Abstract
As global plant trade expands, tree disease epidemics caused by pathogen introductions are increasing. Since ca 2000, the introduced oomycete Phytophthora ramorum has caused devastating epidemics in Europe and North America, spreading as four ancient clonal lineages, each of a single mating type, suggesting different geographical origins. We surveyed laurosilva forests for P. ramorum around Fansipan mountain on the Vietnam-China border and on Shikoku and Kyushu islands, southwest Japan. The surveys yielded 71 P. ramorum isolates which we assigned to eight new lineages, IC1 to IC5 from Vietnam and NP1 to NP3 from Japan, based on differences in colony characteristics, gene x environment responses and multigene phylogeny. Molecular phylogenetic trees and networks revealed the eight Asian lineages were dispersed across the topology of the introduced European and North American lineages. The deepest node within P. ramorum, the divergence of lineages NP1 and NP2, was estimated at 0.5 to 1.6 Myr. The Asian lineages were each of a single mating type, and at some locations, lineages of “opposite” mating type were present, suggesting opportunities for inter-lineage recombination. Based on the high level of phenotypic and phylogenetic diversity in the sample populations, the coalescence results and the absence of overt host symptoms, we conclude that P. ramorum comprises many anciently divergent lineages native to the laurosilva forests between eastern Indochina and Japan.
Highlights
Outbreaks of destructive tree diseases caused by introduced pathogens are increasing due to the growing international trade in plants [1,2,3,4]
We obtained 505 Phytophthora cultures from the 11 stream and river catchments sampled on the northern Vietnam–Yunnan border within diverse Fagaceae, Ericaceae and Lauraceae forest communities at 1200–2100 m above sea level, 64 of which from seven Fansipan streams and the stream at Sau Chua Mountain belonged to a sexually self-sterile Phytophthora later identified as P. ramorum based on morphological characteristics and ITS rDNA sequences (Table S1; Figures 1 and 2A–C)
We obtained 505 Phytophthora cultures from the 11 stream and river catchments sampled on the northern Vietnam–Yunnan border within diverse Fagaceae, Ericaceae and Lauraceae forest communities at 1200–2100 m above sea level, 64 of which from seven Fansipan streams and the stream at Sau Chua Mountain belonged to a sexually self-sterile Phytophthora later identified as P. ramorum based on morphological characteris7toicf 3s2and ITS rDNA sequences (Table S1; Figures 1 and 2A–C)
Summary
Outbreaks of destructive tree diseases caused by introduced pathogens are increasing due to the growing international trade in plants [1,2,3,4]. The origins of the causal agents are often obscure, probably because they cause little harm in their natural habitats, only becoming seriously damaging when introduced into a biogeographic zone containing similar host genera with limited genetic resistance and an environment that favours pathogen establishment and spread. The behaviour and population structure of introduced pathogens are often biological artefacts shaped by founder effects and episodic selection on the over-susceptible hosts [5,6]. A deeper understanding of their natural behaviour including their breeding systems, potential for adaptation and buffering by natural antagonists should come from studying them in their areas of origin. When a forest pathogen causes little observable damage in its native habitats, identifying its geographic origin can be logistically challenging
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