Thursday, September 29, 2011

Bacterial Leaf Scorch symptoms in urban trees


Noticed leaf-browning and scorch-like symptoms on oak this summer?  Bacterial leaf scorch may be the culprit.   The disease bacterial leaf scorch is caused by Xylella fastidiosa, a xylem-inhabiting bacterium that affects water and nutrient transport into leaves and shoots.  Oak, maple, hackberry, dogwood, elm, mulberry, and sycamore are among the long list of hosts.  Pierce’s disease of grape is caused by a different subspecies of the pathogen. 

During the infection process, bacteria are transported from host to host by leaf hoppers, spittle bugs, and sharp shooter insects.  Once infected, bacteria move down branches; details of this transport through vascular tissue are not fully understood.  Once bacteria reach xylem (vascular) vessels, they multiply and are carried throughout the plant fairly quickly. 

Symptom development occurs in urban trees during the summer and fall.  Leaves emerge and develop normally in spring.  However, during mid to late summer, when water or heat stress is highest, water-stressed leaves begin to turn brown at the margins.  Yellow or purple margins may be visible in some hosts, as opposed to typical browning.  Leaves become increasingly more necrotic and fall prematurely from trees as summer progresses.  Each season, symptoms worsen, and disease spreads to additional branches.  This gradual decline is a distinct symptom of bacterial leaf scorch.  Typically, it may be 5 to 10 years after symptoms are reported that trees must be removed.

There is no cure for bacterial leaf scorch, as bactericides and injection treatments have proven inconsistent and overall unreliable long-term solutions.  In landscapes, newly symptomatic branches can be pruned to remove bacterium-infected limbs.  This method may help extend tree life, but will not completely eradicate the disease or pathogen.  Additionally, watering and preventing tree stress, especially during hot dry seasons, may help prolong tree life.  As soon as bacterial leaf scorch is confirmed, replace trees with non-susceptible hosts such as ash, beech, or tulip poplar.  Plant new trees early, so they will have time to mature before diseased trees are removed.  Refer to publication PPFS-OR-W-12 for a list of resistant and susceptible trees and contact your local UK Extension agent for assistance. 

2 comments:

  1. Is there really no found cure for bacterial leaf scorch? I could have sworn I read that there was a cure, a little while ago.

    -Samudaworth Tree Service
    Tree Service Brooklyn

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    Replies
    1. No, there is no cure for infection. Some tree services recommend antibiotic injections, but these treatments simply delay symptoms. Refer to "Evaluation of Therapeutic Treatments to Manage Oak Bacterial Leaf Scorch" by Hartman et. al. in the journal of Aboraculture & Urban Forestry 2010. 36(3): 140-146 for more detailed research data. The abstract is below.

      Bacterial leaf scorch is a very serious tree disease, especially for oaks in Kentucky, U.S. landscapes. From 2003 to 2007, several potentially
      therapeutic disease management treatments were tried on diseased pin oaks (Quercus palustris) growing in golf course, street tree, and horse farm environments.
      Treatments included root flare soil drenches of paclobutrazol, adjuvant-assisted basal trunk applications of anti-microbial compounds, and
      springtime root flare injections of oxytetracycline. Paclobutrazol drenches caused expected growth regulator effects but did not consistently reduce bacterial
      leaf scorch of golf course and street trees. Antibiotics applied directly to trunks of infected trees with an adjuvant had no effect on levels of bacterial
      leaf scorch. Compared to untreated trees, springtime root flare injections of oxytetracycline reduced scorch levels and delayed by about two weeks, the
      time of appearance of late summer scorch symptoms. Injections done three weeks after full expansion of first leaves provided better results than injections
      done earlier or later in the spring. Therapeutic treatments do not provide a cure for trees infected with bacterial leaf scorch, but may prolong tree life.

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