w w w. l a n d s c a p e . o r g

The Landsculptor – September 2016 17

ConColor Fir & rhizosphaera needleCast

Can roundup Control oak Wilt?

The Plant Doctor’sLandscape Tips

By David L. Roberts, Ph.D., Senior Academic Specialist, College of Agriculture and Natural Resources, Michigan State University

w w w. l a n d s c a p e . o r g

photo 2: Herbicide treatments have the potential to be used in situations where traditional root graft disruption or trunk injections may not be economical or feasible. In this aerial view in Northern Michigan, note the Oak Wilt-affected trees near the photo’s center and lower right center. Not readily apparent is a very steep cliff extending from the landscape boundary down to the lake, approximately 80-100 feet in distance. A year or two after this photo was taken, OW advanced down to oak trees on this steep cliff, making trenching virtually impossible (see Photos 3 & 4).

introduCtion:Oak Wilt (OW) is an extremely devastating and costly disease (Photo 1). Two acceptable methods to attempt to contain and eradicate the disease from landscapes and woodlands include trunk injections with propiconazole and root graft disruption (RGD-by trenching or vibratory plowing). For forest situations where economics dictate the most cost effective measures and with no or few revisits, root graft disruption followed by destruction of all trees within the trenched area (according to the Forest Management Model), whether diseased or healthy, is usually the preferred method. For landscapes and high-value trees, either RGD or Trunk Injections or a combination of the two methods may be employed according to the author’s Tier Tree Model, which is far less destructive than the Forest Management Model (see Oak Wilt Part 3: Management Strategies, The Landsculptor, February 2016). In some situations, however, such as fragile steep dune areas, RGD is not easily accomplished or necessarily recommended due to potential for severe erosion and damage to the ecosystem (Photos 2, 3, & 4). In theory, an herbicide could be used to create a root graft barrier to thwart the underground spread of the OW fungus. In other words, the translocation of a systemic herbicide such as glyphosate through the roots, exactly in the same location and manner that the OW fungus is transmitted, might prevent the spread of the OW fungus… if roots are killed. The OW fungus, Ceratocystis fagacearum, is essentially an obligate parasite, meaning it cannot survive without live plant (root) tissue for very long. Killing the roots of oaks trees should hasten the death of the OW fungus. Theoretically, this technique could also be utilized in other locations, for example, dense mixed stands of trees Continued on page 18

photo 1: Oak Wilt outbreaks seem to be becoming more frequent around Michigan. New epicenters of OW are usually caused by improper pruning, as is the case with this site near Holland, Michigan.

photo 3: Ground view of the property in Photo 2 and the advancement of Oak Wilt (OW) from the landscape area over into the oak trees located on the cliff. Due to angle (steep-ness), diversity of tree life and the potential for cliff destabi-lization, trenching (root graft disruption) is not usually practi-cal or recommended, except perhaps in rare circumstances. Glyphosate treatments, which may be very useful for such sit-uations to contain and eradicate OW, are planned at this site.

where systemic glyphosate treatment of oaks should not cause any collateral damage to other species (example: Cherry, Maple, Basswood, etc., due to no root grafting between different genera or species of trees, Figure 2 & Photo 5), especially compared to the highly disruptive trenching method.

18 The Voice of Michigan’s Green Industry.18 The Voice of Michigan’s Green Industry.

Can roundup Control oak Wilt?

w w w. l a n d s c a p e . o r g

Continued from page 17

ConColor Fir & rhizosphaera needleCastThe Plant Doctor’s Landscape Tips

teChnique: A chainsaw cut is made at a downward angle in a continuous trunk-circling fashion, 1-2 inches deep (beyond the bark, Figure 1 & Photo 6). With this girdling method, a circular “stump cup” is created continuously around the base of the tree. Into this “stump cup” is poured concentrated (30-50%) glyphosate. For large trees, at least one cup (8 fl. oz.) is

photo 4: In this ‘birds eye” view, looking down toward the lake from the top of the cliff, we can not only view the steepness of the terrain but also the diversity of plant life, which helps to stabilize this fragile slope in Photos 2 & 3.

photo 5: At this property near Hartland, Michigan, Oak Wilt was introduced by improper pruning in the spring of 2015 (Note dying tree in center of photo). Judy, the owner of the property, protected her landscape trees with trenching and trunk injections but was reluctant to use these same procedures in her adjoining woodland due to expense, density of the trees and potential for damage to other species of trees.

Figure 1: A “stump cup” is made by girdling an oak tree around its entire circumference. Into this “stump cup” is poured concentrated glyphosate. The roots of the tree should be killed by the herbicide, thus creating an effective barrier to Oak Wilt transmission by root grafts.

recommended. For smaller trees, at least ½ cup (4 fl. oz.) is recommended.

order oF implementation: It is assumed that for best efficacy, it is advised that the previously described technique be applied in the following order:

step #1-treat healthy trees 1st: A tier or two (or three) of healthy trees are

photo 6: Judy and her neighbor Bob elected to try the glyphosate/girdling method according to the outline di-vulged in this article. They girdled one tier of trees around the OW-affected tree in Photo 5 and administered con-centrated glyphosate into the “stump cups” in the fall of 2015. Note the several girdled oaks and the non-gir-dled cherry (background). Glyphosate-treated oak trees began to shed leaves and die within days of treatment.

located around the OW-diseased oak trees according to the diagram, Figure 2. Or, a tier or two of healthy trees are located between the diseased trees and the population of trees that are to be saved from OW. The girdling “stump cups” are created on the healthy trees and glyphosate is administered to these trees first. The rationale behind this Step #1 is that cutting into the diseased trees first may cause an immediate transfer of the OW fungus to nearby healthy trees from transpiration pull from the healthy trees. Girdling the healthy trees first should eliminate this sap pull on diseased trees from healthy trees.

