Stenotrophomonas maltophilia causing maize (Zea mays L.) seedling soft rot disease, an emerging threat to maize crop

Maize (Zea mays L.) is a widely cultivated monoecious plant, making it vulnerable to seedling soft rot, a significant disease that results in substantial global losses. During the 2020-2021, various regions of Iran reported a series of symptoms, which included seedling stunting, the browning, and rotting of roots and crowns, and post-emergence dieback. To identify the causal agent of this disease, a total number of 70 symptomatic samples were collected, yielding 117 bacterial strains. These bacterial isolates exhibited small, circular, raised colonies with a yellow pigment on yeast-extract peptone glucose agar (YPGA) medium, 48 h post inoculation (hpi). Identification of the putative pathogen as Stenotrophomonas maltophilia (formerly known as Xanthomonas maltophilia) was based on colony morphology, biochemical features, amplification of the species-specific SmeT gene and nucleotide sequences of the gyrB gene. Koch's postulates were completed by inoculating maize seedlings with bacterial suspensions (10(7) CFU/mL). Two weeks after inoculation, symptoms including decay in the lower part of the seedling, wilting and yellowing of leaves became apparent. Subsequently, the bacteria with typical colony morphology were re-isolated from the crowns of the inoculated maize seedlings and identified as S. maltophilia. In addition, various plant species were tested to determine the host range of S. maltopilia. Sugarcane and sorghum exhibited complete collapse, while onion showed no symptoms. Other hosts displayed varying degrees of symptom severity. This study marks the first documented case of maize seedling rot caused by S. maltophilia. These findings offer valuable insights into the pathogenicity of the disease and its management implications.