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The Trail Companion
Fall 2000
Oak Mortality Syndrome
A report from the Santa Cruz Mountains
Bioregional Council meeting, Sept. 28,
2000
by Jeffrey A. Caldwell
Dear friends,
That is the new name
for what has been called "sudden oak death".
Today I went to the
meetings of the Santa Cruz Mountains Bioregional
Council, which included a panel presentation and
discussion: "Sudden Oak Death -- Dealing With a
Potentially Devastating Forest Disease, and Lessons
Learned from Pine Pitch Canker". The panelists were
David Rizzo, the plant pathologist who recently
identified the water mold causing the disease, Steve
Tjosvold who conducts local field research on it, and
Rick Hawley who studied the effectiveness of control
methods for the unrelated Pine Pitch Canker
disease.
There are about 60
known species of Pytophthora, and the one
causing the disease had not previously been
described. As recent news stories tell us, it seems
to be related to Phytophthora lateralis,
which has caused losses of many Port Orford Cedars in
recent decades. The common name for
Phytophthora is "water mold" and it is related
to the brown algae, but basically acts like what is
commonly known as fungus, so everybody calls it a
fungus. It is believed to be either exotic [possibly
from the Orient] or a new hybrid. Exactly how it
spreads is not known, though the organism has a
waterborne stage, and mud is a major suspect. It
seems somewhat like other species in the
Pytophthora genus that may be vectored by
blowing in the wind in a dormant stage. If that
proves to be so, it is very bad news, impossible to
contain it.
It seems to grow best
at temperatures of about 50 to 60 degrees, hardly at
all over 80 to 90 degrees.
It took so long to
figure it out because usual methods of taking samples
and submitting them for analysis were too slow, and
the organism died before an attempt was made to
culture it. Dr. Rizzo figured out what it was when he
went out, collected a fresh sample and immediately
drove back to his lab to culture it. The ususal
method of taking a sample, keeping it a day, putting
it in the mail, etc. resulted in the organism
responsible for the disease dying before the culture
was attempted.
It is not believed to
be spread by the beetles; they come after the disease
develops. The mystery is that it attacks always above
ground, low on the trunk of the tree. Can't figure
out how it gets there. One theory is animals rubbing
the bark. It was noted, if so, feral pigs are really
bad news!
Worst case scenario is
that it could spread to all the black and red oaks
thoughout North America.
Best case scenario is
that some freak event infected all the trees now
dying several years ago. The "sudden death" is noted
when the foliage begins to turn color. But I saw
coast live oaks in China Camp State Park that were
cracked with the black domes of Hypoxlyon
thouarsianum fungus [one of the secondary
attackers typical of the syndrome, which involves a
number of native diseases and pests going after the
weakened trees] well up their trunks which still had
green foliage. It may be that the coast live oaks can
hold off the Phytophthora longer than the
tanbark oaks; that has proven to be the case in
greenhouse studies where young trees were
deliberately infected.
The Port Orford Cedars
are being protected in part by preventing access to
them in the rainy season, particularly keeping
vehicles on dirt roads from getting near them, which
seems to be preventing new infections, though it is
difficult to get cooperation from the public; some
people crash through the barriers.
It is now believed
that any infected wood should be allowed to dry out
and shouldn't be moved any further than necessary.
The previous recommendation to tarp it with clear
plastic is considered possibly a bad idea that would
favor the disease more than hurt it. A new term that
came up is "waste shed" -- infected wood probably
shouldn't be allowed to leave a watershed. The
infection is almost entirely in the bark and cambium,
and only in the aboveground parts. It may sometimes
enter the wood just a little, never more than an
inch.
The discussion was
largely about what we don't know. The causitive
organism was only discovered in June, and never seen
before. The disease is basically only really active
in cool, wet weather, so we will begin to learn much
more as we get into the rainy season.
What we don't know:
- Is is native or exotic? [though it certainly
seems to be exotic]
- How is it dispersed? How far?
- Does it survive in soil or litter?
- Does the pathogen survive in dead wood?
- Does the pathogen survive in chipped wood?
- How long does it actually take to kill a
tree?
- How does it interact with other diseases and
pests to kill a tree?
- Will fungicides control the pathogen?
- What will the ultimate mortality be and what
species will it spread to?
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