Bretziella fagacearum (Bretz) Z.W.deBeer,
Marinc.,T.A.Duong & M.L.Wingf. (Syn= Ceratocystis
fagacearum) is a fungal pathogen that is responsible for the
widespread decline of oaks across the United States, particularly
in Midwestern and Eastern USA and in Texas. It causes a disease
known as oak wilt (OW) (Fig. 1). It is particularly devastating to
trees in the red oak group, which can succumb to this disease
within four weeks of infection. Spread of this disease is rapid and
occurs on multiple fronts from root-to-root transmission, insect
transmission, and sporadic long-range infections due to movement of
firewood. Long-range spread is difficult to predict but most
recently firewood was linked to increasing the north-eastern limit
of B. fagacearum by 300km.
B. fagacearum is the only member of its genus in the
Ascomycete family Ceratocystidae; it is one of the few true
vascular wilt diseases that occur in trees. An infected tree is
often first noticed due to a sudden drop or browning of leaves in
the summer months (Fig. 1A). Leaves may be brown, somewhat bronzed,
or partially green. Often leaf tips and margins will be bronze or
brown whilst the leaf base will remain green. B.
fagacearum causes rapid death of trees via its ability to
propagate rapidly through the water transport (xylem vessels)
system of the tree. The rapid production of endoconidia (the
asexual spore state) (Fig. 2) and the efforts of the tree to
compartmentalize the fungus (through the production of tyloses)
results in the clogging of xylem vessels and the classical wilting
symptoms observed on the leaves of the tree (Fig. 1A).
Six to 12 months after the tree has died the fungus will complete
its lifecycle and produce spore containing mycelial mats (Fig. 1D)
on the dead tree. These mats form under the bark and as the mats
mature, they produce specialized called “pressure pads”
(in the center of fungal mat) that exert pressure outward causing
the bark to split (Fig. 1B) and thus, provides a route for insects
to reach the mycelial mats (which produce spores at time of
maturity). These mycelial mats have a distinctive odor that makes
them attractive to a variety of beetles (Fig. 1C) that will feed on
the mat then fly to other mats or fresh tree wounds (through which
the fungus then enters the tree and starts the infection process
anew). Infection via beetle activity is known as overland spread.
This form of spread is what creates new infection centers.
Approximately 90% of spread is attributed to root-grafts forming
between oak trees. This form of below-ground spread creates
outwardly expanding infection pockets.
The decline of oaks is an aesthetic blight across the landscape
and also causes devastating ecosystem damage. There is no natural
resistance to this disease in the red oak group and thus the only
means of preserving these valuable trees is to mitigate the spread
of OW to new locations and reduce inoculum load in known OW
positive locations.
