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Bacterial Leaf Scorch

Xylella fastidiosa, “Xf” for short

Though fairly common in our area of southeastern PA, this disease is not thoroughly understood.  The bacterium is difficult to grow in a culture.  It has only been approximately two decades since an immune response (ELISA) test was developed that allowed accurate diagnosis.  Because the same symptoms – browning of leaves, starting at the margins, in late summer – can also be caused by environmental factors like drought, a certain diagnosis can not be made without a lab test.

My experience with this disease dates to 1992, the first year I began having samples tested with the reliable immune-response procedure.  Since then, I have been monitoring the health of trees that have been diagnosed with Xf.  Some clients have opted to treat trees with Oxytetracycline, others to do nothing.  Here are some of my observations from the last 16 years of monitoring specific Xf positive trees:

  • No tree, treated or untreated, has rapidly declined or died

  • Many of the pin oaks that tested positive had other factors negatively impacting their health – poor soils or confined planting sites, pH induced iron deficiency (chlorosis), obscure scale infestations

  •  Some of the trees improved remarkably, regardless of treatments, during years of adequate rainfall.  One tree in West Norriton Township is no longer symptomatic

  • Several trees appear in fairly stable condition after 10+ years of infection – not much better or worse

  •  A red oak in Upper Merion Township that tested positive in 1992 was in extremely poor condition at that time because the root system had been damaged by excavation for a driveway.  This tree was finally removed in 2007.  It was located in a row with 12 other healthy red oaks of similar mature size.  None of the adjacent trees show any symptoms.  However, trees on the other side of the street, in weakened health because of improper pruning (lionstailing and flush-cuts) do show leaf scorch symptoms (I haven’t tested them as they are not owned by my one of my clients)

Currently, the primary research on Xylella is being done at Rutgers University in New Jersey by a team lead by Dr. Ann Gould, so I’ll let Dr. Gould tell you about the disease with this link:  Bacterial Leaf Scorch

For a more in-depth study, you can go here:  Bacterial Leaf Scorch of Amenity Trees

More research is needed.  The Rutgers project is being partially funded by contributions from members of the Penn-Del chapter of the ISA*.  See more about our fundraising here (page 14 of December 2007 newsletter):  Penn-Del ISA Newsletter

*ISA is the International Society of Arboriculture – the professional society for arborists.  See and


If your tree tests positive for Xf, you may wonder what you should do.  Here are my suggestions:

  • Prune dead and dying branches - especially newly or partially infected trees.  STERILIZE PRUNING TOOLS

  • Mitigate drought stress with mulch or watering

  • Identify and manage any additional pests or diseases

  • Analyze the soil, and address any elemental deficiencies.  Treat trees exhibiting symptoms of iron chlorosis

  • If you opt to treat a tree with antibiotic injections, be aware that this will not actually provide a cure for the disease, only single season symptom suppression.

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