Overvåking av askeskuddsyke

Askeskuddsyken har herjet i Europa siden tidlig 1990-tallet. Den ble først oppdaget i Polen og har siden spredt seg gjennom de fleste områdene i Europa hvor vanlig ask forekommer. Askeskuddsyken truer aska på hele det europeiske kontinentet, og aska er nå rødlistet i mange land. Askeskuddsyke forårsakes av en liten begersopp, askeskuddbeger. Askeskuddbeger er et eksempel på en invaderende, fremmed art som har forårsaket store skader utafor sitt naturlige utbredelsesområde i Asia, og selv om den kan spre seg naturlig over store avstander med sine sporer, har spredningen blitt godt hjulpet av import og handel med infiserte askeplanter.

Collage friskt skadd dødt
Foto: Volkmar Timmermann
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Askeskuddbegers fruktlegemer vokser på bladstilker. Fruktlegemene er 2-9 mm i diameter. Foto: Volkmar Timmermann.
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Askeskuddbegers sporer og asci. Sporene er ca 18-19 μm lange. Foto: Volkmar Timmermann.
Les mer om spredning av askeskuddsyke i Norge
Askeskuddsyke ble først oppdaget i Polen tidlig på 1990-tallet. Man observerte da en tiltagende visning hos asketrær i de nordøstre deler av Polen. Sykdommen spredte seg etter hvert til nabolandene. Siden årsaken var ukjent, ble det ikke satt i gang kontrolltiltak som for eksempel begrensning av handel med askeplanter for å forhindre spredningen over landegrensene. Først i 2006 ble årsaken funnet, en sopp som senere har fått navnet Hymenoscyphus fraxineus, på norsk askeskuddbeger.

Soppen utvikler små begre på fjorårets askebladstilker i skogbunnen og den kan spre seg naturlig over store avstander med sine sporer. Sporene infiserer bladene på asketrærne i løpet av vekstsesongen, og soppen sprer seg derfra videre til greinene og til stammen. Infeksjonen forårsaker nekroser i bladene og bladstilkene, og i vinterhvilen i greinene og stammen med bladvisning og skudd- og greinavdøing som følge. Gjentatte angrep over flere år vil til slutt kunne drepe hele treet, ofte i samspill med andre faktorer.

Askeskuddbeger er vanlig i Øst-Asia på bl.a. mandsjuriaask (Fraxinus mandshurica), hvor den kun forårsaker harmløse bladflekkskader. Askeskuddbeger er et eksempel på en invaderende art som oppfører seg annerledes i møte med en ny vert og som har forårsaket store skader utafor sitt naturlige utbredelsesområde. Den har blitt en trussel for europeisk ask (F. excelsior) etter at den har spredd seg med rekordfart gjennom Europa, godt hjulpet av import og handel med infiserte askeplanter. Tilbakegangen av ask og påfølgende treslagsskifte har store implikasjoner for det europeiske skogbruket, men også for biomangfoldet siden det er hundrevis av arter som er knyttet til ask, og ask er nå rødlistet i mange land. I dag er det kun noen få regioner igjen i Europa som ennå ikke er berørt av denne sykdommen.

Siden 2009 har vi fulgt spredningen av askeskuddsyke nordover på Vestlandet og sykdomsutviklingen på enkelttrær og i bestand på våre overvåkingsflater på Øst-, Sør- og Vestlandet. Vi har også overvåket soppens sporespredning i to askebestand i Ås og i Bergen/Fana over flere år.
 
Askeskuddsyke har siden 2006 spredt seg gjennom hele askas naturlige utbredelsesområde i Norge. Resultater fra overvåkingen viser at skadeutviklingen skjer fort, også i områder hvor sykdommen bare har vært til stede i noen få år, eller som er relativt isolerte fra andre askeforekomster. Skadeomfanget øker fra år til år, og mortaliteten er høy, særlig blant de yngste trærne. Svekkete trær angripes lettere av andre skadegjørere, spesielt honningsopp. De fleste døde trær på overvåkingsflatene på Østlandet viste tegn til honningsoppråte. Råteangrepne trær er også mer utsatt for vindfelling.
 
Fortsatt fins det noen få friske asketrær igjen på overvåkingsflatene, noe som gir håp om genetisk betinget motstandsdyktighet mot sykdommen hos noen enkeltindivider. Disse friske individene vil være svært verdifulle for genressursforvaltningen og utviklingen av motstandsdyktig materiale.
 
