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Tree Disease
Oak Wilt Disease | Dutch Elm
Oak Wilt Disease: Prevention and Management
What is Oak Wilt?
Oak wilt is caused by a fungus called Ceratocystis fagacearum. Once inside a healthy tree, the fungus quickly
travels through the water conducting tissues (xylem) located just beneath the bark. The tree's internal defense
system begins to fight off the fungus by producing gums and resins (tyloses) which plug the water conducting
vessels. Unable to contain the fast moving fungus, the tree eventually cuts off its own internal water supply.
This causes the tree to wilt and die.
All species of oaks are susceptible to this disease. Northern red and northern pin oaks (Red
Oak Group) are highly susceptible to oak wilt and do not recover once infected. The initial symptoms of oak
wilt are easily identified in red oaks by the rapid wilting of the leaves in the upper crown of the tree.
Wilting usually begins in the uppermost part of the crown and spreads downward. Individual leaves generally
wilt from the tips to the bases with the edges turning pale green and then light tan or brown. Fallen leaves
are often still green at the base (Figure 1). Once the initial symptoms appear, red oaks generally wilt completely
within several weeks. Also, when oak wilt symptoms are first noticed in red oaks, the fungus has already
spread throughout the tree and into the root system.
Bur and white oaks (White Oak Group) are somewhat resistant to oak wilt. If white oaks become
infected with the disease, they generally wilt very slowly, dying one branch at a time. Leaf discoloration of
infected white oaks can resemble autumn colors. White oaks can live with the disease from 1 to 20 years, or in some
cases, they may actually recover.
How Does Oak Wilt Spread?
Overland Infection
The oak wilt fungus is spread two ways. Initially, healthy oaks can be infected above ground by several species of
sap beetles of the family Nitidulidae. These beetles are attracted to the fruity-smelling spore mats produced by
the fungus between the bark and wood of red oaks that died from the disease.
These spore mats generally develop between April and July on red oaks that wilted the previous
summer. They produce pressure pads that crack open the bark,
providing the beetles access to the spore mats (see
figure to the left). The sticky fungus spores adhere to the beetles when they visit the mats. Several species of sap
beetles are also attracted to fresh wounds on healthy oaks during the spring and summer. Sap beetles infect
healthy oaks with the disease as they inadvertently deposit the fungus spores while they feed on fresh open tree
wounds. Although this activity by sap beetles accounts for less than 10 percent of the spread of oak wilt, this
is the only way new infection centers get started. Once inside the tree, the fungus quickly travels through the
xylem tissues of the tree and into the root system.
Not all oak wilt-killed red oaks will produce spore mats the following spring. If an infected
red oak wilts early in the growing season, it may produce the spore mats in autumn of the same year, or
it may dry too fast to produce them at all. Consequently, these trees are not considered infectious for the
overland spread of oak wilt the following spring. Actually, only about one-third of infected red oaks will
be infectious for the overland spread of the disease. Infectious red oaks are referred to as Potential
Spore Producers (PSPs).
Root Graft Infection
The other way the oak wilt fungus is spread is underground through the root system. When oak trees of the same
species are growing close together (within 50-60 feet), they generally will form a common root system as roots
entangle and graft together. The fungus travels rapidly through these grafted roots, infecting healthy oaks
up to sixty feet away. Fungus transmission through root grafts is responsible for approximately 90 percent of
the spread of oak wilt.
Managing the Spread of Oak Wilt
Prevention
The overland spread of oak wilt can be controlled simply by not wounding or pruning oaks from April to October.
If trees are accidentally wounded or pruning is unavoidable, cover the wounds immediately (within minutes)
with a non-toxic wound dressing. Remember: The only safe period for pruning oak trees is from November
through March.
Root Graft Disruption
Since over 90 percent of the spread of oak wilt occurs through root grafts, this disease can be controlled. By
severing these root grafts, the disease can be isolated in the infected trees and prevented from moving into
healthy trees. This has been shown to be a very effective method of controlling the spread of oak wilt. Root
graft cutting is accomplished by using a vibratory plow with a 5-foot blade (see figure below). Please keep in
mind that removal of the infected trees before root graft disruption will actually hasten the spread of the
disease through the root system. Remember: Cut the Roots BEFORE You Remove the Trees!
Chemical Treatment with Alamo&#reg;
Recently, a systemic fungicide has been successfully used to help control the spread of oak wilt disease. The
fungicide propiconazole (Alamo®) is injected into the root flares below grade via the macro-infusion process.
It has been shown that the use of chemical injections in addition to vibratory plowing can lead
to better control of the disease and can save more valuable oaks. Fungicide injections may be the best option on
sites where root cutting is not possible or when only a single oak tree is at risk. The chemical does not
have a long-term record as a barrier against the spread oak wilt beyond the trees being injected. Therefore,
the use of chemical injections should be used in conjunction with vibratory plowing whenever possible.
Bur, white, and red oaks that are within root graft distance (50-60 feet) of trees infected
with oak wilt are at the greatest risk of getting the disease and can be injected preventatively to save the
healthy trees. Diseased bur and white oaks can also be injected therapeutically, but success will depend on
how soon the injections take place. Remember: wilting red oaks cannot be saved with chemical injections.
There are several private contractors in the Twin Cities metropolitan area that offer this
service. They should be contacted for more detailed information on the use of chemical injections for oak
wilt control.
 Oak Wilt in Minnesota brochure from the University of Minnesota
Dutch Elm Disease
Dutch Elm Disease is back
2004 has been a tough year for trees in Cottage Grove. Dutch elm disease has killed an
estimated three to four times the number of trees in Cottage Grove in 2004 compared to a typical year. The
final losses for the year in the metro area communities that have a higher percentage of elms are much worse
than in Cottage Grove. The trend is expected to continue the next several years.
Dutch Elm Disease
The easiest way to tell if a tree has Dutch elm disease is sudden and severe wilting. Leaves
typically begin turning yellow then brown high in the crown of the tree. The disease is caused by a fungus that
infiltrates the vessels that circulate sap throughout the tree. Over a period of several weeks to a few months,
the fungus travels down the infected branch to the main stem causing all of the leaves to eventually turn brown
and wilt. The fungus, which grows like yeast within the vessels of the tree, causes the elm to produce a toxin
that gums up and blocks the vessels. The blocked vessels are no longer able to carry water to the leaves, which
causes the leaves to wilt and the tree to die.
Dutch elm disease is carried from tree to tree one of two ways:
- By elm bark beetles that pick up the fungus on their backs as they tunnel through an infected tree and carry
the fungus to uninfected elms as they continue to feed.
- Spread through the shared and grafted root systems of adjacent elm trees.
While it's heartbreaking to have to cut down a beautiful mature elm tree, early identification
and prompt removal of elms infected with Dutch elm disease is the best way to slow the spread of the disease
through Cottage Grove's urban forest. With the large number of trees that have been lost to Dutch elm disease
this year, it is important to remove diseased trees. After removal of elm trees, wood shouldn't be stored with
bark on or transported. Trees with bark on them should be buried or burned. Once the bark is removed, the
wood is find to keep and so is the bark. The Cottage Grove compost site will
take Dutch elm diseased trees and dispose of them properly, you can call the NRG Compost site at (651) 458-1026.
If you have any other questions or concerns about trees in your neighborhood please call
Public Works at (651) 458-2808.
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