Ash trees that can survive Chalara dieback

During the last decade an emerging infectious pathogen Hymenoscyphus pseudoalbidus has spread across Europe causing devastating damage on ash trees and associated plant communities.

As a consequence common ash (Fraxinus excelsior) is rarely planted in Denmark any more, and mature trees are being logged intensively in order to harvest timber before the quality deteriorates. This previously common tree species is slowly disappearing from forest ecosystems.

Ash trees with characteristic symptoms of ash dieback were first observed in Denmark around 2003, and subsequent investigation has shown that the very first symptoms were recorded in 2002. In 2006, through the work of Polish and Swedish researchers, it became clear that the disease is caused by a fungus (named Chalara fraxinea), but the pathogen was only properly identified as Hymenoscyphus pseudoalbidus in 2010.

Ash trees with Chalara dieback. Grib Skov, Denmark. Photo: E.D. Kjær

More information can be found on the FRAXBACK Cost Action website

A recent invasive species apparently outcompetes a native saprophytic relative

We have later found Hymenoscyphus pseudoalbidus fruit bodies in a herbarium sample collected in Denmark in 2005. The role of this fungus in ash dieback was unknown in 2005, and fruit bodies were classified at that time as Hymenoscyphus albidus, a species morphologically and genetically very close to H. pseudoalbidus.

Fruit bodies of H. pseudoalbidus on old black leaf stalks in August 2012, Denmark. Photo: L.V. McKinney, 2012

The closely related H. albidus is known to be a harmless saprophytic fungus growing on dead ash leaves. Further analysis of Danish herbarium samples of H. albidus from before the outbreak of the disease in Denmark showed that these were of the original and native H. albidus type. Similar results have been seen elsewhere in Europe.

In 2010 we revisited the localities from which the Danish H. albidus herbarium samples originated, but only found the new H. pseudoalbidus type with no signs of the native type. Apparently, the native type has been outcompeted by the new invasive pathogen. Interestingly, we could only find the native H. albidus type in 2010 collections from Scotland and in 2011 collections from southwest England and south Wales, all from areas with no symptoms of ash die-back at the time.

The observations relating to the Danish and Scottish collections were published in 2012 in Fungal Ecology 5(6): 663-669.

Few trees seem to be able to resist the fungus

Due to the severity of the problem, it has become a priority with the forest genetic

research group at the University of Copenhagen, to study if and how active genetic management can ensure that ash has a future in the Danish forests, cities and landscape.

As one of the few international research groups, we study the degree of inherited natural genetic resistance of the ash trees to the new disease. We have measured the level of susceptibility of 39 F. excelsior clones during the last six years in trials at two sites.

The results are positive in the sense that we do find substantial genetic variation between trees in their level of susceptibility. However, the proportion of trees with a high level of natural resistance seems to be very low, probably less than 5%. Results were published in 2011 in Heredity 106(5): 788-797.

An active defense that limits fungal growth

An intriguing result in the above clonal study was a high correlation between time of autumn leaf colouring of the F. excelsior clones and their degree of susceptibility. We speculated that the variation in susceptibility may be related to time of leaf shedding, because the pathogen infects the ash trees through the leaves in late summer.

Inoculation studies on clones that had already been identified as having either a high or a low level of susceptibility to natural infections showed that bark lesion development varied between clones. There was a close correlation between the general susceptibility of the clones in field trials and their development of necrosis following artificial infection.

An important implication of the results is that the observed high level of resistance in a few trees is likely to be due to an active defense, where resistant ash trees limit the fungal growth, rather than disease escape due to leaf shedding.

In addition, it seems possible to predict the general level of resistance by applying a fairly fast bioassay based on controlled inoculation followed by assessment of necrosis develop around the point of infection.

Young ash trees with Chalara dieback in June 2005, Denmark. Photo: I.M. Thomsen

The results were published 2012 in Forest Pathology 42(1): 69-74.  

Resistance is inherited

Of key importance for both the evolutionary implications and the practical applicability of our findings is the degree to which resistance is inherited from parents to offspring. We therefore studied offspring from 101 selected trees during the last four years based on progeny trials at two sites.

This study demonstrates presence of substantial additive genetic variation with a high level of heritability (hns2 = 0.37-0.52). This means that resistance is likely to be passed on, to a large extent, by parents to offspring. Again, most of the tested native trees were found to produce highly susceptible offspring, and only 1-3% of the trees produced healthy offspring under the present disease pressure.

These results suggest that the genetic variation in susceptibility can provide the species an important ability to recover in the long run, but that ash trees in the present forests most likely will become severely decimated. The findings were published 2012 in Evolutionary Applications 5(3): 219-228.

Ongoing research focuses on studying the actual genes involved in the resistance, and on host-pathogen interactions.

 Save the remaining ash trees

PhD student Lea Vig McKinney in front of a tested ash tree growing at Arnkilsøre, Als. Out of 101 tested trees, this is one of the few trees able to produce predominantly healthy offspring. Photo: L.R.Nielsen, 2009.

Urgent actions are required if resistant ash trees are to be identified and secured in the forests before they are logged. A national Danish campaign has therefore been initiated as a joint effort between researchers, foresters and public authorities. Foresters have been urged to observe and report trees that exhibit good natural resistance.

More than 100 trees have so far been identified by public and private forest managers and documented by researchers. These trees will be DNA tested and propagated by grafting during winter 2012. Clonal trials will be established, and susceptibility of the identified candidate trees will be tested under field and green house conditions supported by genomic studies.

The first artificial crossings between our tested, healthy ash clones were made in the spring 2012.

The objective is to safeguard the few percentages of ash trees that have a reputed high level of health, but are in acute risk of being logged together with the surrounding sick stands. Collections are made from as many trees as possible to ensure the largest possible effective population size. The program will draw on the knowledge generated from past and ongoing research. If successful, the program will allow development of healthy seed sources with high levels of natural resistance to the pathogen.

Hopefully, similar collections will be initiated all over Europe, in order to enable large scale future re-introductions of ash trees into forests. The time horizon for development of new, productive seed sources is 15-20 years or more. 

Ash - an important tree in forests and landscapes
The European Common ash (Fraxinus excelsior L.) is an import species in Denmark. It has been part of the Danish flora for more than 7,000 years, and has until recently been the 5th most important broadleaved species in terms of area coverage in the Danish forests.

The species is dominant in forests on moist, fertile soils with good drainage, and is a key species in a unique natural plant community with many associated plants and insects. The ash tree is also an important tree in landscapes and urban areas.