Abstract
Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot, a major worldwide disease of Prunus species. Very few chemical management options are available for this disease and frequent applications of oxytetracycline (OTC) in the United States peach orchards have raised concerns about resistance development. During 2017–2020, 430 Xap strains were collected from ten peach orchards in South Carolina. Seven OTC-resistant (OTCR) Xap strains were found in 2017 and 2020 from four orchards about 20–270 km apart. Interestingly, the seven strains were also resistant to streptomycin (STR). Six strains grew on media amended with ≤100 μg/mL OTC, while one strain, R1, grew on ≤250 μg/mL OTC. Genome sequence analysis of four representative OTCR strains revealed a 14–20 kb plasmid carrying tetC, tetR, and strAB in each strain. These three genes were transferable to Xanthomonas perforans via conjugation, and they were PCR confirmed in all seven OTCR Xap strains. When tetC and tetR were cloned and expressed together in a sensitive strain, the transconjugants showed resistance to ≤100 μg/mL OTC. When tetC was cloned and expressed alone in a sensitive strain, the transconjugants showed resistance to ≤250 μg/mL OTC. TetC and tetR expression was inducible by OTC in all six wild-type strains resistant to ≤100 μg/mL OTC. However, in the R1 strain resistant to ≤250 μg/mL OTC, tetR was not expressed, possibly due to the presence of Tn3 in the tetR gene, and in this case tetC was constitutively expressed. These data suggest that tetC confers OTC resistance in Xap strains, and tetR regulates the level of OTC resistance conferred by tetC. To our knowledge, this is the first report of OTC resistance in plant pathogenic xanthomonads.
Highlights
Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot, a major worldwide disease on stone fruits and almond (Garita-Cambronero et al, 2018)
STR resistance threshold in four peach orchards 20–270 km apart (Table 2). These OTCRSTRR strains caused similar bacterial spot symptoms on inoculated peach leaves in the greenhouse compared to the sensitive Xap strains (Supplementary Figure 1)
The correlation between OTCR and STRR in Xap strains suggested that the genes controlling these traits are linked
Summary
Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot, a major worldwide disease on stone fruits (e.g., peach, plum, nectarine, apricot, and cherry) and almond (Garita-Cambronero et al, 2018). Pruni (Xap) causes bacterial spot, a major worldwide disease on stone fruits (e.g., peach, plum, nectarine, apricot, and cherry) and almond (Garita-Cambronero et al, 2018). Producers start spraying during “late dormancy” and continue the practice in 7–10-day intervals throughout the season. Despite these efforts, producers experience millions of dollars in yield loss at harvest in form of unmarketable fruit. Twig infections lead to twig cankers ( called “spring cankers”), often observed at the beginning of the growing season, which are considered the major source of initial inoculum for leaf and fruit infection (Garita-Cambronero et al, 2018; Cox, 2020)
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