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. Application 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 ‘ºg/mL ’àí1. However, significant reductions in bacterial growth were observed at a rate of 200 ‘ºg/mL ’àí1, 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.