Devastating Tree-Killing Pathogen Traced to California

A new study by UC Berkeley and Italian researchers may have solved a decades-long mystery behind the source of a tree-killing fungus that affected six of the world's seven continents.

Genetic sleuthing by an international team of researchers has fingered California as the source of the pathogen, Seiridium cardinale, which is the cause of cypress canker disease and has killed as much as 95 percent of native trees in the cypress family, including junipers and some cedars.

The findings were published Thursday in the journal Phytopathology.

“The fungus was likely introduced from California either in the South of France or in Central Italy 60 to 80 years ago, and that introduction resulted in a global pandemic that has devastated the region’s iconic Italian cypress trees,” Matteo Garbelotto, adjunct associate professor and cooperative extension specialist in ecosystem sciences at the University of California, Berkeley, said in a university release.

S. cardinale was first identified in California's San Joaquin Valley in 1928 as the culprit causing the disease. It has lived and thrived in California for a long time and made its way since to Europe, Asia, New Zealand, Australia, South America and Africa.

The fungus attacks trees in the cypress family by entering through cracks in their bark and producing toxins that wreak havoc with the flow of sap, choking off the supply of water and nutrients. The disease has left an indelible mark throughout Southern Europe.

“Italian cypress trees are important to the ecosystem, but they are also considered the quintessential trees of the Mediterranean, the ones that dot the Tuscan countryside and that form the landscape of much of Greece, the South of France and Spain. It is difficult to put a price tag on the impact this pathogen has had. It’s hard to imagine the Tuscan or Provence landscape without cypresses,” said study lead author Gianni Della Rocca, researcher at the National Research Council in Florence, Italy.

The researchers used modern DNA fingerprinting techniques to analyze diseased tree samples from seven Mediterranean countries, eight California counties, Chile and New Zealand.

California emerged as a likely culprit because it hosts populations of the pathogen that are genetically diverse, a strong sign that the pathogen is endemic to the region, the researchers said.

S. cardinale is capable of reproducing asexually by creating genetically identical clones of itself, or sexually when a different variant is available for mating. The authors of the study attribute the diversity to the likely sexual reproduction of two genetic variants of the pathogen found in California.

The researchers found that one of the two variants of S. cardinale endemic to California is responsible for the epidemic of cypress canker in the Mediterranean, saying the fungi there all descended from a founder genotype that made its way to Europe.

The second variant found in California, incidentally, has been linked to the epidemic in countries in the Southern Hemisphere such as New Zealand and Chile.

The researchers said how the pathogen moved from California outward is not yet clear. But it is certain that humans helped the pathogen along in its journey, since air and sea currents alone could not account for the discovery of identical genotypes thousands of miles apart.

The researchers looked through historical catalogues of large commercial nurseries in Italy and France and found records of mature Monterey cypress trees for sale during the late 1920s and 1930s. The records indicated significant imports of the California trees and their seeds at that time.

“It is very likely that the pathogen was introduced during that period,” said Danti, who added that interviews the research team conducted with people who worked in the nurseries then suggested that cypress canker disease was becoming a problem by the late 1930s.

“It could have easily taken 10 to 20 years from the time of introduction for the first major outbreak to occur. In Italy, the pathogen was first identified in 1951, so it could have arrived decades earlier,” Garbelotto said.