AN UNNATURAL union between two fungi has created an aggressive new disease
that is felling hundreds of thousands of alder trees along Europe鈥檚 rivers. Two
species of Phytophthora, relatives of the potato blight, have
hybridised to create lethal offspring that destroy the bark around the base of
the trees, eventually killing them.
So far, the disease has claimed around 10 per cent of the alders in southern
England and Wales, and is steadily killing up to 2 per cent of the population
each year. The blight has also had a catastrophic impact in parts of Sweden,
France and Holland, and is spreading in Germany and Austria.
Forest researchers warn that this might be the first of a rash of new
pathogens to strike as the world trade in plants accelerates the evolution of
new species. 鈥淚鈥檓 concerned that there鈥檚 a lot of this going on,鈥� warns Clive
Brasier of the Forestry Authority Research Station in Farnham.
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Epidemics of fungal diseases are usually caused by introduced pathogens,
which make short work of native species with no resistance to them鈥攁s with
Dutch elm disease in the 1970s. But until now, there has been no evidence that
introduced fungal species were exchanging genes with either resident fungi or
other exotic fungi.
When the alder disease first struck in 1993, Brasier and his colleagues at
the Scottish Crop Research Institute in Invergowrie tried to identify the fungus
by sequencing a region of its DNA called the internal transcribed spacer, a
stretch of genes that provides a signature of the species. They were surprised
to find signatures from two different species, suggesting it was a hybrid.
鈥淭hat鈥檚 very unusual,鈥� says Brasier.
Fungi rarely hybridise in nature, because species that live in the same
environment have evolved barriers to prevent it. 鈥淏ut fungi that are
geographically isolated don鈥檛 accumulate barriers to reproduction. So if you
bring them together there is more risk of hybridisation,鈥� says Brasier.
Further studies have identified one parent as Phytophthora cambivora,
which infects some trees, but not alders. The other parent is close but not
identical to P. fragariae, a blight of strawberries and raspberries
(Proceedings of the National Academy of Sciences, vol 96, p 5878). Both
species have been introduced to Europe.
The hybrids of these species found in different parts of Europe vary greatly
in structure and behaviour and often have reproductive abnormalities. This
suggests that the pathogen is a very recent creation and is still evolving. 鈥淚t
hasn鈥檛 stabilised into a single entity yet,鈥� says Brasier. 鈥淲e don鈥檛 know where
selection will take it.鈥�
Alders are a key species in wetlands and along rivers, where they stabilise
the banks, says John Gibbs, who has surveyed the damage in Britain for the
Forestry Authority. If the fungus reaches the US, which has many native alder
species, the losses could be immense. 鈥淲e don鈥檛 know yet if the American species
are susceptible,鈥� adds Brasier.