In this second edition of “Protect your Oak Trees,” we aim to shine a light on the same signs of fall-like leaf discoloration on oak trees in the summer months, known as “flagging.” When flagging occurs on oak trees in a certain pattern, at a certain time in the summer, it could mean one of two things and possibly be confused with a host of other problems. In the previous edition, the signs of oak wilt and how they manifest is defined. Here we aim to define the signs, symptoms, and possible management of Bacterial Leaf Scorch (BLS).
What is Bacterial Leaf Scorch (BLS)?
Bacterial Leaf Scorch is the condition that strikes a variety of trees and landscape plants caused by the bacterial infection and propagation of the bacterium Xylella fastidiosa. The destructive effects take place when the bacteria spreads and blocks water and nutrient transportation in the xylem. BLS affects many plants and trees, including but not limited to the elms, maples, sycamores, and mulberries, so keep that in mind even though oaks are of highest priority because of the many signs shared with oak wilt. Once again, the red oak group, or group with pointed leaves, seems to be significantly more affected by these vascular blocking pathogens than their close relatives the white oaks, those with rounded tip leaves.
How to differentiate BLS from oak wilt?
The timing and pattern of leaf flagging is the first way to differentiate between BLS and oak wilt. If the flagging pattern occurs more slowly, on isolated limbs, and the leaves display alternating discoloration with a yellowish halo separating the healthy green tissue, these are signs of BLS. Rapid flagging and decline point to oak wilt, where dead limbs may also develop fungal mats quickly and break apart bark. The other difference BLS has is in the timing of symptoms. BLS flagging occurs later in the summer and oak wilt occurs midsummer. The timing and the signs can lead to misdiagnosis as the signs are similar to the symptoms of tree stress leaf scorch caused by drought, salt toxicity, root damage, and sunscald. The heat of the summer, drought, and sunscald are all confusing signs with oak wilt. The fall season entering dormant season can begin early in microclimate sites, confounding flagging signs of BLS. Knowing the variables can help steer diagnosis in the proper direction to mitigate the problem.
How do trees get BLS?
BLS is known to spread above ground by bacteria carrying vectors. Here the vectors are leaf hopping insects (cicadellidae), spittlebugs (cercopidae), and more specifically, sharpshooter (cicadellinae) leaf hopping insects that feed on the xylem of the oaks’ broad leaves. When insects of these families feed on infected vegetation, they pass the bacteria on to healthy trees as they migrate from one to the other. It is not yet known but it is suspected that the bacterium infection can be spread underground via interconnected root systems of trees. Knowing these facts gives us options to manage the destruction caused by the pathogen.
What can be done?
The only way to confirm diagnosis of both BLS and oak wilt, is to send tissue samples to the lab at Rutgers. However, educated decisions can be made in the interim in order to alleviate the symptoms of both pathogens. Mitigating the damage that can be caused throughout the plants in question is the immediate goal in an attempt to save infected trees, but long term may be to extend the life of the tree while limiting the spread of the pathogens to neighboring trees, preserving them. Prevention is of utmost importance, and prevention methods include removal and complete destruction by chipping of infected trees, grinding out stumps and burning remnants, and trenching in between possible interconnected roots of oak trees. Following the confirmation of infection, a systemic trunk injection anti-bacterials or anti-fungals, depending on lab results, are another mode of prevention. For BLS, pruning diseased limbs early on may cure a tree, but this must be done with care so as not to spread oak wilt causing fungus, so the proper diagnosis must be confirmed before this can be done.
There are options to preserve the vegetative value of your landscape. Knowing these signs and symptoms and communicating them to your arborist can go a long way in the prevention and spread of these damaging pathogens. Having this awareness is the first step, the next is the call to your arborist if you should see something that resembles these red flags!
