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The Destructive Tree Pathogen Phytophthora ramorum Originates from the Laurosilva Forests of East Asia

Dr. Thomas Jung, head scientific researcher of the Phytophthora Research Centre (PRC) at the Faculty of Forestry and Wood Technology of Mendel University in Brno, Czech Republic, led the work of an international team of experts into discoveries that will deeply impact phytosanitary measures currently in place in the European Union (EU) and worldwide for regulating global plant trade. These findings were recently published:

Jung, T., Horta Jung, M., Webber, J.F., Kageyama, K., Hieno, A., Masuya, H., Uematsu, S., Pérez-Sierra, A., Harris, A.R., Forster, J., Rees, H., Scanu, B., Patra, S., Kudláček, T., Janoušek, J., Corcobado, T., Milenković, I., Nagy, Z., Csorba, I., Bakonyi, J., Brasier, C.M. (2021).
The destructive tree pathogen Phytophthora ramorum originates from the laurosilva forests of East Asia. The Journal of Fungi, 7, 226. DOI: 10.3390/jof7030226

Phytophthora ramorum is an aerial oomycete pathogen with a wide host range in its introduced state. Since ca 1990, it has caused the deaths of millions of native oaks and tanoaks in western North America (“sudden oak death”) and later of plantation grown larch (“sudden larch death”) in the UK and other European countries. Phytophthora ramorum is included in the EPPO A2 List of pests recommended for regulation as quarantine pests and is considered by the EU as an “harmful organism” and managed as such since 2002 (2002/757/EC); this microorganism quarantine status is also in place in the USA and many other countries worldwide.
The main conclusions of the investigations led by PRC are:
– Based on extensive phenotypic and phylogenetic studies, 8 new lineages were found in Vietnam and Japan, in addition to the already known 4 invasive clonal lineages of P. ramorum.
– Several lines of evidence indicate that these East Asian P. ramorum populations are native and that northern Indochina and south west Japan lie within the centre of origin of P. ramorum:
1. In contrast to the introduced P. ramorum lineages in Europe and North America, the east Asian populations are very diverse. Despite the small, highly localised samples, multiple phenotypically and phylogenetically different lineages were present in both areas.
2. Unlike the introduced clonal lineages, the A1 and A2 mating types co-occur in these Asian populations (indicating possible sexual reproduction).
3. Despite the presence of many potential plant hosts, no obvious host symptoms were observed in any sampled area, consistent with the view that native Phytophthoras cause limited damage to their co-evolved host plants. Nonetheless, P. ramorum was isolated readily from detached leaves and flowers that had fallen into forest streams or onto the forest floor.
4. At some sites, two of P. ramorum’s closest phylogenetic relatives were also isolated, namely P. foliorum and P. lateralis (which, like P. ramorum, are damaging introduced pathogens in Europe and North America).
These findings emphasise the importance of conducting further surveys in underexplored natural ecosystems on a global scale followed by extensive pathogenicity and host range testing in order to analyse the risk posed by unknown exotic Phytophthoras to the worlds forests and improve global plant biosecurity.