Michelle Grabowski, University of Minnesota Extension Educator
Bacterial Leaf Scorch (BLS) is a disease of shade trees common in the southern states and known to occur as far north as Ontario and as far west as Texas. To date, however, it is unclear if this disease occurs in Minnesota and how common it might be. It may seem unusual that a disease of a large tree might go unnoticed for years, but many states, having never reported Bacterial Leaf Scorch previously, have found multiple infected trees once scientists began looking for the tiny bacterial pathogen. Recent reports show that BLS is becoming more common in northern states like New Jersey. Scientists now want to know, does BLS occur in the upper Midwest?
The reason that BLS can slip by unnoticed is that the symptoms it causes look very similar to the symptoms caused by several other diseases and by environmental stress. Scorch is the term used to describe leaves that have singed brown edges. In shade trees infected with BLS, the margins and tips of the leaves turn brown. Unfortunately leaf browning at the edges and tip of the leaf is also commonly seen in trees stressed from drought, compacted soil, restricted root growth, salt damage and many other factors. In addition, diseases like oak wilt and Dutch elm disease can cause browning of leaves that may be confused with BLS. In the end, scientists rely on several laboratory tests, like ELISA, PCR and electron microscopy, to determine if BLS is truly present or not.
Bacterial Leaf Scorch is caused by a tiny bacterium called Xylella fastidiosa. Infection can occur in a wide variety of shade trees including oak, elm, linden, mulberry, maple and many others. These bacteria live in the xylem, or water conducting cells of the infected shade tree. Leaf scorch is the result of the bacteria blocking the flow of water to the leaves and may also be caused by a toxin produced by the pathogen. In BLS infected trees, scorched leaves typically first appear in mid to late summer; August and September in Minnesota. Newly infected trees may have only a few branches with scorched leaves scattered throughout the canopy. As the infection continues year after year, more of the tree's canopy becomes affected. Eventually, the tree's growth slows, branches die and the entire tree may be killed. BLS is spread from tree to tree by leafhoppers and spittle bugs that pick up the bacteria while feeding on infected trees. The BLS bacteria can only survive inside an infected tree or within an insect vector. To learn more about the life cycle, biology and management of BLS visit the American Phytopathological Society education center .
Surveys are being conducted this summer to determine if bacterial leaf scorch is present in Minnesota's shade tree population. If you have a shade tree with leaf scorch symptoms and would like to contribute a sample to the survey please contact Michelle Grabowski, extension educator with the University of Minnesota at 763-767-3876 or firstname.lastname@example.org. There is no charge to send a sample. One pencil width branch with 3-5 leaves attached is all that is needed to test a tree for BLS. Trees of interest include oak, elm, sycamore, maple, mulberry, ash, linden, horse chestnut and other shade trees.
Photo 1. B.Olson OK State University, NPDN
Photo 2. A.B. Gould, APS Education Center