Health Protection, Southern Region
caused by Ceratocystis fagacearum
Importance. - Oak wilt is the most destructive disease
of oaks in the United States. It also occurs throughout most of the South
and can kill oaks rapidly, causing heavy losses. Red oaks are affected more
frequently and severely than white oaks.
Identifying the Fungus. - The fungus can be identified
in the field by the presence of fungal mats which form cushions under the
bark of infected trees. However, these fungal mats are infrequently found in
the South. Identification can also be made by observing laboratory isolates
of the fungus.
Identifying the Injury. - Symptoms are bronzing or
browning of green leaves from tips and margins downward toward the leaf
base, premature defoliation, and eventually death of the tree. The red oaks
develop symptoms over the entire crown shortly after infection, but white
oaks develop symptoms slowly, a few limbs at a time.
Oak wilt infected tree.
(Click image for detail. JPG 37K).
Oak wilt symptoms on live oak.
(Click image for detail. JPG 28K).
Biology. - The wilt fungus is favored by moderate
temperatures. It spreads from infected to noninfected trees through root
grafts. In addition, insects can carry spores of the fungus over long
Control. - In the forest, kill infected trees with
silvicides to reduce inoculum and prevent root graft transmission of the
disease. In urban areas, sanitation by removing infected trees and trenching
to eliminate root grafts will minimize the losses. Effective treatments also
include the use of the fungicide Alamo.