Trenching
plays a key roll in the
oak wilt suppression program, it is designed to circumvent
the disease center disrupting the interconnected tree
roots. Approximately 70 percent of all the oak wilt
trenches installed statewide have held the disease centers
in check, but it is important to understand that trenching
will never stop or slow the spread of oak wilt in the live
oak trees. However, keep in mind, the beetle is not
looking for stop signs or trenches, the next meal is the
only priority. Oak wilt is becoming more wide spread
throughout Texas, in the past 12 plus years Austin has
increased from 70 to over 250 oak wilt centers. That
represents a 350 per cent increase in the disease out
breaks. The number of counties have also increased from 55
to 68 in the recent years.These outbreaks are primarily in
high density populated areas.
Root Transmission:
Regardless, once a tree has the disease root transmission
occurs and it is spread from tree to tree through its
“community” root system. Live oaks grow in Motts of
an inter-connected roots system, they are basically one
tree, living on a common root system. This makes it
difficult to stop or control the disease. Removing the bad
apple does not suffice as the entire roots system of the
Mott is totally infected and it is necessary to remove and
destroy the entire Mott and surrounding trees within the
100 foot proximity.
The Beetle,
overland transmission: If beetles are found at
cracks or wounds in the bark on the Texas Red Oak it
could be a sign that Oak Wilt is not far away. The
fungus, Ceratocystis fagacearum, creates large areas of
mycelium(Spore Mats) under
the bark that invite beetles of the Nitidulidae family
to feed on the bark. This fungus, which is in the same
genus as Dutch Elm Disease (Ceratocystis ulmi)
distributes itself similarly. The Nitidulid beetles
carry the fungus on them and then transport the disease
to other healthy trees. Therefore, healthy trees that
have been wounded or have open cracks are targets for
beetles to spread Oak Wilt.
Nitidulid Beetle (Photo Courtesy
of USDA Forest Service)
Severing the
roots system is one way to stop or slow further
transmission. It is necessary to create a barrier
surrounding the infected trees. Once all of the infected
trees are located it is important to mark the secondary
trench at least 100’ from the last infected tree in order
to make sure the trench fully contains the infected roots.
A primary trench, another 100 feet beyond the
secondary trench and a third buffer trench 50-100 feet
beyond primary trench, is the best strategy to create a
safe zone that should be disease free. (See
Map below) Once the trench perimeters are
established a trench depth of at least 48” needs to be cut
through the roots and in certain cases 6 feet is
recommended.
 Click
on map to enlarge
Removal of the dead or severely infected trees is
of the highest priority in controlling oak wilt, as the
pathogen can remain in the root system for 3-4 years. Once
the water content in the roots drops below 38% the
pathogen is destroyed. Whenever and wherever it is
possible, the best strategy is to remove the infected or
dead trees as soon as possible, using a bull dozer to
uproot the entire Mott. As for disposal, its best to just
burn the uprooted trees as fire will destroy the oak wilt
fungus or pathogen.
In some cases it is not
possible to install trenches. It becomes
necessary to inject high dollar value trees to create a
barrier in-order protect other trees and stop the
progression of the disease. A therapeutic dosage rate or
20 mils per dia. inch is recommended for any suspect trees
or the large trees in close proximity of the disease
center. Annual re-treatment of these trees is also
recommended, as it takes about three years for a tree to
recuperate under perfect conditions. Some trees are just
beyond saving at the time of treatment. It is far better
to inject trees on a preventive basis rather than wait
until your trees become infected and retreat on a
bi-annual basis to protect the new growth wood. However,
some of the treated trees in the yard will survive. You
can also lose some of the otherwise survivable, treated
trees, if they aren’t watered during hot and dry summers.
It is important to deep water during these periods of
drought to help survival.
The application of
a herbicide to the tree stumps as a safety barrier:
The intention is to
kill the root system of the dying or infected tree root
system and stop the spread of oak wilt, which has proved
to be a more dangerous practice than beneficial. The
herbicide applied to the stump is absorbed into the root
system and continues dispersal through the transportation
across root grafts as a result of the movement of water to
transpiring healthy trees from the non-transpiring stump.
In parts of these root zones, all the oaks are grafted
together. Under these conditions the herbicide dispersal
can enlarge rapidly, and become as deadly as the oak wilt
disease itself. This is a practice by unscrupulous, and
uninformed tree services with the intentions for financial
gain only, are in need of very serious scientific
council.
A personal Note: From
the powers to be, our oak trees are not considered a
highly valuable commodity in Texas. Perhaps it would make
much more sense and a more feasible project to consider an
attack on the beetle, to ultimately control and suppress
oak wilt once and for all. Remember the fruit fly
epidemic in California, scientist came to the call and in
a very short period almost totally eradicate the fruit
fly. Why? Because the financial losses to the fruit
industry and other support related industries would have
devastated the entire California economy.
If you can eat
it, don't fix it.
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