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Bur oak blight (BOB) in Minnesota

By Jill Pokorny, United States Forest Service

Update:  Here’s a May 2011 PDF fact sheet on bur oak blight (BOB), including identification and management considerations.

Minnesota tree care companies, their arborists, and urban forestry professionals are advised to be on the lookout for a newly discovered disease of bur oaks.

The first bonafide case of bur oak blight (BOB), confirmed by Dr. Tom Harrington of Iowa State University, has been identified in Minnesota. Previously, symptoms of BOB were reported to occur in portions of southern Minnesota, however, the disease was then called Tubakia leafspot and was cited to be caused by the fungus, Tubakia dryina. Since then, Dr. Harrington has completed DNA and pathogencity testing that confirms this disease is caused by a new, and yet unnamed, species of Tubakia, and he has named the disease bur oak blight (BOB).

It is not clear if this new species of Tubakia is a recent arrival to this region or if a shift in climate (more early-season rain events) have made this disease more noticeable over the last two decades.  To date, BOB is known to occur from eastern Nebraska to central Minnesota and southwestern Wisconsin, and it appears to be spread across all of Iowa.

Plant pathologists and arborists have been on the lookout for the new BOB Tubakia species in Minnesota, particularly in central and more northern counties. Jill Pokorny, plant pathologist with the US Forest Service located symptomatic bur oak trees in Mille Lacs and Sherburne counties, collected leaf samples, and identified the fungus, Tubakia, to be present.  To determine if it was the new species of Tubakia that causes BOB, she submitted samples to Dr. Harrington for further laboratory testing. The samples tested positive for BOB.

In recent weeks, symptoms of BOB have also been reported on bur oaks located in Hennepin and Ramsey counties. These samples have also been submitted for species-level DNA testing, and we are awaiting test results. Jill Pokorny predicts, “As we continue to investigate symptomatic bur oak trees and more samples are tested, it is expected that BOB will be found in additional Minnesota counties.”

Symptoms

Update:  Here’s a May 2011 PDF fact sheet on bur oak blight (BOB), including identification and management considerations. BOB occurs only on bur oaks. Leaf symptoms typically first appear in late July or August, however, this year symptoms began to show up by early July (perhaps because of the frequent rainfall).

Fig. 1: Bur oak leaves exhibiting large, wedge‐shaped lesions and necrotic (brown) veins. Photo by Christine Engelbrecht.

Infected leaves develop necrotic (brown) lesions that are wedge-shaped and often delimited by leaf veins. It is also common for the leaf veins, themselves, to turn brown (Figure 1). Individual lesions may coalesce and cause large areas of the leaf to turn brown, resulting in an overall wilted or scorched appearance to the leaves. Smaller brown lesions may also be present, but are less common.

During the summer, black, pimple-like fruiting structures of the fungus form along the leaf veins and petioles and can be easily seen with the aid of 10X magnifying lens. Leaf symptoms are usually more severe on the bottom half of the affected tree’s crown, but over time, symptoms may spread throughout the crown.

A unique feature of BOB is that some infected leaves will remain on the tree during the winter (healthy bur oak trees shed all of their leaves in the fall). It is important to note, however, that not all infected leaves will remain attached; some leaves will drop off during the growing season and some will be blown off by winter winds. But the fact that some leaves, even a small number, are retained over the winter is an indication that BOB may be present. Also, if leaves are blown off by winter winds, BOB-infected leaf petioles will remain intact. Fruiting bodies that form in late fall on infected leaf petioles are believed to be overwintering stage of the fungus.

BOB appears to be a slow spreading disease, particularly as it relates to disease spread from tree to tree. It remains a mystery as to why BOB does not spread more rapidly because the spores that cause BOB are produced in great abundance and are spread by rain.  Within an individual tree, the disease tends to intensify year to year, generally starting in the bottom of the crown and moving upwards. If a tree is seriously affected one year, it tends to be severely affected the next year.

Over time, severely affected trees may die. In Iowa, BOB has been reported to cause tree death, particularly in upland sites. Within a grove or group of bur oak trees, some trees may be severelyinfected while adjacent trees appear healthy. This is likely due to variation in the degree of resistanceindividual bur oak trees possess to this disease.

