Crawford County Home & Garden
Trees
Hypoxylon Canker
- Drought Related Death of Oak Trees
Oaks are the dominant hardwood tree species in Arkansas, and as a result of
this natural occurrence, are the most common large trees in home landscapes. The
selection of building sites is often due to the presence of large shade trees
such as the oaks. Over 30 species of oaks occur in Arkansas. Loosing one or more
of these large oaks can have a devastating effect on landscape.
A number of stress factors can influence the vigor and health of established
oak trees. Construction related root injury and drought are the most deleterious
of these. Construction damage may be as minor as removing the natural leaf mulch
and installing a lawn or as major as a soil fill. More severe treatments such as
filling over the roots or cutting roots during construction often leads to a
gradual decline and sometime, in the eventual loss of prized trees. Tree losses
following construction may be within a few months or it may extend over five to
eight years. Less severe construction related injury usually results in a
gradual decline in tree growth. In this weakened condition oaks are more
susceptible to secondary disease, insect or environmental problems that would
not normally be serious on healthy trees.
Drought episodes such as the one experienced in 1980 and 1983 have clearly
demonstrated the relationship between drought and oak mortality. Tree loss
increases for two or three years following a drought. Many of the problems are
ascribed to mild pathogens called "secondary" diseases, but such a designation
is of little comfort if a valuable tree is killed.
A fungus disease, called Hypoxylon canker (Hypoxylon atropunctatum),
has been associated with many of these tree deaths. A survey conducted in three
sites in the Ozarks found that over 57% of the dead trees showed symptoms of
Hypoxylon attack following the 1980 drought (E. Bassett, P. Fenn & M. Mead,
1982. Ark. Farm Res. (Jan. – Feb., p. 8). The disease is reported throughout the
state, occurring most severely on sites with poor soils or where construction
related injury has predisposed old oaks to drought stress.
Disease Development: Hypoxylon canker attacks all oak species
occurring in Arkansas, usually killing the tree within a few weeks to months of
first observed symptoms. Oaks of the red oak subgenus (red, black, black jack)
appear more susceptible to the disease than do members of the white oak subgenus
(white and post oaks). Only oaks are attacked by this organism. Trees in all
size classes are killed by the disease, but typical symptoms of the disease
occur only on the larger trees.
Though not conclusively proven, the disease is probably spread by wind borne
spores, which germinate and enter the tree through wounds and broken branches.
The sapwood of the tree is attacked by the fungus, spreading as much as 3 feet
from the infection site the first year. Trees apparently become infected during
drought periods but, if conditions improve, the tree will show no immediate
effect of the disease. The disease may lie dormant for 20 years or more in the
tree, only killing the tree with the return of the next serious drought. A
recent study isolated the fungus from 57% of the branches and 11% of the trunks
of healthy-appearing red and white oaks. This high rate of latent infection
guarantees that the disease will reappear the spring and summer after a severe
drought episode.
Symptoms: As would be expected from disease that destroys the trees’
water conducting tissue, the first symptoms of the disease are the yellowing and
wilting of upper leaves and tip dieback. These symptoms are easily missed. They
usually occur in the summer or early fall. The dieback symptoms progress down
the branches, finally killing major limbs. When a branch or limb dies the bark
is sloughed off, exposing a thin brownish, dusty mass of fungal spores called
conidia. It is common to find a pile of bark accumulating at the base of
infected trees. As the fungal mass matures its color changes to silvery, and
then black as the sexual stage of the disease develops. The disease may first
attack only a portion of the trees’ upper crown, seemingly leaving the remaining
section of the tree unaffected. However, the following year the disease usually
spread to kill the remainder of the tree.
Control: There is no known control for Hypoxylon canker once symptoms
begin to appear. Because the disease is internal and kills the sapwood of the
tree, fungicide sprays are completely ineffective.
Valued trees in home landscapes should receive additional care during drought
periods. Watering should begin before injury occurs. Apply one to two inches of
water per week during the summer. Fertilization in the fall or winter at the
rate of two pounds of 13-13-13 per inch of trunk diameter should improve the
vigor of drought stressed trees and make them less susceptible to disease
attack. The fertilizer should be spread on the soil surface beneath and 20%
beyond the drip line of the crown of the tree. If the tree cannot be watered,
fertilization should not be used.
Good sanitation practices should be followed with severely infected or dead
trees removed as soon as possible. The wood from the tree can be used for
firewood with little concern for further spread of the disease. If the tree is
allowed to stand after it dies, the fungus destroys so much of the sapwood that
the wood is of little value as firewood.
Prepared by Gerald Kingaman, Extension Horticulturist-Ornamentals
Back to
Trees
|
