Abstract
Bacterial leaf streak (BLS), caused by Xanthomonas translucens pv. undulosa (Xtu) and X. translucens pv. translucens (Xtt) on wheat and barley, respectively, persists as a threat to wheat and barley production. Seed is often implicated as a source of primary inoculum, despite conflicting historical reports. To clarify the role of seed transmission and track pathogen movement, seed of BLS susceptible wheat ('Mayville') and barley ('Pinnacle') was inoculated with rifampicin-resistant Xtu and Xtt strains, respectively. Pathogen colonization was assessed in tissue culture box assays and across field trials (2022-2024). Under the controlled environmental conditions of the tissue culture boxes, transmission from inoculated seed into the coleoptile and primary leaf tissues was 100% for both hosts, although the recovery of bacteria was often not accompanied by visual water-soaked symptoms, indicating asymptomatic colonization. In field trials, Generalized Linear Mixed Models confirmed that seed inoculation significantly increased the probability of bacterial detection in plant tissue throughout the growing season. While Xtu successfully colonized inoculated wheat plants during plant development, the rifampicin-resistant strain was not recovered from any of the mature wheat seed that was harvested. In contrast, the Xtt strain was recovered from two-thirds (66.7%) of the barley seed samples harvested at maturity. These results confirm that seed-associated X. translucens can colonize and persist within both wheat and barley, but they highlight a fundamental difference in the long-term persistence and seed contamination rates between Xtu and Xtt. Our results have significant implications for host-specific management and certification programs directed to the control of BLS.