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Feature ArticleJune 29, 2001

Controlling oak wilt

By Jana Albers, DNR Forestry

It's been ten years now since we first began our cost-share programs for oak wilt suppression. So the time seems to be ripe for tallying our accomplishments. Minnesota's cost-share programs have proven that we can do something to control oak wilt and be able to measure our accomplishments at the local level and at the landscape level.

Have we made any progress?

Success can be measured in a number of ways:

1. Here's a concrete way to measure success: look at the basic numbers.

In the last ten years, 6976 acres have been treated. That amounts to 31% of the known acreage. Statewide, 15,359 acres still remain infected. See table below.

Oak wilt is a serious problem in the sandy-outwash plains that make up the Anoka Sand Plain. Here there are expanses of oak monotype and they're interconnected by root grafts. Once oak wilt gets a toe-hold, it runs rampant through the monotype.

In the southeastern counties, the disease situation has hardly changed in ten years; the number of infection centers remain low and in some counties, acreages are decreasing. In most of the region, oak wilt does not pose a significant threat because of soils and cover type. On the dry outcrops, where pin oak becomes infected, an infection center may enlarge to about an acre before being extinguished, usually running out of oaks to infect. However, there are two problem areas, one near Wabasha and another near Rochester. In both situations, oak wilt is a problem on sandy, outwash plains, just as it is in Anoka County.

Note: the most recent statewide data is from 1998, other data for subsequent calculations is from 2000.

Acres of oak wilt
County Controlled Active
Anoka 3182 5712
Chisago 230 386
Dakota 1128 2639
Fillmore 3 41
Goodhue 12 114
Hennepin 0 47
Houston 7 9
Isanti 330 589
Mille Lacs 0 7
Olmsted 51 254
Pine 0 1
Ramsey 509 242
Scott 0 7
Sherburne 1324 3129
Stearns 3 41
Wabasha 20 150
Washington 175 1915
Winona 2 70
Wright 0 16
Total for state 6,976 15,359

44 township area

For the remaining "successes," data from the original 44-township area was used because the 1988 survey data could be compared to the 1998 survey data (a "before" picture compared to an "after" picture).

2. How many acres have we protected from the threat of oak wilt?

Spore Density Changes - chart

From a single infection center, insects can carry oak wilt spores up to 1500 feet away. If we plow and cut down all the oaks inside the plow line, we've removed the threat of oak wilt for all oaks within 1500' of that tree. So its fairly straightforward, where there's a single infection center and all the spores are eradicated/ extinguished from that circular area. Then we can simply add up the acres to get the number of acres protected.

But where there are multiple infection centers, control actions can only reduce the spore load. As the number of infection centers treated increases, the spore load decreases. The graph shows an example from 1996 for several townships in Anoka and Ramsey Counties. By 1998 in the 44 township area, the spore load was reduced on 54,800 acres (10%) of the residential and forested acres.

3. Progress can also be measured by the rate that the disease intensified in a given area over a period of time.

chart - oak wilt intensification

During the ten year period, the local rates only increased from 0.4-fold to 4-fold. See chart. So what does this mean? Was oak wilt controlled or not? Ideally, we would want to compare our treated population with a "wild"/ untreated population. The rate for the Sherburne National Wildlife Refuge can be thought of as the untreated (wild) rate of intensification since no disease control was practiced there.

At the SNWR, the rate of intensification had a 38-fold increase in number of new infection centers. Comparing this rate to the counties' rates, control efforts in all counties successfully slowed disease intensification. Rates in all counties were 10-fold to 100-fold less than the rate for the SNWR.!! Note: Intensification rates for each county appear next to its name in the legend.

Another way of looking at this is to use the Sherburne National Wildlife Refuge's rate of intensification to project the number of infection centers we would now have if the cost-share programs never existed. See table.

Projected acres of uncontrolled oak wilt intensification compared with actual acres

Formula: starting acres x SNWR rate of intensification = projected amount in 1998
County Projected number of acres for 1998 without control Actual number of acres in 1998 with control
Anoka 51,338 ac 5712
Chisago 9,766 386
Isanti 9,804 589
Sherburne 19,304 3129
Washington 20,938 1915

4. Treatment methods improve program effectiveness.

To control oak wilt, a deep vibratory plow line is made to cut root grafts so the fungus can't spread from infected to healthy trees via root grafts AND the oaks inside the trench should be cut down and properly disposed so the fungus can't produce fruiting bodies and be spread overland by insects. Unfortunately, this is often not the scenario that takes place. Homeowners want their high value oaks inside the plow line to live as long as possible and promise to take them down "as they die" from oak wilt. This is "Slow removal". You can see the problem already. In some instances, only half of the prescription is taken because the other half of the medicine is too bitter.

