246 DeStefano et al.: Effect of Growth Regulator Paclobutrazol Arboriculture & Urban Forestry 2007. 33(4):246–252. Effect of the Growth Regulator Paclobutrazol on Growth of the Bacterial Pathogen Xylella fastidiosa Darren A. DeStefano, Arv P. Grybauskas, James L. Sherald, Bahram Momen, Qi Huang, and Joe H. Sullivan Abstract. Xylella fastidiosa is a fastidious, xylem-limited, insect-transmitted, bacterial plant pathogen with a wide host range that causes bacterial leaf scorch (BLS) in shade trees. BLS is a chronic disorder characterized by late season leaf scorch and dieback and is common in urban and suburban areas of the mid-Atlantic and southeast United States. BLS has been recognized since the 1980s and attempted treatments have included antibiotics and plant growth regulators. Appli- cation of paclobutrazol (PBZ), a diastereomeric triazole with both fungistatic and growth regulation properties, has been observed to alleviate symptoms of BLS, but it has not been established whether PBZ has a direct effect on the organism. In this study, we investigated the effect of PBZ on in vitro growth of two X. fastidiosa isolates. Our results showed no significant effect of PBZ on colony growth of X. fastidiosa at the manufacturers recommended rate of 20 g/mL−1 However, significant reductions in bacterial growth were observed at a rate of 200 g/mL−1 . , indicating that high levels of PBZ may have a direct effect on the growth of X. fastidiosa. This direct effect and growth regulator effects of PBZs suggest that PBZ may provide a promising treatment for BLS in shade trees. Key Words. Bacterial leaf scorch; oxytetracycline; paclobutrazol; Xylella fastidiosa. Xylella fastidiosa (Wells et al. 1987) is a xylem-limited, in- sect-transmitted, fastidious plant pathogenic bacterium with a wide host range. It is currently recognized as indigenous to the Americas and causes disease in grape, citrus, shade trees, and other hosts. Pierce’s disease of grape, caused by the bacterium, was first described by Newton B. Pierce in 1892 (Pierce 1892) as an irregular marginal necrosis that resulted in the slow systematic death of the vine. Transmission of the bacterium by arthropods of the Homoptera was confirmed by Houston et al. in 1947. In the 1950s, marginal leaf scorching symptoms similar to Pierce’s disease were observed on American elms (Ulmus americana) in Washington, DC, U.S. (Wester and Jylkka, 1959) leading to a hypothesized relation- ship between the syndromes. The isolation of a xylem-limited bacterium confirmed to be the casual agent of Pierce’s dis- ease (Davis et al. 1978) subsequently enabled scientists to implicate this bacterium as the cause of a variety of syn- dromes, including elm leaf scorch (Hearon et al. 1980), phony disease of peach (Hopkins and Mollenhauer 1973), and almond leaf scorch (Mircetich et al. 1976). Analysis of 25 isolates of the pathogen from 10 distinct hosts in 1987 al- lowed Wells et al. to describe and name the organism X. fastidiosa as a new genus of plant pathogenic bacteria repre- sented by a single species. In 2000, X. fastidiosa became the first plant pathogen to have its entire genome sequenced (Simpson et al. 2000). ©2007 International Society of Arboriculture Xylella fastidiosa infection in shade trees is commonly referred to as bacterial leaf scorch (BLS) and the causal role of the bacterium was first demonstrated by Sherald et al. (1983) in sycamore (Platanus occidentalis). Xylella fastidi- osa has a wide host range affecting over 100 host species in at least 30 families of monocotyledonous and dicotyledonous plants (Hopkins and Alderz 1988), including such important landscape trees as oaks (Quercus), elms (Ulmus), sycamores (Platanus), and mulberries (Morus) in urban or suburban ar- eas of the mid-Atlantic and southeastern United States (Sher- ald 2001). Xylella fastidiosa has been confirmed to infect numerous tree hosts (Table 1) throughout eastern North America and California, U.S. Most hosts are asymptomatic and may serve as reservoirs of the pathogen from which vectors transmit the disease to susceptible hosts. Xylella fas- tidiosa is transmitted by xylem-feeding insects of the Ci- cadellinae (sharpshooters/leafhoppers) and Cercopidae (spittlebugs), which are active throughout the growing season and remain infectious indefinitely after bacterial acquisition. Bacterial leaf scorch is a chronic late season leaf scorch that compromises the growth and aesthetic qualities of shade trees, although it is rarely lethal. Symptoms of the disease include an irregular marginal leaf scorch, early defoliation, delayed budbreak, loss of vigor, stunting, and dieback. Scorch symptoms result from the disruption of water trans- port attributable to bacterial aggregation in xylem vessel el-
July 2007
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