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Home • Bretziella fagacearum C519 v1.0
Oak wilt symptoms
Figure 1. Signs and Symptoms associated with oak wilt A. Dying red oak showing foliar wilt symptoms B. Crack in the bark indicating presence of a mycelial mat. C. Nitidulid beetle visiting a mycelial mat D. Grey colored spore containing mycelial mat and pressure pad. [Photo credit: Monique L. Sakalidis]
Endoconidia from culture (asexual spore state of Bretziella fagacearum)
Figure 2. Endoconidia from culture (asexual spore state of Bretziella fagacearum). [Photo Credit: Karandeep Chahal]

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.