Hymenoscyphus fraxineus is a leaf pathogen of local Fraxinus species in the Russian Far East

Abstract

Dieback of European ash was first observed in Europe in the early 1990s. The disease is caused by the invasive ascomycete Hymenoscyphus fraxineus, proposed to originate from Far East Asia, where it has been considered a harmless saprotroph. This study investigates the occurrence of H. fraxineus in tissues of local ash species in the Russian Far East, and assesses its population‐specific genetic variation by ITS sequencing. Shoot dieback symptoms, characteristic of H. fraxineus infection on European ash, were common, but not abundant, on Fraxinus mandshurica and Fraxinus rhynchophylla trees in Far East Russia. High levels of pathogen DNA were associated with necrotic leaf tissues of these ash species, indicating that the local H. fraxineus population is pathogenic to their leaves. However, the low levels of H. fraxineus DNA detected in shoots with symptoms, the failure to isolate this fungus from such tissues, and the presence of other fungi with pathogenic potential in shoots with symptoms indicate that local H. fraxineus strains may not be responsible (or their role is negligible) for the observed ash shoot dieback symptoms in the region. Conspicuous differences in ITS rDNA sequences detected between H. fraxineus isolates from Russian Far East and European populations suggest that the current ash dieback epidemic in Europe might not directly originate from the Russian Far East. Revision of the herbarium material shows that the earliest specimen of H. fraxineus was collected in 1962 from the Russian Far East and the oldest H. fraxineus specimen of China was collected in 2004.

The susceptibility of Asian, European and North American Fraxinus species to the ash dieback pathogen Hymenoscyphus fraxineus reflects their phylogenetic history

“…In contrast, the low DNA levels of H. fraxineus recorded in symptomatic shoot tissues of F. mandshurica in Russia Far East suggest that the fungus is not a shoot pathogen on this tree. Our results concerning F. mandshurica are in agreement with Drenkhan et al (2016). When grafts of the different species were exposed to high infection pressure by H. fraxineus ascospores in a moist spore chamber, leaves of F. mandshurica exhibited high densities of pathogen DNA and developed brown lesions in leaflet midribs and veins and leaf petioles, symptoms characteristic of leaf infection on European ash (Cleary et al 2013b).…”

“…Collectively, the previous study and our data indicate that when the infection pressure is high, H. fraxineus is able to establish a necrotrophic feeding mode in the leaves of F. mandshurica. The low level of shoot symptoms in arboretum grown F. mandshurica together with the previous observations of low levels of H. fraxineus DNA in necrotic bark lesions in F. mandshurica from Far East Russia (Drenkhan et al 2016) indicates that the tree species is fairly resistant to shoot infection by H. fraxineus and that H. fraxineus is a facultative leaf pathogen of F. mandshurica.…”

“…In another recent study, significantly higher levels of H. fraxineus DNA were July 1st, 3 July 15th, 4 July 29th 2014. Coloration: No color\1 pg/ul, very light blue 1-2.7 pg/ul, light blue 2.7-7.4 pg/ul, blue 7-20.1 pg/ ul, dark blue 20.1-54.6 pg/ul, very dark blue [55 pg/ul detected in necrotic leaf veins compared to asymptomatic leaves of F. mandshurica from Russia Far East suggesting that the leaf symptoms were indeed caused by the pathogen (Drenkhan et al 2016). In contrast, the low DNA levels of H. fraxineus recorded in symptomatic shoot tissues of F. mandshurica in Russia Far East suggest that the fungus is not a shoot pathogen on this tree.…”

“…Both in Korea (Han et al 2014) and in Russia Far East (Drenkhan et al 2016) apothecia of H. fraxineus have been recorded on fallen leaves of F. chinensis subsp. rhyncophylla, indicating that some additional ash species can serve as hosts for H. fraxineus in Asia.…”

Population structure of the ash dieback pathogen, Hymenoscyphus fraxineus, in relation to its mode of arrival in the UK

“…Hymenoscyphus fraxineus is thought to be native to East Asia. It has been identified from indigenous ash species in Japan (Zhao et al ., ), Korea (Han et al ., ), China (Zheng & Zhuang, ) and the far east of the Russian Federation (Drenkhan et al ., ) but the source of the genotypes causing the outbreak in Europe is unknown. However, it is clear that East Asian populations are strongly differentiated from those in Europe and have much greater genetic diversity, as shown by simple sequence repeat (SSR; microsatellite) markers (Zhao et al ., ; Gross et al ., ; Cleary et al ., ), rDNA ITS sequences (Drenkhan et al ., ) and genome sequencing (Yoshida et al ., ).…”