Virulence of Hymenoscyphus albidus and H. fraxineus on Fraxinus excelsior and F. pennsylvanica

Abstract

European ash (Fraxinus excelsior) is currently battling an onslaught of ash dieback, a disease emerging in the greater part of its native area, brought about by the introduction of the ascomycete Hymenoscyphus fraxineus (= Hymenoscyphus pseudoalbidus). The closely-related fungus Hymenoscyphus albidus, which is indigenous to Europe, is non-pathogenic when in contact with F. excelsior, but could pose a potential risk to exotic Fraxinus species. The North American green ash (Fraxinus pennsylvanica) is planted widely throughout Europe and regenerates naturally within this environment but little is known about the susceptibility of this species to ash dieback. We performed wound inoculations with both fungi (nine strains of H. fraxineus and three strains of H. albidus) on rachises and stems of F. excelsior and F. pennsylvanica under field conditions in Southern Poland. Necrosis formation was evaluated after two months on the rachises and after 12 months on the stems. After inoculation of H. albidus, only small lesions (of up to 1.3 cm in length) developed on the F. excelsior and F. pennsylvanica rachises, but with no significant distinction from the controls. Hymenoscyphus albidus did not cause necrotic lesions on the stems of either Fraxinus species. In contrast, H. fraxineus induced necroses on all inoculated rachises of both ash species with mean lengths of 8.4 cm (F. excelsior) and 1.9 cm (F. pennsylvanica). Necroses also developed on all of the inoculated F. excelsior stems (mean length 18.0 cm), whereas on F. pennsylvanica such lesions only occurred on about 5% of the stems (mean length 1.9 cm). The differences between strains were negligible. No necroses were observed on the control plants. Reisolations of H. albidus were only successful in around 8–11% of the cases, while H. fraxineus was reisolated from 50–70% of the inoculated organs showing necrotic lesions. None of the Hymenoscyphus species were isolated from the control plants. Our data confirm H. fraxineus’ high virulence with regards to F. excelsior and demonstrate a low virulence in relation to F. pennsylvanica under field conditions in Poland. Hymenoscyphus albidus did not express any perceivable pathogenicity on both host species.

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

“…All tested species except F. profunda developed relatively long lesions in the direct stem inoculation experiment (short in F. profunda), supported pathogen sporulation and showed generally high levels of pathogen DNA in leaf tissues. Other studies have similarly reported moderate susceptibility to ash dieback in species from section Melioides (Drenkhan and Hanso 2010;Gross and Sieber 2016;Kowalski et al 2015). The moderate disease susceptibility of North American ash species provides further support for the presence of an ancient phylogenetic signal in terms of pathogen host range.…”

“…Based on observations from Estonian parks, F. nigra seems to be highly susceptible to shoot infection, while F. americana and F. pennsylvanica are more tolerant (Drenkhan and Hanso 2010). Inoculation studies have further shown that F. pennsylvanica develops minor lesions after stem inoculation with H. fraxineus (Gross and Sieber 2016;Kowalski et al 2015). North American Fraxinus species are already severely threatened by the introduced emerald ash borer (Kovacs et al 2010).…”

A versatile method for assessing pathogenicity of Hymenoscyphus fraxineus to ash foliage

“…Both measures of symptom development were highly correlated with in vitro growth rate (Table 1b, Figure 1b). The present assay on seedlings took 28 days, which is considerably faster than previously reported experiments on this pathosystem (Kowalski et al, 2015;Schwanda & Kirisits, 2016). The overall preparation time for the assay was therefore reduced from about 6 weeks to about 7 days.…”

“…The method avoids the use of ash wood sticks, which requires availability of a suitable wood source and time to make, autoclave and inoculate the sticks. It also allows statisti- (Kowalski et al, 2015). The present assay on seedlings took 28 days, which is considerably faster than previously reported experiments on this pathosystem (Kowalski et al, 2015;Schwanda & Kirisits, 2016).…”

Pathogenicity of fungi associated with ash dieback towardsFraxinus excelsior

“…The ability of the fungus to produce necrotic lesions after artificial stem wound inoculation has been demonstrated on F. angustifolia, F. excelsior, F. mandshurica var. japonica and F. pennsylvanica (Bakys et al, 2009;Kirisits et al, 2010;Husson et al, 2011;Gross & Holdenrieder, 2015;Kowalski et al, 2015;Gross & Sieber, 2016). The present study has confirmed the high pathogenicity of H. fraxineus towards F. excelsior.…”