step #2-treat oW diseased trees last: Girdle and create the “stump cups” in the OW-diseased trees; the glyphosate is administered similarly as for the unaffected, symptomless trees. Trees that measure 3" dbh or less (or too small to girdle) can simply be cut so that the exposed stump is level (horizontal); glyphosate is subsequently poured over the exposed stump wound. Sprouting stumps left from previously removed trees should be treated exactly as though the entire tree is still present.

step #3-tree removal: Trees that have been administered the glyphosate/girdling treatment can be removed several weeks

Figure 2: In this diagram, the “green” trees are oaks surrounding and adjacent to the dead Oak Wilt-affected tree in the center. The other depicted colors of trees are different species of trees. A tier or two of oaks around the OW-infected tree are girdled and treated with glyphosate. Hopefully, OW is contained within this herbicide barrier and eventually dies out as roots die.

w w w. l a n d s c a p e . o r g

The Landsculptor – September 2016 19

after the treatment or, preferably, the following year, depending on concern for liability and structural integrity issues.

What We might expeCt:The foliage of glyphosate-treated, healthy appearing trees will likely turn brown and begin dropping within a few days to a week or so after glyphosate treatment, based on some field experience by the author (Photo 7). Depending on degree of root grafting of the girdled and treated “healthy, non-OW-affected” trees, other nearby healthy oaks may also exhibit some collateral damage symptoms fairly quickly. The response of the healthy trees affected by the herbicide may mimic symptoms of Oak Wilt … and potentially cause some alarm from property owners. If performed correctly, there will be no emergence of foliage from, or evidence of viability within, these previously girdled and treated healthy oak trees the following spring (photo 7). Expect some collateral damage to healthy trees within root graft range of the girdled and herbicide-treated trees (Photo 8). These collaterally damaged trees may or may not recover. If OW-infected trees are girdled and treated with glyphosate, it is very likely they will not form pressure pads the fall or spring following their death. Obviously, healthy trees treated with glyphosate will not form pressure pads of the OW fungus.

w w w. l a n d s c a p e . o r g

photo 7: Approximately one year later (2016) this epicenter of dead oak trees at Judy’s Hartland home was not due to Oak Wilt but repre-sents the trees that were girdled and treated the year before with glyphosate (the original OW-infect-ed tree has been removed). As of the fall of 2016, no further OW has been noted; it is hoped that the original OW infection will have been contained and eradicated from this “infection center.”

photo 8: At the same location as Photos 5-7, this oak tree is near to the original OW-affected tree and within a few feet of the glyphosate-treated oak trees. Note the collateral damage as evidenced by sparse, stunted foliage (also note normal foliage). This tree is expected to recover from root graft transmission of glyphosate. As expected, no col-lateral damage was noted on other nearby species of trees or plant life. Over the next few years, this site and this tree will be monitored for evidence of further OW outbreak due to root graft transmission. With our fingers crossed, we’ll hope that the glyphosate treatment has worked.

disCussion & preCautions:Some individuals have suggested the use of herbicides other than glyphosate. As examples, these other herbicides may include Picloram (Tordon, etc.), Triclopyr (Garlon 3 & 4, etc.) and Imazapyr (Sahara, Mohave, Assault, etc.). It is possible that these other herbicides may be very effective. The author’s reservation about these alternative herbicides is that they tend to be long lasting—a good attribute for some right-of-way, broad -spectrum vegetation control areas but not necessarily for shared oak tree environments—and soluble in water; in other words, these herbicides may move with water (precipitation and/or irrigation) away from the point of application and cause severe harm to other, non-target plants. Glyphosate products mixed with other herbicides such as Imazapyr (Roundup 365) should definitely be avoided. Concentrated glyphosate tends to be highly effective as a stump/root killer, relatively inexpensive, and far less

likely to move with water and cause harm to other types of plants.

It has also been suggested that OW-infected trees and healthy trees can be cut down and their stumps treated by dabbing or painting glyphosate (or other herbicide) onto the vascular ring of the stumps. This may or may not work for Oak Wilt. This technique is utilized quite commonly in logging operations to prevent re-sprouting of the stumps. With these lower doses of herbicide, there is possibly a benefit of less collateral damage to nearby healthy trees. However, in the author’s opinion, larger concentrated doses of glyphosate are more likely to accomplish our prime objective of killing the oak trees’ roots fairly quickly. The rapid killing of all or most of the roots of treated oak trees should prevent spread of the OW fungus and also hasten the death of the OW fungus (in roots). Without herbicide treatments, the OW fungus may normally survive for 3-4 years in the roots of trees killed by OW.

ConClusions: The use of herbicide exhibits great potential for containing and eradicating Oak Wilt from infection centers. Its ease of application, its lack of soil disturbance, and its comparatively low cost for labor and herbicide may make root graft disruption (trenching, vibratory plowing) and costly trunk injections things of the past for some delicate forest and woodland situations, provided some property owners are willing to sacrifice some oak trees to stop the spread of the deadly OW disease (trees that they would lose anyway if OW was unchecked).

disclaimer: Research with glyphosate (and other herbicides) and the management of Oak Wilt is in its infancy. Therefore, and due to the dangerous effects of herbicides on plant life, extreme precautions must be contemplated before attempting the control of Oak Wilt with glyphosate or any other herbicide. The author makes no definite claims and cannot guarantee any specific outcomes in regards to safety or the control of Oak Wilt by herbicides. Questions? Comments? Please feel free to contact the author at robertsd@msu.edu or (248) 320-7124.

The author, MSU and MGIA do not en-dorse any particular products. If using pesticides, be sure to read and follow label directions.