Cut-to-the-line saves program control costs, actual data.
Routinely use CTL Number of sites controlled Number of treatments Number of treatments per site
Isanti Yes 144 216 1.50
Chisago Yes 200 272 1.36
Sherburne No 617 1499 2.43

Some counties and communities are now strongly encouraging homeowners to take the full dose right away, cutting to the line (CTL) the same year that the plow line was installed. In heavily forested areas, the effectiveness is increased 25-fold, in heavily urbanized areas, it's still a 15-fold increase. The increase in efficacy depends on how much forested land exists close to the infection centers. See table. By strongly encouraging homeowners to cut-to-the-line, there's a greater area protected and a much better return on control dollars spent.

Projected acres protected by control actions, based on 1999 infection centers
Acres of active oak wilt If plowed and CTL If only plowed How much better
Isanti 589 16,347 567 28 times
Chisago 386 12,621 486 26 times
Sherburne 3129 46,174 3298 14 times

5. Success can also be measured on the statewide level.

One of the statewide goals is to restrict or contain the disease to areas where it already exists. In reality that is expressed by slowing the disease spread into new areas. In the last decade, the northern "front" of the disease has only moved north 5-7 miles. The disease front has remained fairly static in the south and southwest, but is spreading west at 12-14 miles per decade along the interstate as oak woodlots and forests are developed for residential use.

6. Success depends on public education.

For several years the jingle, "Don't prune in May or June", appeared on billboards and was heard on the radio in the spring. Local contacts with affected homeowners also tops the list for effective public education. These were extremely effective in educating the public - very little oak wilt was spread out of the Anoka Sand Plain area and this can be credited it to an informed public.

7. Summer storms wreak havoc on control programs.

effects of storms - chart

Tornados and straight-line winds can set back oak wilt control efforts because they create open wounds during the time when insects can carry the disease to them. The number of oak wilt infections increases more rapidly than it normally would. This was the case in Sherburne County which was hit by severe storms in May, June and July of 1997 and 1998. Storm damage essentially tripled the number of infection centers as detected in July of 1999 by aerial photography. See graph.

What has it cost?

Over 120 counties and communities have participated in this program. During the last ten years, counties and communities developed excellent local programs with pass-through dollars from the DNR. $ 5.27 million was spent on the cost-share programs. See table. If you include all dollars spent, it cost $755 per acre to treat oak wilt infection centers.

Summary for oak wilt cost-share programs 1991 to 2000
State and federal sources Community and private matches
Federal CSP $ 2,071,000 $ 1,443,300
Minnesota ReLeaf 912,700 845,000
Subtotal 2,983,700 2,288,300
Total $ 5,272,000

Gazing into the future

Now, let's gaze into our crystal ball to see what the next ten years could bring us.

1. Local disease intensification

projected increase in acres - chart

Here are the projected new (additional) acres of oak wilt if the community cost share programs continue to hold oak wilt intensification to its same rate. See graph. Even this modest increase could easily outstrip current programs' abilities to handle the workload. Note: New (additional) acres are listed next to county name in legend.

2. Regional spread into next tier of counties.

projected oak wilt territory in the next 10 years

Oak wilt will spread regionally because it is easily carried to new, distant locations by transportation of fresh, infected logs. And there is lots of movement, especially firewood after storm damage. The average spread of oak wilt in the last decade was five to seven miles north. The above image is a modeled picture of the territory oak wilt could move into in the next 10 years. Even then, there are more than 1/4 million forested acres in this region that are susceptible to oak wilt.

3. Statewide spread threatens all our oaks.

range of red oak

Range of Red Oak

Again, oak wilt can be carried long distances by transportation of fresh, infected wood. In Aitkin County, for example, a cabin owner brought infected firewood from his home in the Twin Cities and started an oak wilt infection center that expanded for several years before it was extinguished.

High risk areas are

  • where oaks grow on sand, and,
  • where rapid development is occurring in oak woodlots, both on suburban and recreational lands.


The most important messages are that cost share programs control oak wilt and they slow its spread.

We need to realize that this disease is capable of rapid, local intensification and is capable of spreading to new, distant locations. It is a complex disease with complex management. It's a challenge that I'm sure we can meet, given past accomplishments. Here are some suggestions:

  • Continue to support fundamental research to find new control methods, including the use of systemic fungicides on individual, high value oaks and silvicides in more remote areas where vibratory plowing is impractical.
  • Encourage community and county governments to build self-sustaining programs to lessen their dependence on state cost share funds.
  • Encourage private vendors, communities, counties and other cooperators to plan for an increased workload and demand on their time and expertise.
  • Make better use of communication technologies to keep spreading the message to home owners, builders and developers.
  • Seek new opportunities for incorporating the oak wilt message into existing educational efforts, such as during Tree Inspector, Woodland Advisor and Woodland BMPs Workshops.
  • Seek to involve new partners in promotion and on-site detection, such as land surveyors, SWCD technicians, utility crews and private forest management consultants, anyone who has reason to observe oak forests that are slated for or in the process of being developed.
  • Continue to monitor and document our progress and use this info to keep local, state and federal elected officials informed and engaged.
  • Continued public investment at all levels is crucial to maintaining suppression efforts.