The discovery at a San Gabriel Valley nursery of a disease that kills oak trees has raised flags of caution at Descanso Gardens.
State officials last Wednesday confirmed that Sudden Oak Death, a forest disease caused by a fungus-like pathogen, had been found at Azusa-based Monrovia Nursery, which ships plants nationwide and counts Descanso Gardens among its customers.
Officials have said they are unsure if the find was an isolated incident of the disease, which had until last week only been detected in coastal counties of Northern and Central California, or a portent of its spread to Southern California. But horticulturists at Descanso Gardens are doing “everything humanly possible” to ensure that the disease does not infect the Gardens’ multiple acres of oak forest, a Gardens spokeswoman said.
“We’re quite certain that we do not have that pathogen in the garden,” spokeswoman Avery Econome said. “We’re feeling lucky, but forewarned.”
Meanwhile, the city of La Cañada Flintridge is monitoring the disease and has even gone as far as having sap from the city’s 12,000 parkway trees tested for the presence of Sudden Oak Death and other pathogens.
“We’re just kind of watching just like everybody else is,” city Public Works Director Steve Castellanos said.
The disease was found on camellias at Monrovia Nursery and has proven especially harmful to coast live oaks, black oaks and tanoaks, all of which are found in the Southern California.
Descanso Gardens has purchased plants from Monrovia Nursery, Econome said, but postponed acquisitions 18 months ago when the garden’s search for a new director began. Still, the Gardens’ horticulturists are monitoring the 30 acres of oaks and 20 acres of camellias, Econome said.
Horticulturists are in communication with the state and county agricultural departments and get information regarding Sudden Oak Death and other plant diseases, Econome said. The Gardens also keeps a list of plants from certain locations known to have diseases and does not allow those plants to be brought to the Gardens.
“Our people are reviewing that list to make sure that it reflects new information that comes,” Econome said.
The discovery of this pathogen at Monrovia Nursery was an awakening for Descanso Gardens officials, Econome said. Oaks take up a large percentage of the Gardens’ cultivated lands and provide shelter for the camellias that grow below them. They are also somewhat intrinsic to the site’s landscaping.
“They kind of speak to who we are,” Econome said.
The fact that many of the city’s oaks are old and therefore more susceptible to infection is a cause for concern, Castellanos said. Sap tests have revealed some trees show the presence of funguses but not Sudden Oak Death, Castellanos said. The main problem facing city trees at the present is infestation of the bark beetle, Castellanos said.
The disease is spread via wind-blown rain and, in addition to oak and camellia, has been found on Douglas fir, rhododendron and California bay laurel.
According to the Oak Mortality Task Force, a coalition of research and educational institutions, public agencies, nonprofit organizations and private interests that seeks to address elevated levels of oak mortality in the state, the disease is hard to detect.
“Because other organisms and injuries can produce symptoms very similar to Sudden Oak Death on oaks, homeowners will not be able to diagnose their trees by themselves with absolute certainty,” the group’s Web site reads in part.
But when Sudden Oak Death infection does occur, it can be visible on the plant’s trunk, branches and leaves. It causes cankers to form on the woody portions of an infected tree. When cankers expand in the trunk and disrupt physiological functions, the tree dies. Diseased trees are often susceptible to other organisms, such as bark beetles and fungi that decay sapwood, according to the task force.