Bur Oak Blight (BOB)

Bur Oak Blight (BOB)

This past week we were asked to inventory oak trees in a western suburb of Minneapolis.  The charge was to identify and mark trees with oak wilt disease and to identify homes with trees showing symptoms of bur oak blight (BOB).  Based on our observations, homeowners in the community with affected trees will be notified by mail and will also be provided with information on the two fungal pathogens.

What was surprising in our inventory is that over 90% of the bur oaks had symptoms of bur oak blight.  The symptoms ranged from a few yellowing leaves with fungal fruiting structures on the bottom side of the leaf mid-vein (Figure 1) to trees with every leaf affected dried and curling (Figure 2).  While we did have a wet spring that is conducive to fungal pathogen infection, we didn’t see many leaves affected with standard foliage diseases like oak anthracnose.  Every yellowing and scorched leaf was infected with BOB.

Figure 1
Figure 1

 

 

 

 

 

 

 

Figure 2
Figure 2

 

 

 

 

 

 

 

 

 

Bur oak blight is a fungal pathogen that only infects bur oak trees.  Although infection is a spring phenomenon, the foliage does not appear damaged until late summer. Until about ten years ago it was unknown to most arborists, as a disease that impacted bur oaks in urban settings. However, it has become a predominant problem over the last decade to the point where, as in the community we inventoried, it is reaching epidemic proportions.

BOB infections appear to be “locally” systemic.  The fungus stays in the infected branch tissues and re-infects petioles on leaves every season.  Wet springs seem to encourage BOB infections. Every season the infection spreads to more tissues in the tree, until the entire canopy is infected.  Infected leaves either drop to the ground or remain on the tree and shrivel as they dry out.

The easiest way to tell the difference between a severe infection of BOB and oak wilt is to look at the ends of the branches.  Branches infected with oak wilt will be dead at the tips and will continue to die back as the fungus progresses through the water conducting tissues of the tree (Figure 3).  In trees infected with BOB, you can usually see living green leaves at the ends of the infected branches while impacted foliage will be obvious further down the branches closer to the trunk of the tree (Figure 4).  Of course, a professional diagnosis is required to know for certain which pathogen is the real culprit.

Figure 3
Figure 3

 

 

 

 

 

 

 

 

 

Figure 4
Figure 4

 

 

 

 

 

 

 

 

 

There has been limited research with BOB due to its apparently new appearance on bur oaks in the urban environment.  What research there is, is from commercially funded scientific studies at Iowa State University examining management options (Iowa State). Otherwise most of the management options are based on limited trials by arborists trying to find a way to reduce the impact of the disease.  These experiences suggest that in addition to fungicide injections, increasing the vigor of infected trees through modification of their soil environment can help trees tolerate BOB infections.  However, as most of the scientific and professional observations suggest, any treatment whether fungicidal or designed to enhance tree vigor will need to eventually be repeated as the fungus is never eliminated from the host tree.

The US Forest Service developed a fact sheet on BOB that discusses the pathogen in detail and provides photos for identification of infections (US Forest Service). If you think you may have infected oaks, it is a good idea to have an Ostvig Plant Health Care expert examine your trees and provide you with information on the latest mitigation strategies.  Multiple years of BOB infection will reduce tree vigor, making the tree susceptible to other stresses. The reduction in vigor can result in decline and death of impacted bur oak trees.