Management

Because BOB is a late season disease, the impact on the foliage is reduced since the majority of photosynthetic activity has already occurred for the growing season. Although most infected trees will leaf out normally the following spring, they will develop leaf symptoms by late summer, and successive years of heavy leaf damage can result in branch dieback and eventual tree death. Efforts to boost tree vigor may prolong the life of affected trees and ward off invasion by secondary pests such as two-lined Chestnut borer and Armillaria root rot.

Since the fungus overwinters on infected leaf petioles that remain on the tree, removal of fallen leaves is not an effective management tool.

In preliminary studies, injections of the fungicide propiconazole (Alamo formulation) in early June (prior to leaf symptoms) have reduced symptom development in the fall and the following year.

With further study, fungicide treatments may prove to be a valuable management tool for use on high-value landscape trees.  “I’m not usually a big fan of fungicide treatments, but with this disease we are finding very good results. A single treatment may benefit the trees for several years,” Harrington says.

Sample Submission

Urban forestry professionals, now is your chance to have some of those mystery bur oak trees tested for BOB.  Many of you have dealt with bur oaks that have eluded a positive diagnosis because they have exhibited unusual leaf symptoms and/or decline. Samples can be submitted to the University of Minnesota’s Plant Disease Clinic for diagnostic testing.  A routine processing fee will be charged.

For BOB testing

Collect branch tips with symptomatic and healthy leaves from several locations on the tree, wrap them in dry paper toweling (no plastic bags, please), and send them to:

Plant Disease Clinic
Department of Plant Pathology
495 Borlaug Hall
1991 Buford Circle
St. Paul, MN  55108

Enclose a description of the symptoms observed, when they were first noticed and enclose a photo, if possible.  Please ship samples early in the week so they arrive at the Clinic before Friday.

Samples that test positive for Tubakia species and represent a new county record will be forwarded to Dr. Harrington for species-specific DNA testing. This data will assist him in developing a BOB distribution map for the Upper Midwest states.

For more info

  1. 18-minute video on the symptoms and other characteristics of BOB.
  2. Fact sheet from Iowa State University Extension: What’s happening to Iowa’s bur oaks?

This content also available as a printable PDF:  BOB News Brief-Jill Pokorny (PDF)

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18 Responses to “Bur oak blight (BOB) in Minnesota”

  1. Ron says:

    Hello.

    Have follow-up studies been conducted to determine the efficacy of propiconazole treatments?

    Thanks.

  2. Tom Harrington says:

    We did a trial with propiconazole injections in early June 2010. The trees all showed at least some improvement in disease levels in September, some substantial, but there was some immediate leaf symptoms and death, and small branch death, due to phytotoxicity. The 10 ml per inch dbh rate used for oak wilt appears to be a little high for bur oak, especially if the crown is thin or there is branch dieback.

    The beneficial effects of propiconazole injections are expected to be better the following year. A couple of trees treated in June 2009 had very little BOB in the 2010 season, which was a very bad year for BOB in central Iowa. The second season improvement is thought to be due to reduction in overwintering inoculum rather than a carryover of chemical into the next year.

    Arbortech treatments in early 2009 also appeared to be beneficial into the second season. However, it may show even more phytoxicity than propiconazole in bur oak. Bur oak seems to particularly sensitive to chemical injections. An arborist recommended irrigation (an inch of water) to the soil around the tree immediately after propiconazole treatment in order to reduce leaf burn.

  3. Ron says:

    Is there a preferred method of propiconazole injections?

    For example, should soil be removed and the root flare injected or can injections be made above ground level?

    How deep should the injection be made into the Phloem or Xylem?

    In the existing study is the 10mL propiconazole per DBH inch applied at a diluted (with water) rate?

    Has consideration been given to using Debacarb or Tebuconazole for BOB management?

    Thank you.

  4. Jim Gunter says:

    After reading a lot of your info about BOB, I am positive I have it. Looks to be infesting a large number of Burr Oaks in the Spicer/New London area of Kandiyohi County in Minnesota. Should I send in a sample or should I start treating as suggested? They sure look sick!
    Jim

  5. Profile photo of MyMNwoods MyMNwoods says:

    Hi Jim. I would definitely submit a sample. BOB is new in Minnesota, and positive identification of diseased trees at different locations in the state would be of great help determining its spread. Being certain of what you’re dealing with would save you money on unnecessary treatments as well.
    -eli

  6. Sam Kezar says:

    I have just gotten conformation of BOB in Thief River Falls, MN. This is a new county record for Pennignton county and a lot farther north that any other conformations within the state.
    Sam Kezar
    BCMA

  7. Profile photo of MyMNwoods MyMNwoods says:

    Good to know Sam, thanks for the comment. It seems that BOB is more widely established, and has been here for longer, than we’d previously thought… Some of the info in this post needs to be updated.

