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The Spread
of Oak Wilt
The environmental press is
filled with horror stories of despoliation and deforestation. Today's tale,
however, recounts a saga of human-caused forestation and its ruinous consequences.
It is set in Texas, but it can be applied to a broad zone extending north
to Minnesota and east to the Carolinas.
Some say our flora prospered
with the last Ice Age. The cooler and wetter regimen was hospitable to
plants that now inhabit the Colorado Rockies. The pollen of Fir, Blue Spruce
and Pine is well-preserved in the Ottine Swamp along the San Marcos River
near Luling. With the retreat of glaciation, the climate became progressively
warmer and drier and the vegetation changed accordingly.
Ten to nine thousand years
ago, conditions favored the spread of oaks. There are clonal groves of
live oak in the Hill Country which now cover acres, whose progenitor dates
from that epoch. Climatic change continued. The warming and drying trend
advanced. Forest fires, storms and floods left clearings in the woods.
Other open habitats developed when swamps or lakes dried. Now, instead
of reverting to forest, these clearings became the habitat of grasses.
For a while, the forest and grasslands existed in a patchwork, a dynamic
landscape called an ecotone. Gradually, the vegetational truce in this
transitional zone became unstable. Fires destroyed much of the remaining
forest. Grasses, quick to recover after a burn, prospered.
Some live oaks persisted
but in a reduced form. Through selection by fire, only those live oaks
which formed root sprouts remained within the prairies. The prairie realm
was not completely victorious. Woods remained on the steeper sites and
along rivers and creeks. This rich environment was home to many coexisting
species, including the aboriginal Americans.
The landscape was to be transformed
by another invasion. Vast changes would be precipitated by the arrival
of a Genoan adventurer funded by the Spanish Crown. The European conquest
of the New World would eventually alter the natural landscape of Texas.
The prairies in our area
were destroyed by overgrazing in the mid to late-nineteenth century. Once
the cycle of fires was extinguished, the root sprouts of oaks flourished.
The ancient clonal mottes, their arboreal tendencies suppressed for too
many centuries, recovered quickly. The native juniper, formerly occupying
the more broken topography, grew everywhere the birds sowed it. Mesquite
increased in density, though its range stayed the same.
Our present landscape has
a cohort of oaks beginning to show their age. Acres of them are physically
attached through root connections. In some parts, oaks are almost a monoculture.
There exist too many individuals of one species, too close together and
further, many of them are weakened by advancing age.
Given these conditions, the
rapid spread of an epiphytotic disease was inevitable. (Among animals,
such a contagion is said to be an epizootic disease. In the specific
case of the human animal, it is termed an epidemic disease.)
Ceratocystis fagacearum
is the fungal organism which causes oak wilt. There is a different fungus,
Cephalosporium
diospyri which causes another disease, oak decline. Some mistakenly
employ both disease names interchangeably. The oak wilt fungus is not a
recent visitor to our state. Informed opinion holds it has been here as
long as the oaks themselves. It is prospering because of the conditions
outlined in the above paragraphs.
It is spread by spores hitchhiking
on insects, through the vascular connections in root grafts and by human
agency. The reproductive organ, a spore mat, of this imperfect fungus can
develop only in red oaks and their relatives, not in live oaks. Prevention
of oak wilt is possible by injecting them with a systemic fungicide named
propiconazole. Very diluted (one part in five hundred) fungicide
is injected under pressure into holes drilled in the flare roots. A treatment
is thought to be effective for several years. Because of the expense,
only the most valuable trees can be saved. We have learned that once a
tree is infected, propiconazole applications are ineffective and an entire
motte will likely succumb.
The advancing wave of oak
wilt cannot be stopped. Neither rules and regulations, nor the expenditure
of public or private funds will ultimately make a difference. Selected
trees can be saved, but our landscape will be quite different in 20 years.
That future can be seen today in parts of Bandera, Kerr and Kendall counties.
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