First Report of Pecan Bacterial Leaf Scorch Caused by Xylella fastidiosa in Pecan (Carya illinoinensis) in Arizona, New Mexico, California, and Texas
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2017, American Phytopathological Society. All rights reserved. Pecan bacterial leaf scorch (PBLS) is a chronic disease that can debilitate pecan and may cause major yield losses in susceptible cultivars. In the 2015 and 2016 growing seasons, symptoms consistent with PBLS (Sanderlin and Heyderich-Alger 2000) were observed in pecan plantings in AZ, NM, CA, and TX. Symptoms included tan to light brown necrotic lesions, which often started on the leaf margin and expanded throughout the leaflet, eventually resulting in abscission. Some leaflets exhibited tip necrosis with the end of the leaflet curling upward. The USDA-ARS National Collection of Genetic Resources for Pecans and Hickories (NCGR-Carya), located 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 collections and pecan plantings in AZ, NM, CA, and TX were screened for the presence of X. fastidiosa. Symptomatic and asymptomatic shoots (three to four per tree) 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 PCR primer sets using the SSO Polymerase (BioRad), KAPA 3G Plant PCR kit (Kapa Biosystems), or Plant Phire Direct PCR kit (ThermoFisher) (Francis et al. 2006; Minsavage et al. 1994; Rodrigues et al. 2003). Seventy-nine trees from AZ, NM, CA, and TX were positive for the presence of X. fastidiosa based on both ELISA and PCR. Fifteen cultivars, which included Cape Fear, Curtis, Elliot, Lakota, Pawnee, Western, and Wichita, as well as several unknown or seedling trees, tested positive for X. fastidiosa. The generated amplicons of HL (hypothetical protein) and 16S rRNA were sequenced (New Mexico State University and Eton Biosciences) and analyzed by NCBI BLASTn. Comparison of HL amplicons from AZ, NM (gb:KY628016), CA (gb:KY628015), and TX (gb:KY628014) were 94 to 99% identical to X. fastidiosa M23 (gb:CP001011.1) and MUL0034 (gb:CP006740.1). Global alignments of 16S rRNA PCR sequence fragments obtained from sap extracted from symptomatic cvs. Curtis (NCGR-Carya, TX) (gb:KY628012) and Cape Fear (Medina County, TX) (gb:KY628011) were performed using Geneious 9.15 with free end gaps and a cost matrix of 65% (Kearse et al. 2012). These 16S rRNA amplified fragments aligned closely (99 to 100% identity) to X. fastidiosa sub. multiplex Dixon ctg92 (NZ_AAAL02000001.1). Further analysis is required to determine the diversity of X. fastidiosa subspecies observed. 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.
