Thursday, May 21, 2009

Landscape - Bacterial Leaf Scorch in Oaks

The following is a good article on bacterial leaf scorch in oaks and its management using antibiotic injections.

Bacterial leaf scorch, caused by Xylella fastidiosa, affects many 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. Oaks are widely planted in Delaware and the disease is most common in them. Leaves of infected trees typically show marginal necrosis (scorch) late in the summer followed by premature defoliation. Infected trees re-foliate normally in spring and the process of 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 continues to worsen over the years until the tree needs to be removed. Bacterial leaf scorch is a very problematic plant disease in Delaware.

In Kentucky research in 2006, they were able to show in replicated pin oak plots that root flare injection of oxytetracycline antibiotic in late spring delayed the appearance of scorch symptoms by about 2-3 weeks. In 2007, they determined that the best results were obtained when injections were made about 3 weeks after the first emerging leaves were fully expanded. Arborists with clients who want their trees injected with antibiotic may wish to concentrate their efforts for sometime in the next 2 weeks.

In the Kentucky work, they used Bacastat®, a commercial injectable form of oxytetracycline. This antibiotic is used for management of bacterial diseases in agricultural crops and is active against gram negative bacteria such as Xylella. Mycoject® is also an injectable formulation of oxytetracycline, but our results indicated that the more viscous formulation of this product was not as effective.

Although the time of appearance of scorch symptoms was delayed as a result of these treatments, none of the treatments prevented disease or cured trees with bacterial leaf scorch. When bacterial leaf scorch symptoms are delayed with antibiotic treatments, it is not known if this will prolong the life or vitality of the treated trees, but it might. Nevertheless, if clients are demanding a treatment, certified arborists trained in micro-injection techniques will want to make the injections in late May to provide the greatest effect. It appears that the effect of antibiotic injection lasts only one year, so annual injections may be needed. More research is needed to determine these long-term needs and effects.

Adapted from "It Is Near Time for Injecting Oaks for Bacterial Leaf Scorch" By John Hartman in the current edition of the Kentucky Pest News

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