Pathogen native to U.S. but had not infected pines until recently
Fungal pathogens that cause die-back in grape, avocado, citrus, nut and other crops has found a new host and is infecting conifer trees causing pine ghost canker in urban forest areas of Southern California.
The canker can be deadly to trees.
Scientists from University of California, Davis, first spotted evidence that the pathogens had moved to pines during a routine examination of trees in Orange County. Over four years, they found that more than 30 mature pines had been infected in an area of nearly 100 acres, according to a report in the journal Plant Disease.
Akif Eskalen, a professor of Cooperative Extension in the Department of Plant Pathology at UC Davis, suspects drought and other stress conditions brought on by climate change weakened the tree species, making it more susceptible to new threats.
“We have been seeing this on pine trees for the last several years,” he said. “Our common crop pathogens are finding new hosts.”
Pine ghost canker – caused by the fungal pathogens Neofusicoccum mediterraneum and Neofusicoccum parvum – usually infects the lower part of a tree's canopy, killing branches before moving on to the trunks. This dieback in some cases can be deadly.
Points of entry
The pathogens infect a tree by entering through wounds caused by either insects such as red-haired pine bark beetles or pruning – meaning trees in managed or landscaped areas could be at risk. Another route is via tiny natural openings known as lenticels that fungi can make their way through, said Marcelo Bustamante, a Ph.D. candidate in Eskalen's lab who is first author on the paper.
Spores from the fungi can disperse and the higher the prevalence means an increased chance of transmission. Rain, irrigation water and humidity by fog can trigger the right circumstances for the spores to spread, he said.
“The detection of these pathogens in urban forests raises concerns of potential spillover events to other forest and agricultural hosts in Southern California,” Bustamante and others wrote in the report.
Dead branches can indicate a canker. Detecting the fungi is not an emergency but “people should keep an eye on their plants when they see abnormalities,” Eskalen said.
Cankers are localized areas on stems, branches and tree trunks that are usually dead, discolored and sunken. On bark, the spores can look like strings of discolored dots.
The lab has posted a brochure bout how to best manage wood canker diseases.
Tips include:
* Keep your trees healthy: Proper irrigation and maintenance will keep trees strong.
* Prune dead branches to reduce sources of infestation.
Karina Elfar, Molly Arreguin, Carissa Chiang, Samuel Wells and Karen Alarcon from the Department of Plant Pathology contributed to the paper, as did experts from Disneyland Resort Horticulture Department, State University of New York's College of Environmental Science and Forestry, UC Irvine and UC Los Angeles.
Dry, Hot Climate Drives Blister Rust Disease Upslope, Threatening White Pines
Quick Summary
Climate change and drought are shifting how and where infectious plant disease moves in a forest.
Even pathogens have their limits. When it gets too hot or too dry, some pathogens — like many living things — search for cooler, wetter and more hospitable climes. Ecologists have questioned if a warming, drying climate is connected to the spread of plant disease, but detecting a climate change fingerprint has been elusive.
A study from the University of California, Davis, provides some of the first evidence that climate change and drought are shifting the range of infectious disease in forests suffering from white pine blister rust disease.
“Because pathogens have thermal tolerances, we are seeing expansions and contractions in this disease's range,” said lead author Joan Dudney, a Davis H. Smith postdoctoral fellow at UC Davis in the lab of Professor Andrew Latimer, a study co-author. “Climate change isn't so much leading to widespread increases in this disease but rather shifting where it is emerging.”
The study, published today in the journal Nature Communications, found that white pine blister rust disease expanded its range into higher-elevation forests in the southern Sierra Nevada between 1996 and 2016. At the same time, it also contracted its range in lower elevations, where conditions were often too hot and dry for its survival.
“Our study clearly demonstrates that infectious plant diseases are moving upslope, and they're moving fast,” Dudney said. “Few pines are resistant to what is basically a Northern Hemisphere white pine pandemic.”
White pine blister rust disease is caused by a pathogen, Cronartium ribicola, and it has led to a major decline of white pine species throughout the U.S., including whitebark pine, which is in the process of being listed as a threatened species. The study suggests that whitebark pine and many other high-elevation pine species may become increasingly imperiled under climate change.
Expanding and contracting
To collect the data, scientists spent five years resurveying long-term monitoring plots in the remote wilderness of Sequoia and Kings Canyon national parks, measuring stable isotope signatures in pine needles and collecting observations for over 7,800 individual host trees. The data includes two surveys that were about 20 years apart. What resulted is one of the first clear measurements of an infectious plant disease range shift into higher elevations.
