6 Wood Discoloration

8.1 Fungal Damage to Living Trees 171
fungus can both infect red oaks (Quercus falcata var. pagodaefolia, Q. rubra,
Q. shumardii)andwhiteoaks(Q. alba, Q. bicolor, Q. macrocarpa, Q. michauxii,
Q. muehlenbergii). Red oaks become systematically infected and die quickly,
mostly within the year of first wilting symptoms and sometimes within a few
weeks after infection. The economically more important white oaks are more
resistant and show the damage often being restricted to just a few branches.
The lower susceptibility of the white oak is attributed to smaller earlywood
vessel diameter, more intensive tylosis formation resulting in a slower spread
of the fungus in the tree and the ability to “bury” infected tissue by a new
annual ring.
The infection usually occurs via root graft transmissions between the diseased
and healthy trees (Fig. 8.5a), so that the distribution is low with 1 to 2 m
(maximum 8 m) per year. Local spreading via root grafts can be inhibited by
ditches. The fungus invades the vessels of the youngest two annual rings and
stimulates the adjacent parenchyma cells to tylosis formation. Thus, wilt and
defoliation occur in the undersupplied crown regions. Additionally, wilt toxins
are produced. The leaves become flabby and discolor, are light green from the
edge, and later bronze-brown in red oak and pale-light brown in white oak.
After tree death, the hyphae grow inward in the sapwood as well as outward
Fig.8.5. Transmission of the Oak wilt
fungus, Ceratocystis fagacearum, via
root-grafts (a), during maturation feeding
of bark boring beetles (b), and from
sporulating mat by sap feeding nitidulids
(c) (from Rütze and Parameswaran 1984)
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