
Symptoms caused by Phytophthora ramorum vary by host. Non-oak hosts, or foliar hosts, infected will develop ramorum blight, which is not fatal. The leaves or shoots become blighted. Foliar symptoms vary by host and can include dead spots on the leaves, death of the midvein of the leaf, water-soaked lesions, death of the entire leaf, and defoliation. Lesions on the stems and discoloration of the stem’s vascular system do occur, but are less common than leaf symptoms. The foliar hosts do not die, but serve as a source of infection for oaks.
Oaks infected with the pathogen develop the disease sudden oak death. Symptoms include bleeding trunk cankers that arise from infection of the phloem and inner bark of the host tree. These cankers produce a clear, reddish liquid. Over time, the liquid will seep or “bleed” from cracked bark, as well as bark that is still intact. The area where the liquid seeps out becomes stained a dark, red-brown color. Removal of the bark will reveal the dark cankers underneath.
The disease organism can also infect the water-conducting tissue (xylem) of oaks. This restricts water flow to the upper branches of the tree, leading to dieback of the canopy. Oaks do not show any foliar symptoms, but tanoaks, which are native to California and Oregon, may show death of the veins, leaf stalks, and twigs. Despite the name sudden oak death, it may take months or even years for a host oak to die.
Disease Cycle
The pathogen Phytophthora ramorum is not a true fungus, but is classified as a water mold (oomycete). It thrives in moist, humid conditions and in wet soils.
The spores produced by this pathogen can be spread by wind, water, soil, and plant debris.
The life cycle of the pathogen is complicated. On the foliar hosts that get ramorum blight, several disease cycles can occur in one growing season. These hosts do not die, but can serve to spread the disease. The spores that are produced on these foliar hosts can be spread to oaks. Wind and rain can transport spores from the foliar hosts to the bark of oaks.
In time, the spores can penetrate the tree’s tissue and cause sudden oak death.
Oaks that die from sudden oak death do not infect other plants, as the pathogen does not move readily from intact trunk cankers.