First report of Xanthomonas hortorum pv. gardneri causing bacterial spot on tomato (Solanum lycopersicum) in Argentina

Abstract

During December 2020, 20% of the plants in a 3-ha tomato field-crop (cv. Nativo) in the horticultural belt of Buenos Aires-La Plata, Argentina (34°52’23.3"S 58°7’55.1"W) had symptoms of bacterial spot. Leaves with small, dark brown water-soaked lesions were collected, superficially disinfected with 70% ethanol, macerated in sterile distilled water and streaked on yeast dextrose carbonate agar plates (Schaad et al. 2001). Plates were incubated at 28 °C for 48 h. Two isolates (BNM 563 and BNM 564) from different plants were selected for further identification. The isolates produced yellow, round, smooth, brilliant colonies, were Gram negative and rod-shaped. They gave a hypersensitive reaction in Nicotiana tabacum, were oxidase negative, catalase positive, and had no amylolytic nor pectolytic activity. Their DNA fingerprint profiles, generated using BOX-PCR assays (Versalovic et al. 1994), were identical to that of Xanthomonas hortorum pv. gardneri type strain BNM 225 and were confirmed as X. hortorum pv. gardneri-positive by species-specific PCR (Koenraadt et al. 2009), therefore confirming the identity of the isolates. Pathogenicity tests were performed twice on tomato plants (cv. ACE 55), with six plants per strain. Plants were inoculated by immersion of the aerial parts in a bacterial suspension (106 CFU/ml) in water amended with silwet (0.01%). Control plants were mock inoculated with water containing silwet. Plants were placed in individual plastic bags for 48 h and then maintained in the greenhouse under natural temperature (23 °C ±5 °C) and light conditions for two weeks until leaf spots, similar to those observed on naturally infected plants, were observed. Control plants remained symptomless. Bacteria isolatedfrom inoculated plants exhibited identical morphological characteristics and BOX-PCR fingerprints to those of BNM 563 and BNM 564. To our knowledge, this is the first report of X. hortorum pv. Gardneri in Argentina.