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PHYTOPHTHORA, A DANGEROUS GENUS
Phytophthora
ramorum belongs to a large group of
pathogens that cause serious diseases in plants,
as the translation of the scientific name –
Phytophthora means “plant destroyer” –
indicates. The most infamous member of the
family is probably P. infestans, the potato
blight that devastated Ireland in the middle of
the 19th century.
Though most phytophthoras are soil borne
pathogens, attacking plants’ feeder roots, P.
infestans and P. ramorum – the pathogen causing
Sudden Oak Death – are airborne (as was the
fungus that caused chestnut blight, to which
they are not related). Knowledge of the
phytophthoras “has exploded in the last 10
years,” says plant pathologist Steve Oak. “A
large number of new species have been
discovered, described, named and renamed. It’s a
very difficult genus to work with.”
P. ramorum is a complicated
pathogen. First, it produces several kinds
of spores: a zoospore (or swimming spore),
that can be transported in streams; a
thick-walled chlamydospore (or resting
spore), which can survive in soil or plant
debris for a long time, out of the presence
of a host; and sporangia – sacs inside of
which zoospores are produced. Under
favorable conditions, sporangia release
zoospores, but they can also infect
directly. Second, it causes three distinct
diseases, depending on which host it
infects: ramorum leaf blight (brown spots on
the leaves of affected plants); ramorum twig
dieback; and the killing disease known as
Sudden Oak Death (SOD), which expresses
itself in bleeding cankers on trunks and
stems. Cankered trees sometimes survive for
several years, but once crown dieback
begins, leaves turn from green to pale
yellow to brown within a few weeks. Oaks are
“terminal”
hosts of SOD in two senses: they are killed
by the disease (there are a few diseased
trees in monitoring work in the West
that have survived for the duration of the
studies, but most infected trees have died);
and the pathogen does not produce spores on
the trees. In other words, infection isn’t
passed from oak to oak, but to oaks from
plants with one or both of the foliar
diseases. That’s why the abundance of
potential foliar hosts (in association with
oaks) in eastern forests makes them of
particular concern for SOD.
—EH
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