First Report of Bacterial Leaf Scorch Disease of American Elm Caused by Xylella fastidiosa in Georgia, U.S.A.

Abstract

The American elm (Ulmus americana) is a highly desirable deciduous shade tree with moderately dense foliage and a symmetrical broad or upright vase-shaped crown. This plant is susceptible to various diseases because of rapid terminal growth when new springwood vessels are fully functional. The American elm, along with many other shade tree species, can be affected by Xylella fastidiosa (Hearon et al. 1980), and X. fastidiosa has been previously confirmed on American elm in Ontario (Goodwin and Zhang 1997), Washington, D.C. (Harris and Balci 2015), Oklahoma (Olson et al. 2006), and Alabama (Parker et al. 2012). X. fastidiosa colonizes the xylem vessels of plant hosts and can cause bacterial leaf scorch disease (Pearson et al. 1998). On elm, X. fastidiosa causes leaf discoloration and browning, marginal scorching, and dieback. In the summer of 2019, chlorosis and marginal leaf scorch symptoms consistent with those of bacterial leaf scorch disease were observed on an American elm tree in a nursery in Pulaski County, Georgia. This elm tree was propagated from cuttings taken from a mature tree in Houston County, Georgia. Initial identification of the disease was performed by assessing characteristic symptoms of infection like leaf scorch, dieback signs, and so on, as described in Hernandez-Martinez et al. (2006). X. fastidiosa was verified via molecular and serological methods. Genomic DNA was extracted from the petioles and midribs of symptomatic leaves using the DNeasy Plant Kit (Qiagen, Valencia, CA), and real-time PCR was performed in a Cepheid smart cycler II (Sunnyvale, CA) using iQ SYBR Green Supermix (BioRad Laboratories, Hercules, CA). A 25-µl reaction was prepared according to the manufacturer’s protocol with primers XF-F/XF-R, which target the 16s rRNA processing protein (Harper et al. 2010). Real-time PCR results confirmed the presence of X. fastidiosa in all symptomatic leaves. The recombinase-polymerase-amplification technology-based AmplifyRP Acceler8 end-point detection assay (Agdia, Elkhart, IN) was carried out on symptomatic tissue using according to the manufacturer’s instructions, and this assay also confirmed the presence of X. fastidiosa in all symptomatic leaves (Waliullah et al. 2019). For further confirmation, the remaining petiole tissue was tested for X. fastidiosa using the DAS-ELISA Reagent Set for X. fastidiosa (Agdia) with minor modifications to the manufacturer’s protocol. In addition to tests of the symptomatic plant, testing of leaves from five additional young elm trees from the same nursery was also carried out according to the same procedures. These five visually healthy young elms tested negative for X. fastidiosa based upon all three testing methods. In total, three independent tests confirmed the presence of X. fastidiosa in symptomatic elm tissues, whereas asymptomatic elm trees from the same nursery tested negative. Attempts to isolate the bacteria from symptomatic leaf tissue on periwinkle wilt media (Davis et al. 1981) were not successful; however, the strong association of symptoms with the thrice-confirmed presence of X. fastidiosa in symptomatic tissue clearly indicates the role of X. fastidiosa in the observed symptoms. Although X. fastidiosa is most commonly transmitted by xylem-feeding insects, cuttings taken from infected hosts can produce infected plants; however, the mature tree used for propagation in this case could not be located for testing. To the best of our knowledge, this is the first report of X. fastidiosa associated with the American elm in Georgia. The presence of X. fastidiosa in American elm has the potential to impact homeowners, landscapers, and nursery producers within Georgia, and our findings suggest that Georgia tree nursery growers should monitor their nursery stock for bacterial leaf scorch disease.