Bacterial strains used for specificity testing (Table 2; Table 3) were cultured on suitable agar media and incubated at appropriate temperatures and durations. Species verification was performed with MALDI‐TOF (Biotyper Microflex LT; Bruker Daltonics). Isolates of uncertain taxonomic status were classified and confirmed as non‐Y. ruckeri by whole‐genome‐based analyses (Figure S1) as described previously (Riborg et al., 2022 (link)).For spiking experiments, Y. ruckeri CC1 strain NVI‐10705 was cultured in Tryptic Soy Broth at 22°C with shaking until mid‐log phase, from which a dilution series was prepared with sterile phosphate‐buffered saline (PBS) chilled on ice and enumerated by plating on 5% bovine blood agar (BA) in triplicate. For the challenge trials, Y. ruckeri NVI‐10705 was grown in Brain Heart Infusion Broth at 15°C with shaking for 20 hr, harvested by centrifugation and re‐suspension in PBS, followed by enumeration on a cell counter (Casy Inovatis; Roche Diagnostics) and by plating of a 10‐fold dilution series on BA. All BA plates were incubated at 22°C for 2 days prior to counting.