|HILTON, ANGELYN - Texas A&M University
|JO, YOUNG-KI - Texas A&M University
|CERVANTES, KIM - New Mexico State University
|STAMLER, RIO - New Mexico State University
|FRENCH, JASON - New Mexico State University
|HEEREMA, RICHARD - New Mexico State University
|GOLDBERG, NATALIE - New Mexico State University
|SHERMAN, JASON - Arizona State University
|RANDALL, JENNIFER - New Mexico State University
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2017
Publication Date: 6/26/2017
Citation: Hilton, A., Jo, Y., Cervantes, K., Stamler, R., French, J., Heerema, R.J., Goldberg, N., Sherman, J., Randall, J., Wang, X., Grauke, L.J. 2017. First report of pecan bacterial leaf scorch caused by Xylella fastidiosa in pecan (Carya illinoinensis) in Arizona, New Mexico, California, and Texas. Plant Disease. doi.org/10.1094/PDIS-02-17-0298-PDN.
Interpretive Summary: Xylella fastidiosa is a bacteria that causes many plant disesases worldwide, including Pecan Bacterial Leaf Scorch (PBLS). The disease was first observed on pecan in Louisiana, and is known to be present in several southeastern states. We studied samples taken from commercial pecan orchards in AZ, NM, CA and TX, as well as from trees in a special orchard that often sends graftwood to other countries when they request it. In order for us to send the graftwood, those countries must send an import permit that tells what conditions must be met. Often, the trees will be inspected for symptoms of PBLS. When this disease lives in the tree, it can block the flow of water. Leaves from diseased trees look "scorched" with dry, brown edges. Often leaves fall from the tree early making it hard for the tree to get water or food to developing nuts. This reduces nut quality and makes the tree weak, reducing following crops. If this disease is present in graftwood, other countries do not want it. We found that this disease was present in commercial pecan orchards in NM, AZ, CA and TX, and in the special orchard that sends out graftwood to other countries. This is the first report that the disease is present on pecan in the southwestern states. This report is important, because it helps us learn how to protect pecan trees in this country and prevent spreading the disease.
Technical Abstract: Pecan bacterial leaf scorch (PBLS) is a chronic disease that can cause major yield losses in pecan orchards. In the 2015-16 growing seasons, symptoms consistent with PBLS were observed in commercial pecan cultivars in AZ, NM, CA and TX. Symptoms included tan to light brown necrotic lesions, which often started at tip or leaf margin and expanded throughout the leaflet, resulting in abscission. The USDA ARS National Collection of Genetic Resources for Pecans and Hickories (NCGR-Carya) in Texas distributes pecan germplasm internationally within constraints outlined by plant import permits of requesting nations. Permits often require source evaluations for PBLS to prevent the spread of the causal agent, Xylella fastidiosa. Samples from NCGR-Carya and commercial pecan orchards in NM, AZ, CA, and TX were screened for the presence of X. fastidiosa. Symptomatic and asymptomatic shoots were collected and xylem sap was extracted using a Scholander pressure chamber. Sap samples were tested by enzyme-linked immunosorbent assay (ELISA) according to manufacturers' protocols (Agdia). Xylella-specific genes were amplified from DNA or diluted sap samples with previously published polymerase chain reaction (PCR) primer sets. Numerous samples from NM, AZ, CA, and TX were positive for the presence of X. fastidiosa based on both ELISA and PCR. The generated amplicons of HL (hypothetical protein) and 16S rRNA were sequenced and analyzed by NCBI Blastn. Comparison of HL amplicons from TX, NM, AZ, and CA were 94-99% identity to M23 (gb:CP001011.1) and MUL0034 (gb:CP006740.1). Global alignments of 16S rRNA PCR sequence fragments were analyzed using Geneious 9.15 with free end gaps and cost matrix of 65%. Sequence obtained from sap extracted from symptomatic 'Curtis' (NCGR-Carya, TX) and 'Cape Fear' (Medina County, TX) accessions aligned closely to X. fastidiosa Dixon ctg92 (NZ_AAAL02000001.1) subspecies multiplex. This is the first report of the presence of X. fastidiosa in pecan in the southwest region of the United States and in NCGR-Carya and has implications for international distribution of pecan germplasm, as well as pecan nursery and orchard management.