First Report of Bacterial Leaf Scald of Plum Caused by Xylella fastidiosa in Texas.
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Xylella fastidiosa is the etiological agent of Plum Leaf Scald (Greco et al. 2021). The disease was first reported in Argentina (Fernandez-Valiela et al. 1954) and then Brazil and Paraguay (French et al. 1978). In the USA, Plum Leaf Scald has been reported in the Southeastern United States (Wells et al. 1981a) and California (Hernandez-Martinez et al. 2009). In August 2021, during the Stone Fruit Survey of FY2020, plum trees (Mexican variety, Prunus mexicana) with symptoms of leaf scald, were observed in a Central Texas orchard with approximately 7% of trees exhibiting symptoms. Leaf margins were asymmetrically scorched, with necrotic areas that transitioned into chlorotic and healthy green tissues. To detect the presence of the pathogen, leaf sample petioles were tested using a double-antibody sandwich (DAS) ELISA with X. fastidiosa specific antiserum (Agdia Inc., Elkhart, IN) according to manufacturer's guidelines. X. fastidiosa was detected in 20 of the 35 symptomatic samples. To confirm ELISA results, total DNA was extracted from the plant samples using the Plant DNeasy kit (Qiagen Co. Hilden, Germany) following the manufacturer's protocol. All 20 ELISA-positive samples tested positive in a X. fastidiosa-specific real time PCR assay, using the primers XF1F and XF1R and probe XF1p (Schaad et al. 2002). Moreover, the ELISA-negative samples were also negative for PCR assay. Symptomatic samples were used to isolate the pathogen. Samples were debarked, surface-sterilized and xylem fluid collected. The fluid was gently imprinted on buffered charcoal yeast extract (BCYE) (Wells et al. 1981b) or periwinkle wilt modified (PWM) agar plates (Summer et al. 2010). After 10 days of incubation, individual colonies were observed. The colonies were slightly convex, white, opalescent, mucoid, circular with entire margins and with smooth surfaces on both media plates. Isolated colonies were triple-streak single colony purified and archived. Genomic DNA was extracted from four purified isolates using the DNeasy Blood and Tissue Qiagen Kit, to conduct conventional PCR using HL5/HL6 (Francis et al. 2006), which identified the isolates as X. fastidiosa. Using the 16S rRNA primer pair U3/U4 (James 2010), amplicons were sequenced and compared against the NCBI database using the BLASTn algorithm. Comparative sequence analysis of amplicons from the four isolates were identical and indicated that the isolates were 100% identical to X. fastidiosa subsp. multiplex RIV5 (CP064326.1) from cherry plum, and IVIA5901 (CP047134.1) from almond. The sequences of all four isolates were deposited into NCBI GenBank, with the accession numbers OM617940 (467), OM617941 (470), OM617942 (471) and OM617943 (468). To our knowledge, this is the first report of X. fastidiosa associated with plum leaf scald in Texas, extending the geographical range of this important bacterial disease, in the Southern United States. This study highlights the importance of routine scouting of agricultural settings with a view to assessing and detecting early threats from either pests or disease and implementing relevant management strategies.
