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Xanthomonas axonopodis pv. phaseoli (Smith) Vauterin (Xap) and Xanthomonas fuscans subsp. fuscans Schaad et al. (Xff) cause indistinguishable symptoms known as common bacterial blight of bean (Phaseolus vulgaris L.). Seedborne inoculum is important in the survival and dissemination of these pathogens, and they can be transmitted from seed to growing plants, initiating damaging epidemics (4). Several reports have indicated that Xff strains are generally more pathogenic than Xap strains (3). However, there are no published studies comparing frequencies of seed transmission between the two species. Pathogen incidence on seeds is closely correlated with disease incidence in the seed production field (5), and there is usually a positive correlation between seed symptoms and the population of bacteria per seed. Therefore, the ability of the bacteria to survive on stored seed is an important epidemiological factor; if the two pathogens differ in this characteristic, it could represent a meaningful difference in disease risk associated with seedborne inoculum. In order to compare the seed transmission frequency and storage survival of the two bacteria, five strains of each species from different geographic origins were used to inoculate seeds of a susceptible snap bean genotype (cv. ‘Derby,’ Harris Moran Seed Co., Modesto, CA). Bacterial strains (courtesy of R. Gilbertson, V. Grimault, E. Vavricka, and A. Vidaver) originated from the USA (Michigan, Nebraska, or Wisconsin), Australia, Brazil, France, the Netherlands, or the United Kingdom. Strains were stored on silica beads (Microbank, Austin, TX) by shaking beads in a cell suspension, removing excess fluid, and freezing at -80°C. Bacterial suspensions were prepared from nutrient agar cultures grown at ambient temperatures (21° to 24°C) and adjusted to the concentration of 10 CFU/ml in phosphate buffered saline (PBS). Seeds were immersed in the suspension for 5 min with vacuum and air-dried for 2 h in a biosafety cabinet. Control seeds were treated in the same manner but immersed in PBS only. Seed contamination levels were confirmed by culturing on nutrient agar medium. For each strain, three replicate samples of 25 seeds were planted in Cone-tainers (3.8 by 21 cm), one seed per Cone-tainer, in a greenhouse (24° to 28°C) with supplemental high-pressure sodium lighting set for a 14 h photoperiod. An equal number of noninoculated seeds also was planted. Emergence was assessed 4-5 days after planting, and seedlings were left to grow another week to allow symptom development. Symptoms of common bacterial blight such as blight spots, brown necrosis on cotyledons, failure of leaf development and leaf lesions were observed and recorded. Infection was confirmed by culturing surfacedisinfested tissue from both symptomatic and symptomless seedlings on MT semi-selective medium (1). The experiment was conducted twice. Both species reduced seedling emergence compared to the noninoculated control treatment; emergence for the control seeds was 86.7%, whereas it ranged from 65.3% to 70.7% for Xap-inoculated seeds and from 52.0% to 62.7% for Xff-inoculated 6

plant health progress

Comparison of Seed Transmission and Survival of Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans in Common Bean Seeds