First Report of Bacterial Spot Caused by Xanthomonas campestris pv. vesicatoria on Sweet Pepper (Capsicum annuum L.) in Saudi Arabia.

Abstract

In the summer of 2009 and 2010, 18 sweet pepper fruit with blister-like, raised, rough lesions were collected from four greenhouses (total of 0.1 ha) in the Al-Kharj region of Saudi Arabia. All samples were collected from commercial crops of the sweet pepper cv. California Wonder. Disease incidence was ≤5%. Isolations were made from all diseased fruits. A small piece (3 mm2) of symptomatic tissue from pepper fruit was placed in a sterile mortar and macerated in sterile distilled water with a pestle. A loopful of bacterial suspension from each sample was streaked onto Tween B agar medium (3). Plates were incubated at 28°C for 48 h. Single yellow, circular, butyrous, shiny colonies were picked from the plates and transferred to nutrient agar plates containing 5% D+ glucose agar (NGA). Gram-negative, rod-shaped bacteria were consistently isolated from the fruit and 10 of the isolates were identified as Xanthomonas campestris pv. vesicatoria on the basis of morphological, physiological, and biochemical tests (1,2). The isolates were oxidase positive and levan negative, arginine-dihydrolase positive, and did not macerate potato discs. The isolates were also non-fluorescent, grew at 37 and 4°C but not at 40°C, did not liquefy gelatine or starch, but did produce H2S. The identity of the 10 bacterial strains was confirmed by PCR assay using primers RST65 and RST69 (4). Four-week old pepper plants (cv. California Wonder) were inoculated by spraying five potted plants with each isolate using a bacterial suspension (108 CFU/ml). Sterile distilled water was sprayed on an additional five plants as a negative control treatment. The bacterial isolates caused necrotic lesions, each with a yellow halo, on leaves of inoculated plants. Bacteria reisolated from the necrotic lesions using the technique previously described were identical to the original strains according to the morphological, cultural, and biochemical tests described above. Negative control plants inoculated with sterile distilled water did not show symptoms and no bacterial colonies were recovered from them. To our knowledge, this is the first report of bacterial spot on pepper fruits in Saudi Arabia.