Nightmare in the Elm Avenue
Even in ancient times, the elm was a symbol of death and mourning. But this has never been more relevant than today - now with dying trees in the lead role. And they're furthering their own downfall in the process.
Actually, woman descends from the elm tree - at least in the Germanic story of creation, as described in the Edda poetry. Accordingly, the chief of the gods Odin formed the woman from a piece of elm wood - he created the man from the ash tree - and breathed life into her like a ghost. Things were more earthly in southern France: there the shade of their canopy was valued and justice was administered at their feet. And a popular motif in poetry was the marriage of the elm with the vine as a sign of eternal fidelity, starting with the Romans, because they used the saplings of the tree to hold the wine.
Not much of that splendor remains: Elm trees are dying in Europe and North America, killed by the fungal 'Dutch Elm Disease'. The curse on trees was probably brought to Europe shortly before the First World War and probably originated in East Asia, but details are not yet known. The pathogen Ophiostoma ulmi soon spread at breakneck speed, using elm avenues as veritable highways.
But by the time Dutch researchers found out about the fungus - hence the name "Dutch Elm Disease" - the infection in exported veneer wood had long since crossed the pond and was now devastating America's stocks. Canada fought back again in this involuntary war with biological weapons: Ophiostoma novo-ulmi also returned to Europe around 1960 with processed wood products.
This close relative of the original sac fungus was and is still much more aggressive than its predecessor, which lost much of its aggressiveness over time: within a few decades, the new pathogen infected and killed ninety percent of all European elms - alone in the UK it affected twenty million trees and there is no end in sight.
So far, science has shown Ophiostoma to have only one – relative – weakness: In order to spread at all, it usually needs the help of elm bark beetles such as Scolytus scolytus. The insects transmit the spores of the pest when they work their way inside the crops while feeding. The fungus eventually triggers cell proliferation in the vascular bundles of the trunks, which soon clogs the trees' water channels: the elms die of thirst.
However, as might be expected from such an insidious invader, Ophiostoma's sophistication does not end with the Trojan bugs. Because scientists around Geoff McLeod from the Canadian Simon Fraser University in Burnaby have now discovered that the fungi also "force" their hosts to attract their messengers of death themselves.
The pathogen takes advantage of the fact that many plants communicate by means of biochemically synthesized scents - for example to attract enemies of pests or to encourage their neighbors to produce repellents. Elms also behave accordingly and also produce allelochemicals to contain sources of infection.
But this can also backfire, as researchers have found through field and laboratory studies. They captured the gases emitted by crushed pieces of diseased elm wood and presented them to beetles of the species Hylurgopinus rufipes. The guys who play the carrier in America were obviously attracted to it. In the next step, the individual components of this scent signal were analyzed, with the beetles' isolated antennae acting as signal transmitters.
If traces of gas triggered reactions in the biodetectors, they were determined in more detail using a gas chromatograph and mass spectrometer. One mono- and three sesquiterpenes turned out to be animal attractants, which in combination are irresistible to the insects. Only traps in which all four components were released in natural amounts attracted male and female Hylurgopinus rufipes en masse - if one of the components synthesized by the researchers was already missing or was overproportioned, the scent ambushes remained largely empty.
And it's indeed the trees that summon their evildoers themselves: sugar-grown Ophiostoma novo-ulmi didn't make any of the components. And comparisons between he althy and intentionally infected elm saplings clearly showed that only the diseased specimens increase the output of these attractants, also known as semiochemicals, while the production of intact elms is limited. As a result, the beetles prefer to fly to fungus-ridden individuals.
Exactly how the pathogen stimulates the production of fragrances is still unclear to the researchers - so the disaster will continue to take its course. The role of the elms in Greek mythology is more appropriate to today's elms: there, their winged fruits accompanied the messenger of the gods, Hermes, on his way to bring the souls of the deceased before the Last Judgment. Today, however, the beetles take over this tragic part and accompany the elms into the afterlife.
