Bacterial leaf scorch is a bacterial pathogen (Xylella fastidiosa) that invades the vascular system of many plants; including shade trees such us oak, maples (red and sugar), boxelder, flowering dogwood, hackberry, elms and sweet gum among others. This pathogen can also infect edible and economically important host (grapes, plums and pears) as well as weeds (such as Virginia creeper,sumac and many others).
This pathogen infects the plants vascular system, and leafhopper insects transmit the bacteria between plants. Characteristic symptoms are the marginal burn, or scorching, of the leaf that gives the name to the disease bacterial leaf scorch (BLS). The marginal burn can be orange to brown and a yellow border in contact with the green leaf tissue (Figure below). This yellow margin to the lesion can help distinguish BLS from the many other causes of marginal burn. See more pictures at the Bugwood website.
The distribution of the leaf symptoms varies, depending on the host. For example, it can be generalized in oaks (both young and old leaves) or starting on older leaves and moving into new ones in elms. BLS can result in rapid tree decline and death (depending on the tree species).
Unfortunately, the symptoms of bacterial leaf scorch can be similar to physiological and environmental leaf scorch, Verticillium wilt and even oak wilt. Determining if your shade tree is positive for bacterial leaf scorch can help you planning for the future, promoting strong growth to minimize stress and taking inform decisions to manage it. To prevent BLS maintain plant vigor, practice sanitation by removing branches or affected trees when necessary to reduce the source of the pathogen. And if replacing a tree, plants host that are not susceptible to BLS.
Historically, bacterial leaf scorch has been considered of minor importance in Iowa, with only a few suspected instances with no laboratory confirmation.
The ISU Plant and Insect Diagnostic Clinic will be offering testing for bacterial leaf scorch for the next two weeks (9/14 to 9/28). This service will be offered annually for two weeks at the beginning of fall when bacterial populations are high and detection testing is more effective. If your shade tree is showing foliar symptoms, consider sending us a sample as follows:
- Select twigs with leaves that are showing symptoms (leaf scorch in the figure), instead of limbs that are showing advanced symptoms (completely brown or dead).
- Cut several twigs from this area, always making sure the branch has the leaves with foliar symptoms (scorching). We need leaves and petioles (the stalk that joins a leaf to a stem) from scorched leaves (10 to 12 leaves minimum) to run the test.
- Follow our instructions to collect package and drop off/ submit a sample
- Fill out a submission form and indicate BLS testing in the comment section (on the bottom). Include $20.00 testing fee.
Incorrect sampling produces insufficient material for analysis and can lead to false negatives. Avoid:
- Leaves samples with no scorching
- Leaves collected from the leaf litter or fallen on the ground (always collect alive leaves from the tree that are showing scorching)
- Completely dead and dry out leaves (refrigerate samples if possible and deliver/mail promptly)
- Delay in sending samples (sample and submit a sample on the collection day)
Symptoms of Xylella fastidiosa on bur oak. Photo by John Hartman, University of Kentucky. www.forestryimages.org