First report of Pectobacterium versatile causing aerial stem rot on potato crops in Mexico

In February 2023, potato (Solanum tuberosum L.) plants (cv. Fianna) exhibiting symptoms of aerial stem rot (water-soaked and brown lesions on the stem) were observed in a field in the Guasave Valley, northern Sinaloa, Mexico with an incidence of approximately 40%. Ten symptomatic stem fragments from five diseased plants were disinfected, plated on crystal violet pectate (CVP) medium, and incubated at 28°C for 24 h (Hélias, 2012). Single colonies that formed cavities were selected for further analysis. Five representative isolates, showing biochemical and morphological characteristics consistent with Pectobacterium spp. (Palafox et al., 2024), were chosen. These characteristics included catalase activity, pectinolytic activity, Gram-negative bacilli morphology, absence of oxidase activity, and non-fluorescence on King's B medium. DNA extraction was performed according to the protocol of Chen and Kuo (1993), with minor modifications. The recombinase A (recA) gene was amplified by PCR using primers described by Waleron et al. (2002) for members of the former Erwinia genus. BLASTn results of the recA sequences obtained from the isolates (accession numbers: PQ788255-PQ788259) showed 100% identity and coverage with the Pectobacterium versatile type strain 14A (accession number: CP034276.1). Two isolates were selected as representatives for further analysis. Multi-locus sequence analysis (MLSA) of concatenated sequences (1789 bp) from the genes dnaX (PQ788260 and PQ788261), dnaJ (PQ788262 and PQ788263), and gyrB (PQ788264 and PQ788265) was conducted as described by Mainello-Land et al. (2024) on MEGA version 11 (Tamura et al., 2021), comparing sequences with those of Pectobacterium species type strains. Both isolates from northern Sinaloa clustered with the P. versatile 14A strain, supported by a bootstrap value of 100%, confirming their taxonomic identity as P. versatile. The pathogenicity of both isolates was evaluated by inoculating bacterial suspensions (10 mM MgSO₄, OD600 = 0.8, approximately 10⁸ CFU/mL) into the stems of 5-week-old potato plants (cv. Fianna). Three plants per isolate were inoculated, along with negative control plants, and plants treated with sterile 10 mM MgSO₄ as a separate control. The plants were maintained at 28°C with 80% relative humidity. Three days post-inoculation, symptoms of aerial stem rot-including decay, blackening, and tissue liquefaction-were observed in the inoculated plants, while the negative control and sterile solution-treated plants remained asymptomatic. To fulfill Koch's postulates, bacteria were re-isolated from symptomatic tissues as cavity-forming colonies on CVP medium. The identity of the re-isolated strains was confirmed by PCR using specific primers for P. versatile designed by Su et al. (2023). The experiment was performed in duplicates, and the results were consistent across replicates. To our knowledge, this study provides the first report of P. versatile as the causal agent of potato aerial stem rot in Mexico. Recently, Pectobacterium polare (formerly Pectobacterium polaris) has been reported in northern Sinaloa as a causative agent of potato aerial stem rot (Santos-Cervantes et al., 2024). This highlights the emergence of the Pectobacterium genus as a significant threat to local potato producers and underscores the need for extensive sampling to assess its diversity in the potato-producing regions of northern Sinaloa.