ELM trees can now be 鈥渧accinated鈥� against Dutch elm disease, the dreaded
blight which has wiped out tens of millions of elms around the world. The
vaccine tricks the elms into producing antifungal substances that fend off
infection by Ophiostoma novo-ulmi. This fungus is carried from tree to
tree by the Scolytus bark beetle, which burrows beneath the elm鈥檚
bark.
Martin Hubbes, whose team developed the vaccine in the faculty of forestry at
the University of Toronto, isolated its key protein from a mild strain of O.
novo-ulmi. 鈥淏ecause this protein comes from a related fungus, the tree
thinks it鈥檚 under attack,鈥� says Hubbes. 鈥淚t therefore triggers a cascade of
defence reactions.鈥�
Hubbes administers the vaccine to trees by drilling a shallow hole 4
millimetres wide in the outer bark. He then places a soluble capsule containing
the protein into the hole and seals it with beeswax or plastic film. As the
contents leak out, they percolate through the youngest vessels, or 鈥減ipes鈥�, in
the xylem, the tree鈥檚 internal plumbing system. These vessels enable the fungus
to disperse throughout the tree and are most vulnerable to infection.
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Within a few hours of the vaccination, the vessels are awash with antifungal
compounds called mansonones鈥攑art of a family of substances called
sesquiterpene quinones鈥攚hich are lethal to the fungus. 鈥淭hey kill by
attacking mitochondria, the energy powerhouses of the fungus,鈥� says Hubbes.
When Hubbes exposed vaccinated trees to the virulent strain of the fungus, 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鈥攁n
organic polymer in plant cell walls that imparts rigidity. 鈥淢ore wood is put
into the cell wall, making it much thicker, so the fungus can鈥檛 escape,鈥� says
Hubbes. 鈥淏y doing this, the tree seals off the fungal penetration point.鈥�
Hubbes is working out the optimal dosage for different sizes of tree in
trials on elms in Ontario and Manitoba. So far, the results have been
impressive: in one trial on saplings only 1 of 33 vaccinated trees died when
exposed to the fungus, whereas all the control trees died.
The active substance in the vaccine is a glycoprotein, a sugar-laden protein
of unknown function produced by O. ulmi. A world patent (WO 98/43483)
is pending on the new treatment, according to patent data service Derwent
Information, and Phenex, a Toronto company, plans to market it. Hubbes say that
trees would have to be vaccinated each year, but that the treatment is much
simpler to administer and less damaging than fungicides.
Clive Brasier, a specialist in Dutch elm disease research at the British
Forestry Commission鈥檚 research station in Farnham, Surrey, says that the
treatment could supersede today鈥檚 fungicides, which are expensive and difficult
to apply. 鈥淭he approach is valid provided you can sustain the level of
resistance throughout the year,鈥� he says. Brasier, who is developing treatments
based on natural viral enemies of the fungus (see 鈥淲ar in the woods鈥�, 15
February 1997, p 27), adds that while the approach might be applicable to urban
elms, it may be impossible to apply in the countryside. But this limitation also
applies to fungicides.
