|
OAK WILT
TREATMENTS - EXAMINING THE FACTS
By
Mark Duff - Texas Forest Service
Dr. David
Appel - Texas A&M University
08/20/00
Over the past
few years, claims have been made that oak wilt can be
cured or prevented by maintaining vigorous and healthy
oaks. One of the most popular claims is, that by keeping
oak trees in a healthy condition, their "immune systems"
will be "boosted" and therefore "resist" being infected
and killed by the pathogen that causes oak wilt. A
variety of products, ranging from water to organic
concoctions to fertilizers, have been recommended to keep
oak trees disease resistant. The best way to assess the
validity of these claims is by examining the facts about
oak wilt, tree health, and immunity.
Webster's
Dictionary defines immune as, "Having or producing
antibodies or lymphocytes capable of reacting with a
specific antigen." The indices of some of the most
popular college textbooks on plant pathology, plant
physiology, arboriculture, botany and plant biology, make
not a single reference to antibodies, lymphocytes, or
antigens in plants. However, every undergraduate animal
biology textbook, and other sources such as the Mayo
Clinic webpage, is filled with references on human
immunology. Simply put, immunity in the sense of a
antigen / antibody response is a phenomenon exclusive to
the animal kingdom. For example, animals can be injected
with a weakened or dead flu virus in order to prevent them
from developing flu symptoms. The vaccine does not make
the animal sick, but it introduces the immune system to
the flu virus. Memory cells are then placed on guard, and
are ready to produce antibodies when and if the full-blown
flu virus challenges the animal. To our knowledge, you
cannot inject or spray a weakened or dead oak wilt fungus
(or any other material, such as fertilizer) on a live oak
and expect it to produce antigens or antibodies that will
attack and kill healthy oak wilt spores when they
challenge the tree. So, there is no immune system (as a
physiological process) in live oaks to "boost" in order to
prevent oak wilt. However, immunity in plants could be
broadly defined as "exempt from infection". In other
words, oak trees don't catch Dutch elm disease, and elm
trees are exempt from infection from the oak wilt
pathogen.
The oak wilt
fungus is a vascular wilt disease that invades and causes
the plugging of the water conducting tissues of oaks, and
initiates wilt and almost certain death. It is commonly
known as a "primary" pathogen, capable of killing mature,
healthy oaks. The chestnut blight and the Dutch elm
disease are also classic examples of primary pathogens.
Hundreds of plant pathologists and foresters spent their
careers and millions of dollars in an effort to halt the
spread of these diseases. As with oak wilt, some measure
of success was achieved in control of the Dutch elm
disease. Unfortunately, not so for the chestnut blight.
Following my daily observations of oak wilt for nine
years, it has become clear that oak wilt does indeed kill
healthy, mature live and red oaks, just as easily as it
kills stressed ones. A good case can be made that the
higher level of vitality a tree has at the time of
infection, the more quickly the tree will begin it's
response of plugging it's vascular system, thus leading to
rapid mortality. So, the notion that keeping oaks healthy
to prevent oak wilt mortality is misleading, if not
untrue.
From a
general tree health perspective, it is a good idea to keep
trees healthy. Now this can be achieved with
supplemental water, organic concoctions (i.e. mulch,
mycorhizae inoculations), and fertilizers along with other
cultural practices. By keeping trees vigorous, they can
better defend themselves against secondary problems, such
as insect defoliations, hypoxylon fungus and drought
stress. For example, a healthy tree that has just been
defoliated by cankerworms, will have enough carbohydrate
reserves (energy) in its root system to leaf out again
without causing a decline syndrome (not to be confused
with oak wilt) that eventually could result in death to
the tree.
Foresters and
plant pathologists in Texas with the Texas Forest Service,
USDA Forest Service, and Texas A & M University have been
actively working on oak wilt control since 1977. Nearly
2.5 million feet (about 475 miles) of state and federally
cost shared trenches have been installed surrounding
nearly 1700 oak wilt centers in Texas since 1988.
Trenching severs the root connections (which are the major
conduit for oak wilt spread in live oaks) between adjacent
trees. The oak wilt is contained within the boundaries of
the trenches in about 66% of the cases. This was
determined by walking and evaluating these trenches
yearly. Tens of thousands of trees have also been treated
with Alamo fungicide (propiconizole). And while there are
failures, there are significantly greater numbers of trees
that have survived following these recommended treatments,
than if they had not been treated.
So, until a
better "scientifically" -determined treatment is found for
oak wilt, individual macroinjections with Alamo fungicide
and trenching to prevent tree-to-tree spread remain the
most effective (although not perfect) tools known to
combat this disease today. For a treatment to be
considered effective it must be based on science and
thorough research. The research needs to have results
that are replicable and be published in refereed
scientific journals and have withstood the rigors of peer
review. If the research fails to meet these criteria,
then claims cannot be made that they are "scientifically"
proven.
The best
approach to coping with oak wilt apart from total
prevention, may be to treat those individual high-value
trees immediately threatened by the disease with Alamo
fungicide, trench around expanding oak wilt centers where
feasible, and plant a great variety of native Texas tree
species that are resistant to oak wilt.
See TreeLife articles
located in
Oak Wilt Section on Info
Desk
|