Men det haster: Som følge av de omfattende skadene og den høye dødeligheten i hele det naturlige utbredelsesområdet for ask i Norge, har askas status på Norsk rødliste for arter gått fra å ikke være oppført i 2006, til nær truet (NT) i 2010, sårbar (VU) i 2015 og sterkt truet (EN) i 2021.

Les mer om askeskuddsyke i våre årlige rapporter fra skogskadeovervåkingen og i vår serie med faktaark (NIBIO POP 3(2–7) 2017).

Publikasjoner

Sammendrag

In Norway the common ash (Fraxinus excelsior L.) has its northernmost distribution in Europe. It grows along the coastal range as small fragmented populations. The first occurrence of ash dieback caused by Hymenoscyphus fraxineus in Norway was reported in 2008. At that time, the disease had already spread through large areas of southern and south-eastern parts of Norway. Since then the disease continued spreading with a speed of about 50- 60 km per year along the western coastal range. To monitor the disease development over time, we established eight permanent monitoring plots in south-eastern and western Norway in 2009 and 2012, respectively. In all plots tree mortality was high, especially among the youngest trees in south-eastern Norway. The extent of crown damage has continually increased in all diameter classes for both regions. In 2009, 76.8 % of all trees on the five monitoring plots in south-eastern Norway were considered to be healthy or slightly damaged, and only 8.9 % to be severely damaged. In 2015, 51.7 % were dead, 13.5 % severely damaged and only 25.7 % remained healthy or slightly damaged. To assess the infection pressure and spore dispersal patterns of the pathogen, we used a Burkard volumetric spore sampler placed in an infested ash stand in southern Norway. We examined the airborne ascospores of H. fraxineus and H. albidus captured on the sampling tape microscopically and with real-time PCR assays specific to these fungi. We detected very few ascospores of H. albidus, whereas ascospores of H. fraxineus dominated throughout entire sampling periods of 2009, 2010 and 2011. Spore discharge occurred mainly between the hours of 5 and 8 a.m., though the distinctive sporulation had yearly variation between 5-7 a.m. We observed the same diurnal pattern throughout the entire sampling period, with a seasonal peak in spore liberation between mid-July and midAugust, after which the number of ascospores decreased substantially. Similar diurnal patterns were observed throughout the sampling period except that after mid-August the number of trapped ascospores substantially decreased. To compare the genetic pattern of common ash in the northern and central ranges of Europe we analyzed the Norwegian samples together with available samples from central Europe by using chloroplast and nuclear microsatellite markers. We found that the northern range of common ash was colonized via a single migration route that originated in eastern or south-eastern Europe with little influence originating from other southern or western European refugia. In the northern range margins, genetic diversity decreased and population differentiation increased, coherent with a post-glacial colonization history characterized by founder events and population fluctuations. Based on our findings we discuss the future management and conservational implications.

Sammendrag

Dieback of European ash (Fraxinus excelsior L.), a disease caused by the ascomycete Hymenoscyphus fraxineus (previously referred to as H. pseudoalbidus or Chalara fraxinea), was first observed in Poland in the early 1990ies, and is currently present almost throughout the entire distribution area of European ash. The characteristic symptoms of the disease include dead shoots with necrotic lesions in the bark and discoloration of xylem and pith but the seasonal dynamics of pathogen spread in shoot tissues remain poorly understood. To investigate whether the internal spread of the fungus involves season-specific patterns, saplings with necrotic bark lesions in 1-2 -year-old stem regions were collected during 2014-2015 at time intervals in spring, summer, autumn and winter at several localities in western Ukraine and at two localities in south-eastern Norway. Tissuespecific presence of H. fraxineus was determined by a highly sensitive quantitative real-time PCR assay that is specific to DNA of H. fraxineus. The relatively high proportion of bark samples positive for H. fraxineus in the saplings collected during spring provides support to a model that H. fraxineus can be a primary causative agent of bark lesions and that other fungi may eventually replace it in old infection areas.

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Sammendrag

Ten saplings of European ash (Fraxinus excelsior L.) naturally infected by the invasive ash dieback pathogen Hymenoscyphus fraxineus were collected in Ukraine and Norway and examined for bark necrosis and extension of discoloration in sapwood and pith in a stem region. Tissue-specific colonization profiles were determined by spatial analyses of symptomatic and visually healthy stem tissues using a H. fraxineus-specific qPCR assay and light microscopy. Our data suggest that hyphal growth in the starch-rich perimedullary pith is of particular importance for both axial and radial spread of H. fraxineus, but that most of its biomass accumulates in sapwood parenchyma. The study confirms the results from earlier work and presents new information that refines the current stem invasion model.

