140 Hartman et al.: Therapeutic Treatments to Manage Oak Bacterial Leaf Scorch Arboriculture & Urban Forestry 2010. 36(3): 140–146 Evaluation of Therapeutic Treatments to Manage Oak Bacterial Leaf Scorch John Hartman, Ed Dixon, and Shawn Bernick Abstract. Bacterial leaf scorch is a very serious tree disease, especially for oaks in Kentucky, U.S. landscapes. From 2003 to 2007, several potentially therapeutic disease management treatments were tried on diseased pin oaks (Quercus palustris) growing in golf course, street tree, and horse farm en- vironments. Treatments included root flare soil drenches of paclobutrazol, adjuvant-assisted basal trunk applications of anti-microbial compounds, and springtime root flare injections of oxytetracycline. Paclobutrazol drenches caused expected growth regulator effects but did not consistently reduce bacte- rial leaf scorch of golf course and street trees. Antibiotics applied directly to trunks of infected trees with an adjuvant had no effect on levels of bacterial leaf scorch. Compared to untreated trees, springtime root flare injections of oxytetracycline reduced scorch levels and delayed by about two weeks, the time of appearance of late summer scorch symptoms. Injections done three weeks after full expansion of first leaves provided better results than injections done earlier or later in the spring. Therapeutic treatments do not provide a cure for trees infected with bacterial leaf scorch, but may prolong tree life. Key Words. Oxytetracycline; Paclobutrazol; Root Flare Injection; Streptomycin; Xylella fastidiosa. Bacterial leaf scorch, caused by Xylella fastidiosa, affects many Kentucky, U.S. landscape trees, including oaks (bur, Quercus macrocarpa; pin, Q. palustris; red Q. rubra; scarlet, Q. coc- cinea; shingle, Q. imbricaria; and white, Q. alba); maples (Nor- way, Acer platanoides; red, A. rubrum; silver, A. saccharinum; and sugar, A. saccharum); planes (American sycamore, Plata- nus occidentalis, and London plane, P. × acerifolia); sweetgum, Liquidambar styraciflua; hackberry, Celtis occidentalis; elm, Ulmus americana; and mulberry, Morus rubra (Hartman et al. 1991; Hartman et al. 1992; Hartman et al. 1995; Hartman et al. 1996; Beale et al. 2002). Leaves of infected trees typically show marginal necrosis (scorch) late in the summer followed by pre- mature defoliation. Infected trees re-foliate normally in spring and the process of late summer scorch and premature defolia- tion 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 be- gin to appear in the tree and the condition continues to worsen until the tree is in serious decline and needs removal. Bacteri- al leaf scorch is a very problematic plant disease in Kentucky. The disease-causing bacterium, X. fastidiosa, has a wide host range, and symptoms vary with host plant (Hopkins 1989). X. fastidiosa is vectored by leafhopper insects (Bentz and Sher- ald 2001; Pooler et al. 1997). This pathogen exists as sev- eral subspecies including X. fastidiosa subspecies multiplex, the probable cause of bacterial leaf scorch of most landscape trees (Schaad et al. 2004). More than 40 tree species grow- ing throughout North America have been identified as hosts of X. fastidiosa, which makes bacterial leaf scorch a wide- spread and serious threat to urban forests (Anonymous 1993; Gould et al. 2004; Gould and Lashomb 2005; Sherald 2007). ©2010 International Society of Arboriculture Paclobutrazol (PBZ) is a plant growth regulator chemical which inhibits synthesis of the plant hormone gibberellin. The chemical is said to suppress shoot growth and stimulate root development which could reduce bacterial leaf scorch symptoms. At relatively high rates in the laboratory, PBZ suppresses growth of X. fastidiosa (DeStefano et al. 2007). Preliminary evidence suggests that appli- cation of PBZ may improve tree health and reduce the effects of plant disease. The chemical suspension is applied as a soil drench around the base of the tree and is taken up into the tree systemi- cally. PBZ effects may not appear until the year after the treat- ments, but its beneficial effects are thought to last for three years. Organosilicate surfactants (surfactant) serve to promote the penetration of applied chemicals through the bark of treated trees to the vascular system. Phosphite-containing materials (phosphite) such as Agrifos® (Agrichem Manufacturing In- dustries Pty. Ltd., Loganholme, Queensland, Australia) have been shown to have activity against some bacterial diseases such as fire blight (Brannen and Garner 2008). Surfactant- containing mixtures are sprayed on the bark surface, wetting the tree bark as far as the application will allow, normally on the lower 2 m of the trunk. Surfactant plus phosphite is useful in control of sudden oak death, caused by Phytophthora ramo- rum (Garbelotto et al. 2007; Garbelotto and Schmidt 2009). Antibiotics such as streptomycin sulfate and oxytetracycline are active against gram negative bacteria such as X. fastidiosa. These antibiotics are already used for management of bacterial diseases of many agricultural crops. Oxytetracycline has been shown to delay bacterial leaf scorch symptoms in elm (Kostka et al. 1985). The objectives of this research on therapeu- tic treatments of oaks already infected with bacte- rial leaf scorch were to test the following hypotheses:
May 2010
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