When Fungi Become Pathogenic

Plants and fungi have evolved together for a long time and, if we take woodlands as an example, these fungi live in a delicate balance with the trees and other organisms which make up a complex, thriving web of interactions.
However, all around us there are examples of organisms which have moved out of their native range, often through human activities, and have thrived as “invasive species”.
Prominent examples are Japanese knotweed and Himalayan balsam, which are almost out of control, or Australia’s destructive cane toads.

The ash dieback pathogen (Hymenoscyphus fraxineus), is a fungus that spells doom for native ash forests across Europe after first being detected over twenty years ago in Poland.
This is just one example of what is in store in the rapidly changing climate, allowing organisms to thrive where once they could not and is exacerbated by human induced translocation.
The increasing rate of pathogen invasions makes understanding pathogen invasion and adaptation a big concern, in conservation and agriculture and disease management.

But we can’t lay blame solely on the invader where human activity is at the root of the problem.
The ash dieback pathogen might not even be a pathogen at all in its native habitat.
This fungus lives part of its life inside Manchurian ash leaves, often without symptom.
While we still haven't discovered how the tree and the fungus interact, the current evidence shows the ash dieback fungus seems to live as a saprophytic fungus does, decomposing leaf litter as the fungus matures only when leaves are dropped.
The native habitat for this fungus is East Asia, where it stays in the leaves and does its job of decomposition when the leaves fall to the forest floor.
In Europe however, this fungus, for some reason doesn't stay in the leaves but instead invades the woody parts of the tree, which results in death for the infected tree.
The genetic diversity suggests that as few as two spores were the likely cause.
These spores, traveling outside of their native range, gave rise to this deadly disease.
What happens if more spores arrive from East Asia?
What does this mean for the scientists trying to breed more resistant ash trees?

Understanding Hymenoscyphus fraxineus population genetics, along with how much diversity exists between different populations of the fungus in different environments and between different host species, will give us more insight into what might happen to the fungus in Europe in the future.

Ash dieback fungus is a conundrum.
It's difficult to understand the advantages, to the fungus, of invading and killing its host because the fungus completes its life cycle by decomposing dead tissue in the leaf litter.  
This means that by invading the woody parts of the host, the organism doesn't get to reproduce in the leaf litter and it kills it's host.
It’s important to study the fungus in its native range to better understand its evolution.
Perhaps this scenario has already played out in East Asia in the past and the host trees there have adapted.

It's important that we rapidly improve our understanding of the evolution of virulence between pathogen and host.
More than just understanding ash dieback.
With the alarming increase in invasive pathogens spreading to new areas, the study of this European pathogen invasion, arising from a Far Eastern non-pathogen, will help us better understand how plants and their pathogens evolve.
Then we can apply this knowledge to the benefit of conservation efforts and the well being of our global food supply.

Redhawk

**Note; this is not part of the soil series but is more a commentary on the current plight of our planet **

Is the Emerald Ash-Borer a known vector for this(outside of incidental exterior contamination)? If so, this is even more worrisome; hopefully whatever genetic resistance they can develop helps protect against both threats...

Does that mean that, with proper control and hot-composting of leaves, for instance, that the fungi can be killed? And how is the life cycle of the fungus impacted by the action of earth worms eating leaf litter that would have otherwise been decomposed by the fungi? Is this a potential control strategy?

-CK

Dustin Rhodes wrote:Is the Emerald Ash-Borer a known vector for this(outside of incidental exterior contamination)? If so, this is even more worrisome; hopefully whatever genetic resistance they can develop helps protect against both threats...

No, the emerald ash borer is not a known carrier, it is presumed (at this point) that the spores did hitch a ride but most likely on something other than an insect.
I think if this fungus had come with the EAB there would have been a far faster outbreak, the EAB did hitch rides on traded goods, so it is probable that the same thing happened with this fungus.

Chris Kott wrote:Does that mean that, with proper control and hot-composting of leaves, for instance, that the fungi can be killed? And how is the life cycle of the fungus impacted by the action of earth worms eating leaf litter that would have otherwise been decomposed by the fungi? Is this a potential control strategy?

-CK

While fungi can be killed by hot composting, the problem is that in the "invaded" areas, the fungus is eating the whole tree, not just the leaves as it does in its natural habitat.
Earth worms might possibly be one method of transference of the spores of the fungus, so it wouldn't be an effective control. (The natural method of spreading is airborne spores from mushroom blooms)
The fungus fruits on stems and thus the spores are usually sent airborne to land on new host trees, thus once the leaf has hit the soil, the new spores are already in new homes.

The issue is that in other species of ash, instead of the natural host species, the organism sends mycelium through the leaf stem, into the branch and it continues into the trunk where it decimates the cambium layer, leaving diamond shaped dead tissues, which is how we can identify the culprit.
These diamond shapes first show up at branch junctions both on the branches and at the branch to trunk junction, they spread down and around the cambium until the tree is effectively girdled by dead cambium and the tree dies.
At that point, the fungus will not be able to spread unless it can fruit, and since it killed its host, there isn't the proper nutrient profile for it to fruit again. This is why it hasn't spread like a wild fire in a wind.

It's often invoked as the "evolutionary arms race" between host and pathogen....but that kind of visioning tends to lend itself to thinking in terms of winners and losers versus your better version of "....a delicate balance with the trees and other organisms which make up a complex, thriving web of interactions".   It's easier for me to grasp the migration of a pathogen to a new region where the local host is not adapted to that pathogen.....without resistance that host might be devastated by the new fungus, depending on all factors at play.  More 'stochastic' it seems is the acquisition, locally, of new virulence genes in a pathogen that it did not have before. These can be incorporated from other resident fungi or even other bacteria into a fungal genome under study and give it a new virulence spectrum that it did not have previously. This has been documented in several systems to date, even as the mechanisms remain enigmatic.

Thank you John, great info in your post.  I agree, the issue in this particular case is the way it is being approached in Europe, more investigation of how the fungus works should give us help in finding a way to create resistance.
Trying to kill off any fungus is usually doomed to failure, management of biology, while easier to do in the lab is quite hard to accomplish out in nature.
Hopefully the current studies will give enough of a profile of this species that resistant trees can be created or perhaps we will find some bacteria that can simply keep it in the leaves where it belongs instead of going into the wood of the European Ash trees.

Bryant RedHawk wrote:
Hopefully the current studies will give enough of a profile of this species that resistant trees can be created or perhaps we will find some bacteria that can simply keep it in the leaves where it belongs instead of going into the wood of the European Ash trees.

It's a tough problem for sure and one that has had researchers on American Elm (and Dutch Elm Disease; DED), American Chestnut (and Chestnut Blight), Hazelnut (and Eastern Filbert Blight) scratching their heads on where to find non-GMO resistance. ( Heck, Ash (Fraxinus) was planted in droves in our region south of Canada to counter the ravages of DED on the elm population....now ash borer is on the way.)  

For various reasons, the hybrids with exotic counterparts that *do* have resistance are not very 'satisfying'....the hybrids often lack traits that gave the domestic parent its particular characteristics.  Thus, the American X Chinese elm hybrids that have greater resistance to Dutch Elm Disease are more bushy and less stately than those few remaining domestics.  But this latter phenomenon offers some hope and my wife has actually transferred cuttings and seed from the few remaining American elms along our river (an important point>> these older trees stand alone in a graveyard of elms that succumbed to the disease) with hopes that they may have some resistance.  I believe there are movements across the US to identify resistant stock within all such threatened tree species in a conservation effort to *possibly* save what is left.