Sudden Oak Death Pathogen - Phytophthora
ramorum
The genus Phytophthora
The name of the genus Phytophthora is
derived from the Greek “phyto” (plant) and “phthora”
(destroyer).
Phytophthoras were traditionally placed
in the Fungi but are now assigned to the Chromista kingdom. They are fungus
like microbes, grouped with pythiums,
both of which are considered water molds (oomycetes). Water molds are ecologically
favored by free water, being motile through waterfilms both in the soil and
on foliage.
There are currently some 60 described species of Phytophthora.
This genus is of great importance due to the severe impact it has caused historically.
The most significant example was the late blight of potato (P. infestans)
that caused the catastrophic Irish famine in the 1840’s. Another phytophtora,
P. cinnamomi caused one of the most severe tree epidemics in Australian
forests. P. Cinnamomi has the potential of parasitizing over 900 host
species as does the equally virulent P. parasitica. Phytophthoras
are of great concern due to their impact on food and fiber production, and
are the most often cited fungal pathogens in literature.
Many Phytophthoras have reached the status of major “epidemics”.
Epidemic is defined as “affecting or tending to affect a disproportionately
large number of individuals within a population, community, or region at the
same time”. These epidemics either occur because of invasion into a
new area, changes in agronomic practices, or environmental fluctuations.
Phytophthora ramorum
The organism responsible for Sudden Oak Death has been identified
as Phytophthora ramorum, a species
that has previously only been reported in Germany and The Netherlands on ornamental
rhododendrons and viburnums.
P. ramorum is an unusual Phytophthora; it is uniquely
different from the 60 described species. The sporangia are deciduous which
is characteristic of only 2 other species including the potato late blight
pathogen. This may be significant because it means the spores can be transmitted
through the air.
Most Phytophthoras that affect trees are
soil organisms which attack the roots. P. ramorum is unique because
it apparently spreads aerially by way of foliage on host plants.
Researchers suspect that the organism can survive
in rain water, soil and leaf litter. It infects foliar hosts (eg Bays, laurels)
and can then be splashed to the oaks. Once spores get into the trunk through
the bark, it destroys the cambium, girdling and killing the tree within months.
P. ramorum growing on V-8
agar in a petri dish (Source)
Optimal growth appears to be around 20 deg C (68 deg F). Movement
of the organism is believed to occur in the cool wet, months. Since its discovery
in the U.S. in 1995, P. ramorum has not
been located outside of Coastal California and Oregon (see map). The potential
range of the organism can only be predicted with a small degree of confidence.
(see US Forest service map).
Identification of P. ramorum
based on morphology is extremely difficult. There is overlap of morphological
characteristics across species as well as within species variation. Molecular
and biochemical techniques are being increasingly applied to aid in the process
of identification.
There is a great deal that is still unknown about the
pathogen that causes SOD. It is not known where it came from, how it spreads,
how serious it will become, or how to control it. Numerous studies are underway
nationally to learn more about this potentially dangerous disease.
For technical assistance with this page contact the Technology Assisted Learning Lab at (979) 862-3449 or via e-mail at prabha@neo.tamu.edu. For more information about Sudden Oak Death at Texas A & M University, contact Dr. David Appel at 979-845-8273 or by e-mail at appel@ppserver.tamu.edu; or Sara Service at 979-587-2701 or by e-mail s-service@neo.tamu.edu