Michelle Grabowski, UMN Extension Educator
A new fungal leaf blight of bur oak (Quercus macrocarpa) trees has been confirmed in Minnesota. Bur Oak Blight, also known as BOB, was collected by Jill Pokorny of the USDA Forest Service in several central Minnesota counties and confirmed by Dr. Harrington of Iowa State University.
How to Recognize BOB
• Bur oak blight only infects bur oak trees. Leaf spots or blights on other types of oak tree are caused by different pathogens.
• On bur oaks, symptoms first appear in July or August.
• Infected leaves have brown angular or wedge shaped lesions. Leaf veins turn brown and small black spots (fungal spore producing structures) can be seen with a magnifying glass within the brown lesions.
• Infected leaves are often found first on the lower branches. After many years of infection, however, the entire canopy can be infected.
• Healthy bur oak trees typically drop all of their leaves in the fall. Trees infected with BOB, however, often have many leaves that remain attached throughout the winter.
Will BOB Hurt my Oak Tree?
Bur Oak Blight affects leaves late in the season. Because the leaves have had several months to do photosynthesis before disease becomes severe, the tree has had an opportunity to store up energy. One year of infection, therefore, will not hurt the tree.
Unfortunately trees that are infected one year are likely to have the disease in following years as well. Over several years, the disease becomes more severe, slowly affecting more and more of the canopy. This repeat infection puts stress on the tree, weakening it and making it more susceptible to secondary pests like two lined chestnut borer and Armillaria root rot. Both of these secondary pathogens can cause severe damage to the tree.
Where Did BOB Come From?
Reports of damage to bur oaks similar to BOB were reported in Minnesota as early as the 1990's. Before this time, the problem had not been noticed on bur oaks. It is only recently that the fungus that causes BOB was identified as a new species of Tubakia. It is possible that this fungus was present in Minnesota but never noticed, or perhaps was present but the environment was not conducive to disease prior to the 1990's.
On individual trees, spores of Tubakia sp. overwinter on infected twigs, leaves and petioles that remain attached in the canopy. Rain splashes spores from these old infections to the new emerging leaves, resulting in year after year of disease. New trees can be infected by spores splashing onto leaves from a neighboring infected tree. Overall the spread of the disease within a canopy and within a group of trees is surprisingly slow given the large number of spores produced by the fungus.
What Can I Do To Protect my Tree from BOB?
There are currently no fungicides recommended for treatment of BOB, but research is being conducted to determine if a fungicide injection or spray could protect uninfected oak trees and reduce disease in trees already suffering from BOB.
For now the only thing gardeners can do is reduce stress on infected trees as much as possible. Removing turf grass from below a tree and replacing it with wood chip mulch will help to keep the soil moist. In years of low rainfall, trees should be watered to reduce drought stress. Care should be taken not to wound trees with lawn mowers or other yard equipment. Avoid driving cars or other heavy vehicles over the root zone of mature trees. This can compact the soil and cause root damage.
For more information about bur oak blight read 'BOB in Minnesota' by Jill Pokorny, USDA Forest Service.