In April of 2008, lettuce (Lactuca sativa L. cv. Darkland) plants grown in the Al-Ouunia Region of Saudi Arabia were observed with numerous lesions typical of bacterial leaf spot. Leaf lesions were irregular, small, pale green to black, and 2 to 5 mm in diameter. Bacteria were isolated from diseased leaf tissues by cutting leaves into small pieces (0.5 mm) and soaking them in 2 ml of sterile distilled water. The resulting suspension was streaked onto yeast dextrose calcium carbonate agar (YDC) (1) and plates were incubated at 28°C. Large, round, butyrus, bright yellow colonies typical of Xanthomonas spp. formed after 48 h and five strains were selected for further tests. A yellow, mucoid bacterium was consistently isolated from lettuce samples with typical bacterial leaf spot symptoms. All five strains tested in this study were gram negative, oxidase negative, nitrate reduction negative, catalase and esculin hydrolysis positive, motile, and strictly aerobic. All were slightly pectolytic but not amylolytic. All were identified as Xanthomonas campestris pv. vitians. The bacterium was identified with specific oligonucleotide primers (2). This primer pair directed the amplification of an approximately 700-bp DNA fragment from total genomic DNA of all X. campestris pv. vitians strains tested. Pathogenicity tests were performed by using bacterial cultures grown on YDC for 48 h at 28°C. Each strain was suspended in sterile distilled water and the bacterial concentration was adjusted to 106 CFU/ml. Leaves of 5-week-old lettuce plants (cv. Darkland) were sprayed with the bacterial suspension. The inoculated and sterile-water-sprayed control plants were covered with polyethylene bags for 48 h at 25°C, after which the bags were removed and plants were transferred to a greenhouse at 25 to 28°C (1). All strains were pathogenic on the lettuce cv. Darkland, causing typical bacterial leaf spot symptoms by 2 weeks after inoculation. All inoculated plants showed typical symptoms of bacterial leaf spot and symptoms similar to those observed on the samples collected. No symptoms developed on the control plants. The bacterium was reisolated from inoculated plants and identified as X. campestris pv. vitians by morphological, physiological, and biochemical tests as described above. To our knowledge, this is the first report of bacterial leaf spot of lettuce by X. campestris pv. vitians in Saudi Arabia.
References: (1) F. Sahin and A. Miller. Plant Dis.81:1443, 1997. (2) J. D. Barak. Plant Dis.85:169, 2001.