'The recently described ascomycete fungus Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) causes the current dieback of ash (Fraxinus excelsior) in large parts of Europe. The origin of this species and its relation to the native cryptic species Hymenoscyphus albidus are still enigmatic. The spatiotemporal pattern of the epidemic is typical for an introduced invasive species. However, the presence of two cryptic species indicates that hybridization or mutation might have been involved in driving speciation in this case.'

http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0329.2011.00751.x/abstract  (1)
Cryptic species are species that are similar but believed to be incabable of interbreeding.

'Our objective was to investigate the genetic structure of H. pseudoalbidus and to examine its relationship to the species H. albidus, known as a saprotroph. The study comprised 181 isolates of H. pseudoalbidus collected within the diseased area, 17 H. albidus isolates from six apothecia, collected outside the diseased area in Norway, and nine apothecia of H. pseudoalbidus collected in Sweden. By analysis of microsatellite markers developed for this study, combined with AP-PCR using the M13 primer, we demonstrated sexual heterothally in H. pseudoalbidus, detected high gene flow and low geographic structure of the H. pseudoalbidus population and found indications of a founder effect. Also, substantial genetic differences were detected between the two species of fungi; only four of seven microsatellite markers developed for H. pseudoalbidus were amplified for H. albidus, and no alleles were shared among the species. Furthermore, AP-PCR banding patterns were distinctly different for the two species. We conclude that even though the two fungi have a similar habitat and are morphologically virtually identical, they do not share a recent common ancestor.
Alleles harbour DNA codes.

 'Some caution is warranted, however, because information about the ecology and evolution of transgenic microbes in the wild is limited.'
'Two viridin-related B-norsteroids, B-norviridiol lactone (1) and B-norviridin enol (2), both possessing distinct unprecedented carbon skeletons, were isolated from a liquid culture of the ash dieback-causing fungus Hymenoscyphus pseudoalbidus.'
'Analysis of RNA transcript abundance, enzyme activity, cell wall composition, and soluble carbohydrates revealed significant changes in the  transgenic lines. All lines showed significantly increased SuSy enzyme activity in developing xylem. This activity manifested in altered secondary cell wall cellulose content per dry weight in all lines, with increases of 2% to 6% over control levels, without influencing plant growth. The elevated concentration of cellulose was associated with an increase in cell wall crystallinity but did not alter secondary wall microfibril angle. This finding suggests that the observed increase in crystallinity is a function of altered carbon partitioning to cellulose biosynthesis rather than the result of tension wood formation.'
'The genetic modification of carbon skeleton production had been thought to be difficult, because many enzymes are involved in carbon skeleton production.'
From the available information, I'd reckon it likely that a genetically modified microbe has crossed with a natural fungi. It appears to me that the alleles (2) are key. The layman's question (the only way out?): If a genetically modified microbe took on DNA from a natural fungi, could 'unprecedented carbon skeletons' result in different alleles, the DNA making the fungi 'morphologically virtually identical' while the alleles would be different (same/similar code, encrypted differently)? 


1. http://onlinelibrary.wiley.com/doi/10.1111/j.1439-0329.2011.00751.x/abstract molecular toolkit for population genetic investigations of the ash dieback pathogen Hymenoscyphus pseudoalbidus. A. Gross, C. R. Grünig, V. Queloz, O. Holdenrieder
2. http://www.sciencedirect.com/science/article/pii/S175450481100122X  Population structure of Hymenoscyphus pseudoalbidus and its genetic relationship to Hymenoscyphus albidus. S.B.K. Bengtsson, R. Vasaitis, T. Kirisits, J. Stenlid
3. https://docs.google.com/viewer?a=v&q=cache:ME-JLT-gIhoJ:www.sgm.ac.uk/news/hot_topics/sgm-gm-draft-statement.docx+genetically+modified+fungus+1980&hl=en&gl=uk&pid=bl&srcid=ADGEESgNN28MW2_mu9b2iCr6pKXnnNjSGOgBtT3A14W2lepmT6DvZWh58TJL48v2IpvOLMnv0Z5trxmLDAPZQlFnlIqHDzaNy6nQ1qSzwZIzoMLKwaYKw0iJCw8ReHCM3pi5bXWBnpJc&sig=AHIEtbRdtT_Qqx6w9_jMIudkPJhmol4_wA   SGM draft statement on genetic modification and microbiology
4. http://www.nap.edu/openbook.php?record_id=10880&page=159 Biological Confinement of Genetically Engineered Organisms. Committee on the Biological Confinement of Genetically Engineered Organisms, National Research Council
5. http://www.ncbi.nlm.nih.gov/pubmed/22732888 B-norsteroids from Hymenoscyphus pseudoalbidus. Andersson PF, Bengtsson S, Stenlid J, Broberg A.
6. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2722352/  Sucrose synthase affects carbon partitioning to increase cellulose production and altered cell wall ultrastructure. Heather D. Coleman, Jimmy Yan, Shawn D. Mansfield
7. http://www.isb.vt.edu/articles/sep0401.htm IMPROVED NITROGEN ASSIMILATION USING TRANSCRIPTION FACTORS.   Shuichi Yanagisawa