Research Accomplishment Reports 2007

Pin Oak Bacterial Scorch Management

J. Hartman
Department of Plant Pathology

 

Project Description

Bacterial leaf scorch, caused by Xylella fastidiosa, affects many Kentucky landscape trees including oaks (pin, red, scarlet, shingle, and white) maples (norway, red, silver, and sugar), planes (american sycamore and london plane) sweetgum, hackberry, elm and mulberry.  Leaves of infected trees show marginal necrosis (scorch) late in the summer followed by premature defoliation.  Infected trees re-foliate normally each spring and late summer scorch and premature defoliation is repeated.  The disease begins on one or a few branches and over several years gradually spreads throughout the tree.  After many years, dead twigs, then dead branches and limbs begin to appear in the tree and the condition worsens over the years until the tree needs to be removed.  Bacterial leaf scorch is a very problematic disease in Kentucky.  Kentucky landscape industry professionals and tree owners are in need of a remedy for bacterial leaf scorch.  Arborists here have difficulty knowing if certain commercially available treatments actually work.

Bacastat® is a commercial injectable form of oxytetracycline, an antibiotic used for management of bacterial diseases in agricultural crops and is active against gram negative bacteria such as xylella.  In 2006, we were able to show that root flare injection of bacastat in late spring delayed the appearance of scorch symptoms by about 2-3 weeks.  The best time for application of bacastat is not known, so the objective of this research was to determine the springtime application timing that provides the best results in reducing summer and fall bacterial leaf scorch disease symptoms in pin oaks.

Tree disease research is often limited to a few replications, making it difficult to interpret results.  This research was conducted at stone Street Farm in Lexington, a horse farm with a large number of pin oaks and a history of bacterial leaf scorch disease.  Antibiotics were injected during May and June, 2007, into trees using a micro-injection technique with an M3 device, a reusable, pressurize-able applicator capsule.  Applications were made through small holes drilled into major root flares at the base of the tree and spaced 6-8 inches apart around the tree.  Chemical uptake time using this system takes 10 to 30 minutes.  Eight different treatments were each replicated 10 times and the experimental design was a randomized complete block.

Prior to 2007 treatment applications, on 18 October, 2006 when symptoms were prominent, trees were evaluated for bacterial leaf scorch symptoms so that the effect of 2007 treatments could be evaluated.  Beginning with first symptom appearance, trees were evaluated for percent scorch symptoms at about two-week intervals on 7 and 20 August, 4 and 18 September, and 2, 16 and 30 October 2007.

Impact

Bacterial leaf scorch symptoms gradually increased from August through October.  None of the treatments “cured” infected trees or prevented scorch symptoms from appearing by the end of the growing season.  However, treatments using antibiotics generally delayed the onset of symptoms by about 2 - 3 weeks.  It appears that the late May Bacastat® treatment provided slightly better results than the early May or mid-June treatments.  By mid-October, the antibiotic-treated trees showed less scorch than the same trees showed at about the same time in October, 2006.  At the same time, untreated trees were more diseased than the previous year, which was expected since bacterial leaf scorch disease worsens progressively each year.

Although treatments used in this experiment did not cure trees of bacterial leaf scorch, treatments that can delay symptoms by even a few weeks might prolong the useful life of infected trees.  Arborists statewide are provided with results of these experiments so that they can make informed decisions on tree health management.  This information will have an impact on how Kentucky urban forests are managed.

Publications

Hartman, J., Dixon, E., Bernick, S., Moore, G., and Hanks, L.  2008.  Evaluation of Root Flare Injection Treatments to Manage Oak Bacterial Leaf Scorch - 2007.  Nursery and Landscape Program 2007 Research Report, U.K. Agricultural Experiment Station PR-554, pp 27-29.