  8. chris dvorak says:

    At a Tree Inspector recertification workshop in February, we learned about Bur Oak Blight. After the class, I spoke with the DNR’s Val Cervenka explaining to her the exact symptoms she had just described.I live in Hennepin County and this is the second year I have seen this.It is not affecting the mature tree anymore than last year,but some of the seedlings are completely affected.All of their leaves have the symptoms.I will continue to monitor them. Love the Website.

  9. Profile photo of MyMNwoods MyMNwoods says:

    Chris, have you submitted leaf samples to be tested for BOB? If not, I’d encourage you to do that, as it’ll help to get a handle on where BOB is in Minnesota. Glad you like the site!
    -eli

  10. Tom Elmer says:

    The U has confirmed BOB in at least one of our trees in Chanhassen. We would be interested in treating. Given the relative newness of BOB in Minnesota, are there vendors that are more experienced in treatment?

    Given the questions posed by Ron, I would prefer not to be an experiment.

  11. Profile photo of MyMNwoods MyMNwoods says:

    Hi Tom. You may need to contact arborists directly to find that out, but a call to the UMN Department of Forest Resources’ Tree Info Line might be worth a try as well. They’re at 612-624-3020 or treeinfo@umn.edu .
    -eli

  12. Rick Reimer says:

    I didn’t see Kandiyohi County on the B.O.B list for 2011. I had sent samples to the Uof M and on to Iowa State and they confirmned B.O.B

    Thanks

  13. Liz Christy says:

    Our Wayzata/Minnetonka neighborhood has dozens of trees that a couple arborists have indicated have BOB. We are being told we ALL need to treat affected trees with injections or we won’t be able to save our trees (if just a couple owners treat and others don’t we are being told it won’t be worthwhile). Is this true?

    Has anyone treated their trees and had success?

    If we don’t treat will they die for sure or can they survive?

  14. Delwyn Colwill says:

    There have been a couple of cases reported in Wright county. I have justed started treatments with Alamo for my customers here who have confirmed through the u of m that it is in fact bob.(last summer). One thing to be noted, that Bob is a slow progressing disease, however the added stress on the tree is lowering its resistance to pests such as wood wasp, (clear wing borers) and the ever present twolined chestnut borer, (agrilus blinateus). In treating with alamo is it a good idea to treat for secondary pests as well? I would think it would be the only prudent course of action…at least until the tree can ward off theses native pests again on its own. LIterally every case of Bob I have seen or suspected has included a secondary pest infection. Also due to the trees’ lack of resistance is it more susceptible to other fungal infections such as verticilium wilt, or anthracnose?

  15. Jeff Johnson says:

    After reading this information, I’m quite sure we have BOB here in Minneapolis at the golf course. Treatment is a must to save our 150 year old Oaks. Seriously there is always something to deal with regarding tree management.

  16. Mary Lou says:

    I live in Des Moines, IA and have 8 huge Bur Oaks. We moved to this property in the fall of 2010. Four of our old trees have BOB. The oldest tree, over 200 yrs. old,, was treated in June 2011 with Alamo and Cambistat. It has not improved, at all. In fact, last year, 2013, half the tree is dead with very sparce leaves. It is too far gone and will sadly be removed. The three others that tested positive last summer, were treated with Alamo and I am praying they will live. After spending a fortune on this treatment, I am not too hopeful. It is so disheartening to see such majestic trees die. I suspect the previous owner knew of these issues and sold out, not disclosing the tree issues.
    If the remaining three trees do not look well this summer, 2014, a year after their treatment, does that mean they, too, are lost? Losing them will drastically, damage the look of our beautiful yard.
    Please advise.
    Thanks,
    Mary Lou

  17. Profile photo of MyMNwoods MyMNwoods says:

    Hi Mary Lou. I’m sorry to hear of what I’m sure is a sad and demoralizing problem. Having no direct experience with bur oak blight research or management, my knowledge is limited to what is available in the post above and the linked fact sheet. Both suggest that the propiconazole treatment seemed effective for several years in early trials, but it is possible that it has proven less effective since. Good luck with your trees!
    -eli

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