The Rural Voice, 2019-04, Page 53the early 20th century.
Oak wilt is cause by a fungus,
originally known as Ceratocystis
fagacearum and now (since late
2018) known as Bretziella
fagacearum. It grows in the vascular
tissues of oak trees, resulting in
clogging of these tissues which
blocks the flow of moisture and
nutrients, eventually causing wilting,
browning and thinning of the foliage
and eventual death of the tree.
Browning of foliage usually starts at
the tips of the leaves and works its
way back towards the base.
The disease can be spread in
several ways. Trees growing adjacent
to one another often form root grafts,
which are an ideal conduit for spread
from tree to tree. Sap beetles are a
primary means of spread in the
northern part of the range. The
fungus produces sticky, sweet-
smelling, sporulating mats on
infested, dead or dying red oak trees.
Nitidulid beetles (aka sap beetles) are
attracted to these mats and can then
carry spores to infect healthy trees
that have fresh wounds with sap
present that also attracts the beetles.
In the southern part of the range, oak
bark beetles are thought to be another
vector. Also, the disease range can
expand by movement of infected
nursery stock or firewood and logs
cut from infested trees.
Oak wilt affects all species of oak
but the red oak group is more
susceptible and usually die once
infected. The red oak group includes
species that have pointed leaf lobes,
e.g. northern red, pin and black oak.
White oak species are somewhat less
susceptible, usually taking several
years to die or sometimes recovering.
The white oak group are generally
those with rounded leaf lobes and
include white, bur and swamp white
oak.
There is really no control;
however, high value trees may be
treated by injection with a soluble
fungicide, similar to the treatment
used in elm trees to ward off Dutch
elm disease. The treatment is costly
and must be repeated every couple of
years.
The best option for control is to
prevent spread of the disease
wherever possible. An obvious, but
challenging option is to restrict the
movement of infested nursery stock,
firewood and logs. Other options are
to avoid any activities such as
pruning that may cause wounding of
oak trees during the time of year
when there is likelihood of spread,
i.e. from April to October, or even
more particularly in the mid summer
period (May to July). Of course
there’s nothing that can be done to
prevent wind storm damage in that
period that results in sappy wounds
attractive to the spore carrying
nitidulid beetles. One approach that
may reduce potential for spread is
immediate coverage of wounds with
wound dressings. Another option is
removal and destruction of infested
trees to reduce the amount of
available infectious material.
Unfortunately this disease may be
one more addition to the ever-
growing list of issues that continue to
stress our forest health and
diversity.◊
April 2019 49
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