Many gardeners are all too familiar with environmental leaf scorch. Environmental stress, root injury, drought, and many other factors may cause leaf margin necrosis, a condition we call scorch. The extended periods of drought in 2005 have caused leaf scorch to be rather common. Environmental scorch is usually widespread in a tree and is fairly uniform, although it may appear more severe on the more exposed sides of the plant. Environmental leaf scorch does not necessarily return each year. It is not caused by an infectious agent. Bacterial leaf scorch (BLS) is an infectious disease caused by a bacterium that spreads systemically and causes a slow decline and death of the tree. This disease is not new but is beginning to appear more frequently in the Midwest. Details can be found in issues no. 13 and 16 of this newsletter.
BLS is caused by the bacterium Xylella fastidiosa, which is fastidious, or limited, to the xylem of the plant. It cannot be isolated by conventional lab techniques and is not observed in simple tests for bacterial exudates. To identify this pathogen, fresh leaf and petiole tissue is macerated and extracted sap used to do an ELISA (enzyme linked immunosorbant assay) test. The Plant Clinic had 12 BLS suspect samples in 2005. Eleven oaks and one maple sample were tested for BLS. Three of the oaks tested positive. These samples were from oaks in Champaign, Vermilion, and St. Clair counties. Over the last 5 years, samples submitted through the Plant Clinic have yielded 28 positive cases of bacterial leaf scorch, as indicated in the following table .
I have watched symptom development on three oaks in Champaign that tested positive for BLS. These oaks showed symptoms in July this year and have rapidly progressed to mostly brown leaves by mid-September. Surrounding oaks are still green. It is not difficult to see trees potentially infected with BLS. Make note of them in your area. If BLS is present, the trees can be expected to leaf out normally next spring. Scorch symptoms will appear in mid- to late summer and be more intense than this year. August is the perfect time to sample these suspect trees, as detailed in issue no. 13 of this newsletter. Know the symptoms of this disease. An excellent resource on BLS can be found on the U.S. Department of Agriculture Web site, http://www.usna.usda.gov/Research/BacterialLeafScorch.html.