Hymenoscyphus fraxineus

Hymenoscyphus fraxineus is an Ascomycete fungus that causes ash dieback, a chronic fungal disease of ash trees in Europe characterised by leaf loss and crown dieback in infected trees. The fungus was first scientifically described in 2006 under the name Chalara fraxinea. Four years later it was discovered that Chalara fraxinea was only the asexual (anamorphic) stage of a fungus that was subsequently named Hymenoscyphus pseudoalbidus and then renamed as Hymenoscyphus fraxineus. Trees now believed to have been infected with this pathogen were first reported dying in Poland in 1992. It is now entrenched in Europe. It is closely related to a native fungus Hymenoscyphus albidus, which is saprotrophic and grows on the dead leaves of ash trees.According to a report published in the Journal of Ecology a combination of the disease and emerald ash borer attacks could wipe out European ash trees. The fungus Hymenoscyphus fraxineus was first identified and described in 2006 under the name Chalara fraxinea. In 2009, based on morphological and DNA sequence comparisons, Chalara fraxinea was suggested to be the asexual stage (anamorph) of the ascomycete fungus Hymenoscyphus albidus. However, Hymenoscyphus albidus has been known from Europe since 1851 and is not regarded as pathogenic. In 2010, through molecular genetic methods, the sexual stage (teleomorph) of the fungus was recognized as a new species and named Hymenoscyphus pseudoalbidus. Four years later it was determined that 'under the rules for the naming of fungi with pleomorphic life-cycles', the correct name should be Hymenoscyphus fraxineus. Hymenoscyphus fraxineus is 'morphologically virtually identical' to Hymenoscyphus albidus, but there are substantial genetic differences between the two species. Hymenoscyphus fraxineus has two phases to its life-cycle: sexual and asexual. The asexual stage (anamorph) grows in affected trees attacking the bark and encircling twigs and branches. The sexual, reproductive stage, (teleomorph) grows during summer on ash petioles in the previous year's fallen leaves. The ascospores are produced in asci and are transmitted by wind; this might explain the rapid spread of the fungus. The origins of the disease are uncertain, but researchers are investigating the theory that the fungus originated in Asia, where ash trees are immune to the disease. Genetic analysis of the fungus Lambertella albida which grows harmlessly on petioles of the Manchurian ash (Fraxinus mandschurica) in Japan, has shown that it is likely to be the same species as Hymenoscyphus fraxineus. Teams from The Sainsbury Laboratory (TSL) and the John Innes Centre in Norwich sequenced the genome of the fungus in December 2012. The sequence has been published on the website OpenAshDieBack and offers clues to how the fungus infects trees. The study has uncovered toxin genes and other genes that may be responsible for the virulence of the fungus. In the long term researchers aim to find the genes that confer resistance to the pathogen on some ash trees. Trees now believed to have been infected with this pathogen were reported dying in large numbers in Poland in 1992, and by the mid 1990s it was also found in Lithuania, Latvia and Estonia. However, it was 2006 before the fungus’s asexual stage, Chalara fraxinea, was first described by scientists, and 2010 before its sexual stage was described. By 2008 the disease was also discovered in Scandinavia, the Czech Republic, Slovenia, Germany, Austria and Switzerland. By 2012 it had spread to Belgium, France, Hungary, Italy, Luxembourg, the Netherlands, Romania, Russia, Britain and Ireland. There are few official figures available, but the disease has caused a large-scale decline of ash trees across Poland, and the experience there suggests that in the long term '15 to 20 per cent of trees do not die, and show no symptoms.' In 2012, the disease was said to be peaking in Sweden and Denmark, and in a post-decline (or chronic) phase in Latvia and Lithuania. The disease was first observed in Denmark in 2002, and had spread to the whole country by 2005. In 2009 it was estimated that 50 per cent of Denmark's ash trees were damaged by crown-dieback, and a 2010 estimate stated that 60–90% of ash trees in Denmark were affected and may eventually disappear. The disease was first reported in Sweden in 2003. A survey conducted in Götaland in 2009 found that more than 50% of the trees had noticeable thinning and 25% were severely injured. A Danish study found that substantial genetic variation between ash trees affected their level of susceptibility. However, the proportion of trees with a high level of natural resistance seemed to be very low, probably less than 5%. A Lithuanian trial based on the planting of trees derived from both Lithuanian and foreign populations of European ash found 10% of trees survived in all progeny trials to the age of eight years.