ETH-Zurich researcher Valentin Queloz describes the species of fungus responsible for the ash dieback recently observed in Switzerland as a "familiar stranger." However, researchers still don't know why a harmless leaf colonizer could turn into an aggressive pathogen.
The uncanny phenomenon began in eastern Poland. In the nineties ash trees started dying apparently out of the blue, and entire forests shrank. Then, in 2006 a Polish scientist succeeded in isolating a previously unknown fungus from 70 percent of the dead ash trees. Only consisting of hyphae, he called his discovery Chalara fraxinea -- a vegetative form that produces sticky spores, known as conidia, asexually. It wasn't until later that researchers in Poland found little yellow-white fungal fruiting bodies on the leaf remains of ash trees that they were able to identify as Hymenoscyphus albidus. It later transpired that Chalara fraxinea and the fruiting bodies were one and the same species but, as is often the case in mycology, carry two different names: one for the anamorph and one for the teleomorph.
H. albidus wasn't a stranger after all, having first been characterized in 1851. But why should this fungus, which had always been regarded as a harmless litter decomposer, suddenly turn on its host, the ash?
Mass infestation in the Jura, scarce in Ticino
H. albidus, and therefore ash dieback, first appeared in Switzerland in 2008. In the summer of 2009 ETH-Zurich PhD student Valentin Queloz from the Institute of Integrative Biology specifically began to scour areas already affected by the mysterious ash disease, as well as those it hadn't yet reached, such as south of the Alps and Ticino.
He didn't need to look very far for the culprit north of the Alps: one sample area near Porrentruy in the Jura was teeming with the little white fungi. "I saw them as soon as I stepped out of the car," says the environmental scientist. The forest was littered with them. An infestation of this magnitude is exceptional and even surprised the experts; "A mast year, this humid, warm summer," says Queloz.
The story was quite different south of the Alps, however: he did eventually find the fungus, but only smaller fruiting bodies, and twenty times less frequently than in Jura.
Surprise in the lab
It wasn't until he was back in the lab that the researcher became aware the mystery behind the unequal distribution: what he had found south of the Alps was in actual fact the harmless H. albidus, whereas he had only collected the pathogenic form of the fungus north of the Alps. Finally, a genetic test yielded a surprising result: the aggressive, pathogenic fungus was in fact a different species, which Queloz re-classified as H. pseudoalbidus in a recent publication. Using herbarium specimens, the PhD student from ETH Zurich even discovered that mycologists had already collected H. pseudoalbidus, but documented the specimens as H. albidus.
Where does pathogenicity come from?
The scientist is now puzzling over whether and why H. pseudoalbidus developed into a pathogen in the space of a few decades, and why it acquired its pathogenic properties. In other words: why a harmless endophyte, a fungus that lives in plants, turned into a germ that is deadly for ash trees. One hypothesis is that climate change is weakening the trees and that H. pseudoalbidus happens to be the stronger of the two species of fungus.
After all, H. pseudoalbidus hasn't changed anything in its life cycle and is thus akin to H. albidus. After overwintering in the litter, the fungus forms fruiting bodies on the partially decomposed midribs of ash leaves when summer comes. The fungus looks like a little white mushroom, no bigger than a thumb tack. It produces spores that are dispersed by the wind; the spores find their way onto the still-green leaves of standing ash trees, germinate and form a mycelium that spreads inside the leaf. When the leaves drop off the tree in the fall, the fungi return to the soil, where the cycle starts all over again.
Setting up a seed bank
Now, H. pseudoalbidus is threatening to throw this well-established equilibrium off balance. Compared to its sister species, it is more aggressive, penetrates the ash leaves more easily and grows into the wood more rapidly -- penetrating it right to the core. Young trees are hit the hardest; the leaves wither and the bark changes color around the spot where the fungus got in. However, the damage goes unnoticed for quite some time as the symptoms mostly begin at the shoot tips before spreading towards the trunk.
Meanwhile, H. pseudoalbidus has decimated entire ash populations in Poland. In Switzerland, however, there is still time to fight the fungus -- by collecting a large enough stock of ash seeds, for instance. "We have to try and preserve the genetic diversity of the ash before the fungus destroys it," warns Queloz. We also need to keep an eye on the germ. "We urgently need to find out where the pathogenicity comes from."
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