by Jeannette Warnert, ANR
University of California scientists recommend the public dispose of their Christmas trees quickly and properly after Christmas to help stem the spread of pitch canker, a disease that is now affecting Monterey pine Christmas tree lots, landscape plantings and native coastal forests in 16 California counties.
"People can buy healthy-looking trees that are infected with pitch canker. If they are left for a long period in the backyard, insects could visit and spread the disease to landscape pines," said Tom Gordon, UC Davis plant pathologist.
Infected Christmas trees have been found in San Mateo, Los Angeles, Orange and San Diego counties at choose-and-cut tree lots. After the holiday, Gordon suggests all Christmas trees be promptly turned over to community recycling programs, or chipped and spread as a thin mulch or composted.
Scientists' biggest fear is that pitch canker will spread to the Sierra Nevada. In research studies conducted at UC Davis, Gordon found that many trees native to Sierra Nevada forests are susceptible to the disease.
"These are ongoing studies," Gordon said. "Some trees are being tested now, and other trees are growing to a size where they can be tested."
However, his research has already determined that gray pine, coulter pine, Torrey pine, ponderosa pine, shore pine and Douglas-fir are susceptible to pitch canker when they are exposed to the disease in growth chambers.
To help prevent the spread of pitch canker to the Sierra Nevada, scientists recommend that no Monterey pine or other pine firewood, cones, logs and chipped pine material be transported from west of Interstate 5 to east of Interstate 5. The state has also designated a "Zone of Infestation," which counties may use to place restrictions on the movement of potentially infested materials.
Pitch canker, caused by the fungus Fusarium circinatum, was first discovered in the pine plantations of the Southeastern United States 60 years ago. The disease was not known before that anywhere in the world. In the Southeast, pitch canker was not much more than a curiosity until the mid 1970s, when it became a serious problem on slash pine.
In 1986, foresters found pitch canker in California. It quickly developed an association with native insects and spread to many native pines, including Monterey pine, Bishop pine and knobcone pine. Infected trees in landscape plantings and native forests are found in coastal areas from San Luis Obispo to Mendocino counties.
While young Monterey pines are bushy and can be shaped into a cone for Christmas, mature Monterey Pines grow 50 to 100 feet tall, are often multi-trunked and carry all their vegetation high in tufts.
Monterey pines are greatly prized for their esthetic value in the three California locations that comprise the only native populations of Monterey pine on mainland anywhere in the world. Stands are found at Año Nuevo in San Mateo County, on the Monterey Peninsula and near the quaint coastal town of Cambria.
"All those populations became infested with pitch canker and are still today," Gordon said. "Although the situation from a statewide perspective has more or less stabilized, on a local scale, it is becoming more severe."
Gordon and UC Berkeley entomologist David Wood have found some Monterey pine trees to be naturally resistant to pitch canker and some trees observed over many years have only a very limited amount of damage caused by the fungus. These trees are not expected to die from the disease unless new strains of the fungus are introduced into the state and are able to overcome the natural levels of resistance. The scientists believe pitch canker that where young trees are exposed to pitch canker, many of those that survive will be resistant.
For that reason, they say natural regeneration is the first choice for pitch-canker-infested forests. Monterey pine, Gordon said, regenerates best after fires. However, since prescribed burning is not an option for these highly populated areas, Gordon and his colleagues are conducting studies to determine whether mechanically removing or incorporating vegetation in the forests will improve Monterey pine regeneration.
At Asilomar State Beach and Conference Grounds and at Pebble Beach Golf Links, both on the Monterey Peninsula, UC scientists are working with landscape specialists to take a more proactive approach to Monterey pine regeneration.
"The landscapes are carefully managed at these facilities. The staff is actually screening seedlings as they grow up to see what looks resistant before they are planted," Gordon said. "We provided them with the methodology, gave them the inoculum and helped them get their programs established."