Symptomatology and Host Range of Clavibacter Species Associated with Small Grain Cereals

Bacterial mosaic of small grain cereals is caused by five Clavibacter species, i.e., Clavibacter davisi, Clavibacter rahimiani, Clavibacter tessellarius, Clavibacter vidaverae, and Clavibacter zhangzhiyongii. While taxonomic range of the causal agents has only recently been investigated, biological features and host range of the pathogens remain unexplored. In this study, phenotypic features, symptomatology, and host range of the five Clavibacter species were evaluated. The host range assays were performed on seven small grain cereals’ species, i.e., barley, maize, millet, oat, sorghum, triticale, and wheat under controlled greenhouse conditions. The five Clavibacter species showed considerable differences in colony color and morphology on agar-based culture media. The range of field symptoms induced by Clavibacter species under field conditions expands beyond mosaic patterns and yellowing, including necrotic lesions and leaf desiccation. Whereas water-soaking and oozing were not observed in the bacterial mosaic infections. Under greenhouse conditions, symptoms were similar to those recorded in the field. Clavibacter tessellarius strains were the most aggressive agents with the widest host range, infecting all evaluated plant species except maize. Clavibacter rahimiani and Clavibacter zhangzhiyongii strains induced noticeable symptoms only on sorghum and oat plants, while Clavibacter vidaverae and Clavibacter davisi did not induce any symptoms on oat and millet. Sorghum plants became infected with all strains of the five Clavibacter species, while only Clavibacter tessellarius strains infected millet plants. These findings provided novel insight into the biology of bacterial mosaic pathogens and highlighted the importance of precise symptomatology of the disease in the field surveys and quarantine inspections.