Virulence of the invasive ash pathogen Hymenoscyphus fraxineus in old and recently established populations

Abstract

Following its introduction from Asia in the 1990s, the ascomycete Hymenoscyphus fraxineus has caused a severe dieback of Fraxinus excelsior in Europe. In this study, the virulence of 200 H. fraxineus isolates were assessed and compared. These isolates equally represented (i) two geographically distant populations with a different disease history (Switzerland, recently established populations at the epidemic front versus Lithuania, old established populations), and (ii) isolates from two different types of host tissue (necrotic bark lesions as dead‐end tissue versus fallen leaf petioles as primary host tissue). Inoculations conducted on 3‐year‐old F. excelsior seedlings showed that the vast majority of the isolates (98%) were able to induce necrotic bark lesions after 10 months. Although a high variation in virulence was observed among isolates, no significant differences were detected between the older and the epidemic‐front populations. Decline in virulence of populations of invasive organisms is generally assumed with increasing age of epidemics. However, this does not appear to hold true for H. fraxineus. Either the Lithuanian population is still too young (15 to 20 years old) to show a decline in virulence, or the size of the host population may still not be critical for pathogen survival. Given that bark lesions represent an epidemiological dead end and do not benefit the survival of H. fraxineus, a trend towards reduced ‘bark virulence’ of isolates originating from leaf petioles compared to isolates from the bark lesions was expected. However, such a trend was observed neither in old, nor in recently established populations.

Variation in aggressiveness of Hymenoscyphus fraxineus genotypes amid the ash dieback epidemic

“…Differences in aggressiveness among strains of H . fraxineus have been observed in previous studies (Junker et al , ; Kowalski et al , ; Gross and Sieber, ; Schwanda and Kirisits, ; Lygis et al , ; Orton et al , ), but in all cases, the tested isolates were obtained from lesions, thus omitting potential nonaggressive genotypes. In addition, these studies dealt with isolates of different ages from the time of initial spore germination, where variation in age and cultivation may have generated variation in aggressiveness.…”

“…It is encouraging that we observed genotypes of the ash dieback pathogen with low aggressiveness. Nevertheless, Lygis et al () compared isolates of H . fraxineus obtained from fallen petioles and necroses of infected trees at the forefront‐ and post‐epidemic populations, but found no evidence of decreasing aggressiveness in isolates from the post‐epidemic populations.…”

Population genetic analysis of a parasitic mycovirus to infer the invasion history of its fungal host

“…Data of a recently conducted virulence testing of Swiss and Lithuanian H. fraxineus isolates on wound‐inoculated Fraxinus excelsior seedlings (Lygis et al . ) were used to test for a relationship between HfMV1 group and fungal virulence. We used mean lesion length produced by HfMV1‐infected isolates as a measure of virulence.…”

An overview of ash ( Fraxinus spp.) and the ash dieback disease in Europe.

“…Population studies also indicate that the fungus is outcrossing and high-genotypic diversity was detected within the European populations [26,29,34,35,42]. These results may explain part of the variations in virulence, growth rates, temperature optima for growth and exoenzyme profiles, which have been observed among isolates [47][48][49].…”