Sammendrag

Shoot dieback disease of European ash caused by the ascomycete Hymenoscyphus pseudoalbidus threatens ash on a continental scale. A spore sampler placed in a diseased ash forest in Southern Norway, coupled with microscopy and DNA-based fungal species-specific real-time PCR assays, was employed to profile diurnal and within-season variation in infection pressure by ascospores of H. pseudoalbidus and the potentially co-existing non-pathogenic Hymenoscyphusalbidus. Hymenoscyphus pseudoalbidus was found to be predominant in the stand. Massive simultaneous liberation, by active discharge of pathogen ascospores in the morning, peaked in mid-Jul. to mid-Aug. Accumulation of pathogen DNA on leaflets of current-year leaves reached a high level plateau phase before appearance of autumn coloration, suggesting that pathogen establishment in leaves is terminated before the onset of leaf senescence.

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Sammendrag

A large database of invasive forest pathogens (IFPs) was developed to investigate the patterns and determinants of invasion in Europe. Detailed taxonomic and biological information on the invasive species was combined with country-specific data on land use, climate, and the time since invasion to identify the determinants of invasiveness, and to differentiate the class of environments which share territorial and climate features associated with a susceptibility to invasion. IFPs increased exponentially in the last four decades. Until 1919, IFPs already present moved across Europe. Then, new IFPs were introduced mainly from North America, and recently from Asia. Hybrid pathogens also appeared. Countries with a wider range of environments, higher human impact or international trade hosted more IFPs. Rainfall influenced the diffusion rates. Environmental conditions of the new and original ranges and systematic and ecological attributes affected invasiveness. Further spread of established IFPs is expected in countries that have experienced commercial isolation in the recent past. Densely populated countries with high environmental diversity may be the weakest links in attempts to prevent new arrivals. Tight coordination of actions against new arrivals is needed. Eradication seems impossible, and prevention seems the only reliable measure, although this will be difficult in the face of global mobility.

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Sammendrag

Dieback of European ash (Fraxinus excelsior), caused by the ascomycete Hymenoscyphus pseudoalbidus (anamorph Chalara fraxinea), started around 1992 in Poland and has since then spread over large geographical areas. By November 2010, the disease had been recorded in 22 European countries. The gradual expansion and high intensity of the ash dieback epidemic in Europe may suggest that H. pseudoalbidus is an invasive alien organism. In Norway, ash dieback was first reported in spring 2008, and a survey in early summer of the same year revealed that the disease had spread over large parts of the southern and eastern regions of the country. The distance from the southernmost to the northernmost infected stands was, at that time, about 400 km. Some old necrotic lesions were also observed, indicating that the ash dieback pathogen is likely to have been present in Norway since at least 2006. In 2009, a spore sampler was installed in a diseased ash stand at Ås, South-Eastern Norway. Sampling started in late July and continued until late September. Large numbers of ascospores resembling those of H. pseudoalbidus were observed, with the maximum number of spores occurring from the end of July to mid-August. The deposition of ascospores occurred mainly between 6 and 8 a.m. Ascospores are most likely to be the primary source initiating host infections and responsible for the rapid recent spread of H. pseudoalbidus in Europe.

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Sammendrag

The causative agent of dieback on European ash (Fraxinus excelsior) was first described as Chalara fraxinea based on cultural morphology because no sexual stage of the fungus was known. Later, based on culturing of ascospores of a candidate teleomorph, morphological comparison and nuclear ribosomal internal transcribed spacer sequencing, the sexual stage of C. fraxinea was assigned as Hymenoscyphus albidus, a native and widespread species in Europe. Recently, the morphological species concept of H. albidus was shown to cover two species that cannot be separated from each other based on teleomorph characters, but which can be distinguished by several DNA markers. As a result, the strains causing ash dieback were reassigned as H. pseudoalbidus. The closely related H. albidus is presumably a non-pathogenic endophyte, but pathogenicity tests to confirm this hypothesis have not yet been performed. Genotyping of herbarium specimens has shown that H. pseudoalbidus was present in Switzerland for at least a decade prior to the epidemic outbreak in Europe. The origin of the ash dieback pathogen, and the general importance of correct pathogen identification to development of effective disease control, are discussed.