The earliest samples of <Emphasis Type="Italic">Hymenoscyphus albidus</Emphasis> vs. <Emphasis Type="Italic">H. fraxineus</Emphasis> in Estonian mycological herbaria

In 2006, the alien and pathogenic Hymenoscyphus fraxineus was differentiated by molecular methods from the long recognized saprotrophic H. albidus . Today, H. fraxineus seems to have replaced H. albidus in several countries, but the exact year of arrival of H. fraxineus in northern Europe is still debated. Investigations of herbaria specimens might help to ascertain it. Before the epidemic, H. albidus was not of significant interest to mycologists in the Baltic area and was not eagerly sampled and deposited. Nevertheless, the TAAM herbarium in Estonia holds 13 putative H. albidus specimens that were collected 1966–2006. Using newly developed species-specific PCR primers to differentiate between H. albidus and H. fraxineus, all available herbarium samples from TAAM were identified. In addition, the primers were also tested on pure cultures and ascocarps of H. fraxineus and H. albidus, symptomatic petioles of F. excelsior, pure cultures of 10 non-target fungal species, and 10 different soil samples. With the exception of the oldest specimen in TAAM from 1966 (collected in Lithuania), all herbarium samples were identified as H. fraxineus. The first record of H. fraxineus in Estonia dates back to 1997 and apparently represents the oldest record of the species in Europe. In spite of this, symptoms based on reliable observations were not detected until 2003 in Estonia. This temporal shift is difficult to explain. Possible reasons may represent (i) overlooking of symptoms during the first years; (ii) typical lag phase at the start of an epidemic; (iii) an abrupt shift towards higher virulence of H. fraxineus; or (iv) other environmental (climatic) factors. Closer investigations are needed to disentangle these possible reasons.