They found that the optimal climate for blister rust moved into higher elevations between 1996 and 2016 — a warmer, hotter period than the previous two decades. Climate change decreased the prevalence of blister rust disease by 5.5% in arid, lower elevations and increased its prevalence nearly 7% in colder upper elevations. This amounted to an area expansion of about 200,000 acres, which exposed the majority of hosts in Sequoia and Kings Canyon national parks.
Though infection risk increased in the parks, the overall prevalence of the disease declined in the area. That surprising result is partly because many of the infected trees in the lower elevations died between surveys, and it became too warm and dry for new infections to develop there. Meanwhile, the secondary hosts the pathogen requires — such as currant and gooseberry plants — are not abundant at higher elevations, although that could change as the climate warms.
An evolutionary race
For white pines, the forests above Sequoia and Kings Canyon national parks have long served as a small refuge from white pine blister rust, but the projected expansion of the disease under climate change threatens that refuge, the study suggests.
The authors said that white pines in the study area's upper elevations are “disease-naïve.” The same harsh conditions they adapted to also restricted most diseases and pests. Climate change is shifting those constraints quickly, leaving the trees more vulnerable.
“It's kind of a race between evolution and climate change,” Latimer said. “So far, climate change is winning.”
While the white pine outlook appears grim, Dudney said being proactive about disease prevention could help slow the spread and detect new invasions.
“Once they've experienced an epidemic, we have little recourse but to triage the area,” Dudney said.
Additional study co-authors include Claire Willing of UC Berkeley and Stanford, Adrian Das of U.S. Geological Survey, Jonathan Nesmith of Sierra Nevada Network Inventory and Monitoring Program, and John J. Battles of UC Berkeley.
The study was funded by the U.S. Forest Service and National Park Service.
Planting Genetic Resilience Into Forests in the Face of Climate Change
California's drought and bark-beetle infestation killed more than 129 million trees between 2012 and 2016 in the Sierra Nevada. But amid the devastation stood some survivors.
At the time, UC Davis biologist Patricia Maloney and a team of researchers entered the forest to collect seeds from 100 surviving sugar pine trees. Alongside other parched sugar pines etched with the tell-tale tunnel marks of bark beetles, were green, healthy trees. The researchers spent the past two years raising 10,000 seedlings from 100 surviving mother trees around the Lake Tahoe Basin. They were first cultivated at the USDA Forest Service's Placerville Nursery and then moved to the UC Davis Tahoe City Field Station.
This week, between 4,000 and 5,000 of the seedlings are being planted around Lake Tahoe's North Shore as part of a restoration project funded by the Tahoe Fund and the California Tahoe Conservancy. About 1,500 will be used to study and identify important adaptive traits, and the remainder will be given to private landowners to plant.
Survivors matter
If the seedlings turn out to be as genetically resilient as Maloney thinks and hopes they will be, these trees could represent the future forest, one better able to withstand the threats of climate change, including more droughts and bark beetle outbreaks.
“These survivors matter,” said Maloney, a scientist in the UC Davis Department of Plant Pathology and Tahoe Environmental Research Center. “Essentially, these are the offspring of drought survivors. This is hopefully the genetic stock of the future.”
Maloney is working with a crew of volunteers from the California Conservation Corps to plant the seedlings around the Tahoe Basin in micro-climates and habitats conducive to their growth and survival, just in time for the coming rain and snow season. After all, they will no longer be hand watered by attentive scientists in a nursery.
“This project not only fosters restorative growth in California's forests, it also plants seeds of forestry awareness and experience in our Corpsmembers that can lead them to careers in forest conservation,” said Bruce Saito, director of the California Conservation Corps.
Planting genetic diversity
While the project is bringing more trees to the forest, it also aims to promote more genetic diversity within the trees.
Sugar pine trees with their giant, foot-long cones, once covered a quarter of Lake Tahoe's forests. Comstock Era logging removed all but 5 percent of them, removing a significant share of their genetic diversity at the same time, Maloney's lab found. Over the years, white pine blister rust and bark beetles have impacted those that remain. By restoring native sugar pines' genetic diversity, Maloney thinks the overall forest will benefit.
It's a small-scale start, but Maloney is excited to track each mother tree to see how it survives.
“Forest tree species have a large capacity for gene flow; they can move long distances,” Maloney said. “Our native tree species have the potential to change.”
Media contact(s)
Patricia Maloney, UC Davis Tahoe Environmental Research Center, 775-881-7569, pemaloney@ucdavis.edu
Kat Kerlin, UC Davis News and Media Relations, 530-752-7704, kekerlin@ucdavis.edu