Genetic detective work by an international group of researchers may have solved a decades-long mystery of the source of a devastating tree-killing fungus（真菌） that has hit six of the world's seven continents. In a study published Sept. 1 in the peer-reviewed journal Phytopathology, California emerged as the top suspect for the pathogen Seiridium cardinale（基数，红衣主教） that is the cause of cypress canker disease.

It was in California's San Joaquin Valley in 1928 that S. cardinale was first identified as the culprit causing the disease. The fungus has made its way since to Europe, Asia, New Zealand, Australia, South America and Africa. In many regions, the pathogen has infected and killed up to 95 percent of native trees in the cypress（柏树） family, including junipers（杜松） and some cedars.

"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," said Matteo Garbelotto, adjunct associate professor and cooperative extension specialist in ecosystem sciences at the University of California, Berkeley.

The fungus kills a tree by entering through cracks in its bark, producing toxins that wreak havoc with its flow of sap and choke off its supply of water and nutrients. The disease has left an indelible（难忘的） mark throughout Southern Europe.

"Italian cypress trees are important to the ecosystem, but they are also considered the quintessential（精髓的） 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," said study lead author Gianni Della Rocca, researcher at the National Research Council in Florence, Italy. "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."