BLS (Bacterial Leaf Scorch, a.k.a "Brown Leaves in Summer")
Erik Lindberg, Rainbow Treecare Scientific Advancements
Bacterial leaf scorch (BLS) is a unique, systemic disease in that the culprit behind it isn’t fungal or insect related, but rather the result of a pesky bacterium known as Xylelia fastidiosa. This bacterium is spread by a variety of xylem-feeding insects in the subfamily Cicadellinae (i.e. Leafhoppers, Sharpshooters, and Treehoppers). These insects feed on the terminal shoots of susceptible host trees, which is where the bacteria gains entry. From here the bacteria multiplies and spreads throughout the tree, clogging the xylem vessels along the way which impedes water and nutrient uptake. The worst part is that they overwinter in protected areas within the xylem of the tree. This means that come growing season, they’re ready to go and wreak havoc once again!
What symptoms look like
Leaves of an infected tree will develop normally early in the season, but come June/July, premature browning of the leaves will start becoming evident. This leaf necrosis begins along the leaf margin and spreads inward toward the veins and petiole in an irregular pattern (an identifying characteristic). Additionally, as the necrosis starts to spread further along the leaf, there will be a yellow/reddish brown band/halo that separates the healthy green tissue from the necrotic brown tissue (another identifying characteristic). These symptoms become progressively worse over a period of 3-8 years until the entire tree eventually turns brown prematurely.
Determining a solution to the problem
As it currently stands, there is still a lot to learn about this problem. That said, the existing research help us to better understand the disease, and provides guidance on how to effectively manage it. Here at Rainbow, we offer two products to help you manage your trees with BLS: Bacastat™ and Cambistat™. Before we look into those products, it’s important to have an understanding of the research that went into determining the effectiveness of the active ingredients in each product.
In 1979, George Nyland conducted a study to see if he could find a solution to the almond leaf scorch disease he was seeing throughout California. His answer for the problem was oxytetracycline (OTC) injections. Injected trees from his study had their average disease rating change from 11.9 to 1.8 one year after injections. In response to that study, Kostka et al. published a paper in 1985 demonstrating that injecting American elms with OTC helped reduce signs of BLS in symptomatic trees or kept BLS at bay relative to the previous season. After this, OTC injections were a common practice within the tree care industry, but it wasn’t until 1996 that work done by Schnappinger et al. established that tetracyclines worked on the bacteria by inhibiting protein synthesis.
That being said, not all OTC applications are the same! The industry standard for OTC products involved a calcium complex OTC formulation that was used as the active ingredient. This formulation took a long time to inject due to its viscous nature, and arborists were not seeing the results that they were hoping for in terms of control. Rainbow Treecare Scientific Advancements heard these concerns from arborists and worked with a chemical manufacturer to create a new formulation of OTC (oxytetracycline hydrochloride), which is the version of Bacastat™ we use today. A study done by Hartman et al. in 2010 determined that the previous industry standards of calcium complex OTC treatments were not as effective as the OTC formulation in Bacastat™ when comparing treatments made on similar application dates. Additionally, this new formulation decreased the uptake time from 1-2 days to only 10- 30 minutes! This allows multiple trees to be treated within a day, and is safer as the chemical does not have to sit overnight. Understanding the physiological mechanisms by which the antibiotic works on the tree allows us to determine the most effective treatments for the problem.
While OTC has been commonly used throughout the industry to effectively manage BLS symptoms, paclobutrazol (PBZ) has also been investigated as a potential treatment option. Communications with industry leaders in the plant health care (PHC) field regarding anecdotal evidence of our Cambistat™ product decreasing the severity of BLS led to a series of internal research studies and collaborations with other tree care companies in order to understand this further. Additionally, DeStefano et al. published a paper in 2007 demonstrating significant reduction of X. fastidiosa occurrence with high rates of paclobutrazol (200 µg/mL) in a laboratory setting. That study found applications of OTC at 50 µg/mL and paclobutrazol at 200 µg/mL provided similar control when examining the colony-forming units (CFU) grown in vitro, and that both treatments significantly reduced the number of CFU’s relative to the untreated controls.
What we suggest here at Rainbow
Here at Rainbow, we suggest using a two-pronged approach to effectively manage BLS. Applying Bacastat™ and Cambistat™ at their respective label rates will effectively reduce the symptoms associated with bacterial leaf scorch. These treatment strategies should be paired with other sound cultural practices such as mulching and supplemental irrigation during dry periods.
Now that you know a little more about the science behind oxytetracycline and paclobutrazol, you can treat your client’s properties with confidence. Here at Rainbow, we want to make sure that your promise to your clients is trusted and backed by science. If you have any questions at all, or would like to learn more about BLS and the research, you can contact Erik Lindberg at firstname.lastname@example.org.
DeStefano, D. A., A. P. Grybauskas, J. L. Sherald, B. Momen, Q. Huang, and J. H. Sullivan. 2007. Effect of the growth regulator paclobutrazol on growth of the bacterial pathogen Xylella fastidiosa. Arboriculture and Urban Forestry 33:246. Full Text (PDF)
Hartman, J., E. Dixon, and S. Bernick. 2010. Evaluation of therapeutic treatments to manage oak bacterial leaf scorch. Arboriculture and Urban Forestry 36:140–146. Full Text (PDF)
Kostka, S. J., T. A. Tattar, and J. L. Sherald. 1985. Suppression of bacterial leaf scorch symptoms in American elm through oxytetracycline microinjection. Journal of arboriculture (USA). Full Text
Nyland, G. 1979. Chemotherapy of diseases of deciduous trees associated with mycoplasma and rickettsialike organisms. Pages 139–142 Proceedings of ROC-United States Cooperative Science Seminar on Mycoplasma Diseases of Plants. Full Text (PDF)
Schnappinger, D., and W. Hillen. 1996. Tetracyclines: antibiotic action, uptake, and resistance mechanisms. Archives of microbiology 165:359–369. Full Text