Trees fight back

After killing trees by the millions, the Dutch elm disease finally meets its match

Published: Monday 15 February 1999

Vaccinating elms against a fun DUTCH elm disease (DED), a dreaded fungal disease that afflicts elm trees, has had a ghastly record. Spread by bark beetles, DED has wiped out tens of millions of trees around the world. It first appeared in the Netherlands in 1917, from where DED reached the UK through infected elm logs in 1927. US contracted the disease in the 1930s and since then, DED has claimed some 43 million trees in the US alone. The disease, however, is known for another reason - for exposing pesticides as the killers that they really are.

In the 1950S, DED afflicted thousands of elm trees across the us. Farmers, in their bid to stop it from spreading farther, used pesticides liberally. The chemicals did curb the disease but, on the flip side, they also killed thousands of birds - all unintentional victims. The next year, spring was eerily silent in the US: the pesticides had wiped out a formidable percentage of the country's avian population. Then, in 1963, came Silent Spring, a book by Rachel Carson shook the country's conscience by detailing how the pesticides had claimed birds and even several species of small animals as their unintended victims.

Still alive and killing, DLD is now moving west from central us and Canada, say plant biologists. But this time, nations will be prepared to fight back. Elm trees can now be 'vaccinated' against DED. This new vaccine tricks the elms into producing anti-fungal substances that can effectively fend off the infection. The DED fungus, Ophiostoma novo-ulmi (O novo-ulmi), is carried from tree to tree by the Scolytus bark beetle, which burrows beneath the elms' barks.

Martin Hobbes, whose team developed the vaccine in the faculty of forestry at the University of Toronto, Canada, isolated its key protein from a milder version of O novo-ulmi. "Because this protein comes from a related fungus, the tree thinks its under attack," says Hobbes. "Therefore, it triggers a cascade of defence reactions."

Hobbes administers the vaccine to trees by drilling a shallow hole some four millimetres wide in the outer bark. Then, a soluble capsule containing the protein is placed in the hole which is then scaled with beeswax or plastic film. As the contents leak out, they percolate through the youngest vessels or "pipes", in the xylem, the trees internal 'plumbing' system. These vessels enable fungus to disperse throughout the tree and are most vulnerable to infection.

Within a few hours of the vaccination, the vessels are awash with anti-fungal compounds called mansonones which are lethal to fungi "They kill by attacking the mitochondria, the energy powerhouses of the fungus," says Hubbes.

When Hobbes exposed vaccinated trees to the more lethal forms of O novo-ultni, he found that the trees were ready to mount a defence, trapping the fungus in infected cells by hardening the walls of these cells with lignin - an organic polymer in plant cell walls that imparts rigidity. "More wood is put into the cell wall, making it much thicker, so the fungus can't escape," says Hobbes. "By doing this, the tree seals off the fungal penetration point."

Hubbes is working out the optimal dosage for different sizes of trees in trials on elms in Ontario and Manitoba. So far, the results have been encouraging: in one trial on saplings, only one out of 33 vaccinated died when exposed to the fungus. Others came out of the test unscathed.

The active substance in the vaccine is a glycoprotein, a sugar-laden protein of an unknown function of O novo-ulmi. A Toronto-based company, Phenex, is even planning to produce the vaccine commercially.

Hobbes says trees would have to be vaccinated each year, but that such a treatment is much simpler to administer and definitely less damaging than spraying fungicides and pesticides.

Clive Brasier, a specialist in DED research at the British Forestry Commission's research station in Surrey, UK, says the treatment could supersede today's fungicides, which are both expensive and difficult to apply. "The approach is valid provided you can sustain the level of resistance throughout the year," he adds.

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