Scientists suggest European-tree-killing pathogen endemic to California An international team of scientists has potentially unearthed the source of the fungus that has been killing trees in six continents. Published in the journal Phytopathology, the study reveals how the state of California in the United States is the leading suspect for hosting ... An international team of scientists has potentially unearthed the source of the fungus that has been killing trees in six continents. Published in the journal Phytopathology, the study reveals how the state of California in the United States is the leading suspect for hosting the pathogen Seiridium cardinale - the culprit behind cypress canker disease. Researchers first identified S. cardinale in the San Joaquin Valley in 1928. This fungus then travelled from the United States to Africa, Asia, Australia, Europe, New Zealand and South America. Some regions have reported that up to 95 % of their native trees (part of the cypress family), including cedars and Junipers, have been infected or even killed by this disease. '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,' explains Professor Matteo Garbelotto of the University of California, Berkeley in the United States. The fungus moves in on the tree and kills it by producing toxins that destroy the tree's sap flow and stop it from obtaining water and nutrients. Southern Europe in particular has been affected by this disease. Says lead author Gianni Della Rocca of the Institute for Plant Protection of the National Research Council (IPP-CNR) in Florence: '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.' While researchers believed an exotic pathogen was responsible for both the appearance and destructiveness of the disease in European countries, the question of its origin remained a mystery. In this latest study, Professors Della Rocca and Garbelotto and colleagues used modern deoxyribonucleic acid (DNA) fingerprinting techniques to evaluate 96 S. cardinale isolates of diseased tree samples from 8 California counties, Chile, 7 Mediterranean countries and New Zealand. S. cardinale has the capacity to reproduce asexually: it either creates genetically identical clones of itself, or sexually when a different variant is present for mating, the researchers say. The team selected California as the leading suspect because it hosts populations of the pathogen that are genetically diverse. This, say the researchers, indicates that the pathogen is endemic to the region. The diversity is probably due to the sexual reproduction of two genetic variants of the pathogen found in California. Their work also led them to the discovery that one of the two variants of S. cardinale endemic to California is responsible for the epidemic of cypress canker in the Mediterranean. This hints that the fungi there all descended from a 'founder' genotype that travelled to Europe, according to the researchers. They also suggest that the second variant is linked to the epidemic in countries south of the Equator. While it is not 100 % clear as to how the pathogen left California and travelled abroad, the team says that people helped the pathogen move. The scientists believe genetic screening of plants can help stop the disease from spreading. 'We can develop tests to screen for the presence of S. cardinale on plants that are traded, and even to test for the presence of strains that are currently not present in Europe or in the southern hemisphere,' Professor Garbelotto says. 'Technological advancements of the last few years allow for the easy development of such tests, but it was essential to figure out the source of the pathogen in order to know what to look for.' Noting how European researchers for years have been working on developing cypress trees resistant to the current variant of S. cardinale, Professor Garbelotto says: 'There is no assurance those resistant trees will be resistant to other strains from California, if they are introduced. If another strain were to be introduced into the Mediterranean or in the southern hemisphere, this could accelerate and worsen the epidemic in those regions. It could nullify a 30-year long effort to develop trees that are resistant to the pathogen. It is imperative to stop the movement of infected plants in order to avoid further introductions that could completely overcome those lines of resistant trees.'For more information, please visit: Phytopathology: http://apsjournals.apsnet.org/loi/phyto Institute for Plant Protection of the National Research Council (IPP-CNR) in Florence: http://www.ipp.cnr.it/en/home.html Countries Italy, United States
