Hymenoscyphus pseudoalbidus and Hymenoscyphus albidus: viridiol concentration and virulence do not correlate

Abstract

Hymenoscyphus pseudoalbidus is the causal agent of ash dieback, a disease that is presently endangering Fraxinus spp. throughout most of Europe. The phytotoxin, viridiol, was previously isolated from culture extracts of H. pseudoalbidus and found to be toxic to leaves of F. excelsior. Thus, we were interested in learning to what extent viridiol is responsible for pathogenicity of H. pseudoalbidus and investigated this using twelve isolates of H. pseudoalbidus. We also included five isolates of the closely related avirulent species, Hymenoscyphus albidus, in our studies. Some, but not all, isolates of H. pseudoalbidus and H. albidus produced measurable quantities of viridiol in culture. Three tests were used to determine to what extent viridiol concentration correlates with virulence: culture extracts were tested for activity in leaf segment tests and for inhibition of germination of seedlings of Fraxinus excelsior; virulence of the isolates was tested following infection of axenically cultured ash seedlings. Activity of the culture extracts varied, as did virulence of the isolates following inoculation into seedlings. No correlations were found between viridiol concentration and activities of culture extracts in leaf segment tests or in the germination test, nor between viridiol concentration and disease symptoms when inoculated into seedlings. However, activities of culture extracts in leaf segment and in the germination test correlated, as did the results of each of these tests with virulence in the infection experiment. Apparently, as yet unidentified factors other than the concentration of viridiol play important roles in the virulence of H. pseudoalbidus.

Hymenosetin, a 3-decalinoyltetramic acid antibiotic from cultures of the ash dieback pathogen, Hymenoscyphus pseudoalbidus

“…As hymenosetin (2) has thus far only been obtained from laboratory cultures, its role in the pathogenesis of ash dieback remains unclear. The fact that production rates of this new compound in the three tested strains correlate with virulence of the isolates (Junker et al, 2013), suggests that the metabolite might constitute an important, hitherto unknown, pathogenicity factor. In that case, the tests for phytotoxicity used were not the correct ones to the function of hymenosetin (2) in vivo.…”

“…The origin of these three strains as well as their rDNA ITS GenBank accession numbers are listed in Table 1. This information as well as the viridiol concentration of their culture extracts, the activity of these culture extracts in tests for virulence, and the virulence of the isolates after inoculation into seedlings of F. excelsior were previously reported by Junker et al (2013). filled each with 200 ml media.…”

“…Screening for antibiotic activities was performed with three strains of H. pseudoalbidus that had been selected from a previous study (Junker et al, 2013; hybridized quaternary carbons and three carbonyls ( Table 2). Analysis of the COSY spectrum resulted in the identification of three spin systems (Fig.…”

“…To identify these potential virulence factors, we studied three strains of H. pseudoalbidus that varied in their virulence (Junker et al, 2013) for their secondary metabolite production in submerged culture. The crude extracts were analysed by HPLC-MS and subjected to a classical antimicrobial screening using fungal and bacterial test organisms in a serial dilution assay.…”

“…Junker et al (2013) recently showed that the concentration of the phytotoxin, viridiol ( Fig. 1, 1; Moffatt et al, 1969), which is produced by isolates of both H. pseudoalbidus and H.…”