In Oklahoma, during the late summer of 2004, an elm tree (Ulmus americanus L.) located in the Oklahoma Botanical Gardens near Stillwater showed symptoms of marginal leaf scorch bordered by a yellow band between necrotic and green tissues, indicating possible Xylella fastidiosa infection. Three leaves from the symptomatic tree and one leaf from an asymptomatic nearby elm were sampled. DNA was extracted with the Extract-N-Amp kit (Sigma, St. Louis, MO). Samples were tested for X. fastidiosa using real-time polymerase chain reaction (PCR) with Xylella genus specific primers XfF1/XfR2 and dual-labeled TaqMan probe XfP2 (2). Infected oleander from California was used as a positive control. All three samples from symptomatic leaves and the positive control were PCR positive, and the sample from the asymptomatic tree was PCR negative. Attempts to culture an isolate of the bacteria from petioles and branch tissues on PD3 and PW, media selective for X. fastidiosa, failed. For more detailed molecular characterization of the putative pathogen, DNA from additional symptomatic petioles from the same tree was isolated using the cetyltrimethylammoniumbromide (CTAB) extraction. X. fastidiosa specific primers BBXFOUTF1 (5′-AAGCGCCTCCGTGAGTTATC-3′) and BBXFOUTR1 (5′-CCTTCACGCATATCATCACC-3′) were used to PCR amplify the gyrB gene. The amplification product was recovered after gel electrophoresis with QIAquick gel extraction kit (Qiagen, Valencia, CA) and was subjected to automated sequencing (Oklahoma State University Recombinant DNA/Protein Resource Facility). BLASTN alignment (1) of the obtained 381 bp sequence revealed 100% identity with the gyrB gene from elm (GenBank Accession No. AF534966) and mulberry (GenBank Accession No. AF534965) isolates of X. fastidiosa. During 2005, petiole samples from the tree were collected and serological diagnosis was confirmed using enzyme-linked immunosorbent assay (Agdia, Inc., Elkhart, IN). Some strains of X. fastidiosa have very wide host ranges and many of the hosts may be asymptomatic. Therefore, the economic importance and implications of the detection of X. fastidiosa in the state of Oklahoma remain to be determined. To our knowledge, this is the first report of X. fastidiosa in Oklahoma. References: (1) S. F. Altschul et al. J. Mol. Biol. 215:403, 1990. (2) N. W. Schaad et al. Phytopathology 92:721, 2002.