First Report of Bacterial Leaf Scorch Caused by Xylella fastidiosa on Gleditsia triacanthos var. inermis in Pennsylvania
In the summer of 2019, three thornless honeylocust trees (Gleditsia triacanthos var. inermis) in Gettysburg, PA, were observed in a general state of decline. The trees were 50 to 64 cm in diameter at breast height. Symptoms of the decline included branch dieback, stunted growth, and swollen woody tissues where leaf petioles attached on the branch. Symptoms were randomly distributed in the canopy and progressed from the outer portions of the canopy inward. Based on communication with local arborists who have observed these symptoms on other honeylocusts in the area, from the onset of symptoms, mature, established honeylocust trees died in a period of 3 to 5 years. Bacterial leaf scorch disease, which is caused by the xylem-limited, gram-negative bacterium Xylella fastidiosa, is widespread in Gettysburg, PA, on Quercus palustris and Q. rubra. The dieback symptoms and symptom progression on the honeylocust trees are similar to the disease symptoms on the Quercus species. However, the classic scorching symptom was not evident on the pinnately compound leaves. In August of 2019, tissue samples were taken from three symptomatic and two asymptomatic thornless honeylocust trees in Gettysburg, PA. One hundred grams of petiole tissue was excised from each sample, and DNA was extracted using the Qiagen DNeasy Plant Mini Kit (Qiagen, Hilden, Germany). PCR reactions as described by Minsavage et al. (1994) using the X. fastidiosa species-specific primers RST31/RST33 confirmed the presence of X. fastidiosa in the three symptomatic trees but not in the healthy, asymptomatic trees. A representative sequence of the RNA polymerase sigma 70-factor has been deposited to GenBank (MN535097) and shared 100% identity with X. fastidiosa reference sequences deposited in GenBank (MF401541 and CP000941). Samples could not be processed immediately, which is needed for X. fastidiosa isolation, so culturing was not attempted. In addition to PCR, the samples were processed with the commercially available AmplifyRP XRT X. fastidiosa recombinase polymerase amplification detection kit according to the manufacturer’s instructions (Agdia, Elkhart, IN), which amplifies the Disulfide isomerase gene. X. fastidiosa was also confirmed with this method in the symptomatic trees but not the asymptomatic trees. Because isolates were not obtained, pathogenicity tests were not attempted. However, based on the lack of X. fastidiosa detection on healthy trees, and detection on symptomatic trees, we feel confident that X. fastidiosa is associated with the dieback and decline. Thornless honeylocusts are a commonly planted street tree, historically used as a street tree and replacement for the American elm after Dutch elm disease became established. Thornless honeylocusts make good street trees because they are affected by minimal pests and diseases and have general survivability in the stressful, urban environments. G. triacanthos var. inermis represents a new host for this disease in Pennsylvania and an underdocumented host nationwide. As summer temperatures continue to warm and droughts become more common, the severity of bacterial leaf scorch of G. triacanthos var. inermis will continue to worsen. Based on our current knowledge, this is the first report of X. fastidiosa causing bacterial leaf scorch and general decline of G. triacanthos var. inermis in the eastern United States.