BP-28-W
Oak wilt is a serious, widespread disease of oak trees in Indiana, especially red and black oaks. There is no cure. However, proper care of oaks and knowledge of the appropriate preventive measures can keep oak wilt from spreading from diseased to healthy oaks through fresh wounds or through graft connections.
Red and Black Oaks
Oaks of this group, which includes red, black, pin, shingle, and many
other oak species, are much more susceptible to oak wilt than are
white or bur oaks. Infection of red or black oaks in early summer will
generally result in tree death by late summer. The first symptoms of
oak wilt occur in the top portion of the tree where leaves become
bronze-brown in coloration and then wilt. These initial symptoms
usually occur in June, but can appear as early as May. Wilted leaves
have the bronze or dull tan tissue beginning at the leaf tip and
edges. The bronze tissue progresses toward the leaf base, which is
green. The separation between bronze and green tissue may be
abrupt. Leaves that drop may be either totally green or the bronze and
green described above. The streaking of the sapwood, typical of wilt
diseases, can be found in the twigs. To find the brown streaks, remove
the bark and examine the sapwood surface. The brown streaks may not
be present in all trees; thus it is best to examine an actively
wilting and defoliating tree. Defoliation may begin at any time after
symptoms appear, and by late summer the infected tree may be bare of
leaves.
White and Bur Oaks
These oaks are much more resistant to oak wilt and the disease
progresses more slowly; thus, symptom appearance is of a more
"gradual" nature. They often remain healthy after surrounding black
and red oaks have been killed. (This factor helps diagnose the
disease.) Often, only single, scattered limbs will show symptoms and
the disease will progress downward for only a short distance in one
growing season. Leaf fall is generally not pronounced, and an infected
tree may survive several years before its eventual death. This
gradual dieback will result in dead limbs throughout the crown of the
tree and become more noticeable each year as the infection spreads.
Other Problems of Oak
Oak wilt may be confused with other disorders. For example, oak
trees are very prone to injury resulting from soil compaction, grade
changes, root restriction, etc. (See BP-37, Tree Decline).
Often these other problems are mistaken for oak wilt,
causing unnecessary concern and/or the use of improper control
measures. Also, many older oaks are killed by an apparent complex of
factors involving drought, insect borers (especially the twolined
chestnut borer), and root decay. Thus , identifying oak wilt often
requires expert assistance. Contact your local Extension office for
help in submitting samples to the Purdue Plant Pest Diagnostic Clinic.
Select several 4- to 8-inch sections from freshly wilting branches
(not dead ones) about finger-width in diameter. Wrap in dry paper
toweling, place in plastic bag, and keep cool and dry. Mail early in
the week so there is little time for deterioration.
Cause
A fungus, Ceratocystis fagacearum, is the cause of oak
wilt. The fungus enters the tree through fresh wounds or by root
grafts connecting healthy and diseased trees. Once inside the tree the
fungus permeates the vascular system, causing the water-conducting
units of the plant to become plugged. Water and nutrient flow is
disrupted, causing the oak to wilt.
Knowledge of how oak wilt spreads is important since the control of oak wilt depends upon preventing its spread. There are two ways the oak wilt fungus moves from a diseased to a healthy tree -- root grafts and sap feeding beetles.
Root grafts occur when roots of nearby trees grow into one another, forming a connecting bridge between the two trees. Root grafts often unite red and black oaks growing within 50 ft of one another. Root grafts are much less common between trees in the white-bur group, and never occur between the two oak groups.
Sap feeding beetles, especially "picnic beetles" can pick up spores of the fungus on their bodies or mouth parts during feeding. The beetle may then carry the spores to fresh wounds on healthy trees. This often starts the first infection in new areas.
Once a tree becomes infected with oak wilt there is no cure. The only way to stop the disease from occurring is to prevent its spread from diseased to healthy oaks. The following suggestions will help in the prevention of oak wilt
(1)Wound Prevention and Treatment. Fresh wounds represent one way the fungus enters healthy trees; therefore, caution should be used to avoid wounding when working in the vicinity of oaks. If wounding does occur, immediately treat the wound with an appropriate tree wound dressing. Pruning of oaks should be done in the winter if feasible and only after mid-summer. Do not prune in spring and early summer when insect activity is high.
(2)Root Grafts, Their Prevention and Disruption. The only way to prevent root grafts is to plant oak trees more than 50 feet apart. When trees are already planted within 50 feet of each other all possible root grafts should be disrupted between healthy and infected trees, or trees suspected of being infected. Root grafts can be disrupted either mechanically or chemically; choose the method best suited to your particular situation. Cut or kill roots midway between the diseased and healthy trees. If possible, the zone of disrupted roots should be made well beyond the spread of branches. If sidewalks or other obstacles are in the way, extend the zone of disrupted root grafts along and on the other side of the obstruction so that all root grafts are removed where possible. Oak trees that are within 50 feet of a tree infected with oak wilt may not show symptoms of the disease, but may be infected; it is, therefore, best to also disrupt root grafts between such suspect trees and neighboring oaks. Remember--root grafts do not occur between red-black oak and white-bur oak group.
Mechanical Disruption. Roots may be cut by digging a narrow trench or by passing a root cutting blade through the soil between the healthy and infected trees. Whichever method is used, it is important that all roots be severed to a depth of 3 to 4 feet. If buried pipes, power lines, telephone cables, sidewalks, or other obstructions are present which interfere with digging or cutting, it would be best to use chemicals to disrupt suspect root grafts.
Chemical Disruption. The simplest method of disrupting root grafts is with chemicals. Certain chemicals used to kill roots are hazardous to both human and plant life and can be used only by professional applicators such as arborists or licensed nursery operators. Contact a professional applicator if you wish to disrupt roots with chemicals.
(3)Tree Removal. Once a tree is infected with oak wilt, a fungal mat is often produced beneath the bark. This fungal mat is often the source from which sap beetles pick up spores and thereby spread the disease. To help prevent such fungal mat formation, completely girdle diseased red oaks and black oaks through the outer sapwood as soon as possible. It is best not to remove the tree until winter if surrounding trees may be wounded in the removal process. Fungal mats are produced only rarely on white oaks; therefore, girdling is not necessary. Diseased trees can be used for firewood or harvested for lumber in areas where oak wilt is already present However, process the wood as soon as possible. Firewood should be split; stacked off the ground in a single tier in a dry, open area; and protected from moisture to hasten drying.
Sanitary Pruning. For trees in the white and bur oak group, sanitary pruning of infected limbs might save the tree. At the first indication of oak wilt, remove the infected branch. Make your cut as close to the main trunk as possible to avoid leaving infected wood behind. Treat the wounds promptly and fertilize the tree to increase vigor. Clean and disinfect the cutting tools before trimming each branch or another tree.
REVISED 6/90
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