International trade in Monterey cypresses caused a pandemic

Massive die-offs of Italian cypresses from Portugal to Greece have been linked to California’s Monterey cypresses (Cupressus macrocarpa), solving a long-standing puzzle in the world of plant pathology and surprising researchers. “When I was a student, this was one of the big mysteries”, says study co-author Matteo Garbelotto of the University of California, Berkeley. “The cypress canker epidemic was first seen in California in the 1920s – 100% of the Monterey cypresses in several San Joaquin Valley sites suddenly died – so the most credible theory was that the pathogen was exotic here”, he explains.

Identified as the fungus Seiridium cardinale early on, the pathogen has since hit Europe, Africa, Asia, Australia, New Zealand, and South America, killing up to 95% of native cypresses, cedars, and junipers locally. Genetic analysis of S cardinale from the Mediterranean, the Southern Hemisphere, and California pointed to the last as the source. Whereas the Mediterranean samples were clones, the California samples were genetically diverse. Moreover, there are two distinct genetic variants of the fungus in California, but only the first was found in the Mediterranean and only the second was detected in the Southern Hemisphere in this study, which appeared in September in Phytopathology (2011; doi:10.1094/PDIS-01-11-0052).

Garbelotto attributes the initial California epidemic to the fact that the Monterey cypresses were planted “out of place” – not in their cool, coastal native range, but in the hot interior of the state. “We know they don’t belong there”, Garbelotto points out. Indeed, S cardinale is largely benign in cool areas but causes cankers in hot areas, which is where Italy’s native cypresses grow best. Monterey cypresses were popular in Italy beginning in the 1920s, and eyewitness accounts put the start of the canker epidemic there in the 1930s.

Canker-resistant cypresses have now been developed in Europe. However, even these would be vulnerable to California’s second genetic variant, making testing cypress imports a priority. “Legally, they can’t be regulated because the pathogen species is in both continents – the law doesn’t account for genetic variants”, Garbelotto explains. “But before we didn’t know what to look for, and now we do.”

Frontiers in Ecology and the Environment, 2011