Occurrence and distribution of wheat bacterial mosaic in southern Iran

The bacterial mosaic induced by Clavibacter tessellarius is one of the wheat seed-borne diseases that causes mosaic and chlorotic leaf spots on infected plants. The disease has previously been reported in a few regions within Iran. There is still no information on the disease's prevalence and distribution in the major wheat-growing areas of southern Iran. The disease may have gone unnoticed in part due to its mild symptoms and resemblance to those caused by viruses and certain non-infectious wheat diseases. This study aimed to determine the occurrence and distribution of wheat bacterial mosaic in wheat fields in southern Iran.

Between 2019 and 2020, surveys in wheat fields in four southern Iranian provinces, including Fars, Khuzestan, Kerman, and Bushehr were conducted, and 192 plant samples with symptoms resembling the bacterial mosaic were collected. Bacteria were isolated and purified using semi-selective media, and Gram-positive isolates were selected using Gram staining and KOH solubility tests. The isolates were identified using a polymerase chain reaction (PCR) with Clavibacter-specific primer pair CMR16F1/CMR16R1, as well as their phenotypic and pathogenicity characteristics. Additionally, a fragment of the gyrB gene from the representative isolates was amplified by PCR using primer pair 2F/4R. The PCR products were purified and directly sequenced. The obtained sequences were aligned, trimmed, and compared to those of related strains in the GenBank using the BLAST search method. A phylogenetic tree based on the gyrB sequences was constructed using the MEGAX software employing the neighbor-joining method.

In total, 400 bacterial isolates were obtained. Based on positive PCR results with specific primers for the genus Clavibacter along with the pathogenicity test on wheat, and the phenotypic and biochemical characteristics, 61 isolates were identified as C. tessellarius. In ten representative isolates, a ~530 bp fragment of the gyrB gene was amplified by PCR. The gyrB gene partial sequences were edited and deposited in GenBank under the accession numbers MZ014474–MZ01483. A BLAST search confirmed that all ten isolates shared the highest nucleotide identity (98.8% to 99.5%) with their corresponding sequences in C. tessellarius strains in GenBank. In the phylogenetic tree, these isolates with C. tessellarius strains from GenBank were clustered in a clade distinct from Clavibacter species and subspecies of Clavibacter michiganensis.

C. tessellarius was isolated from 53 out of 192 wheat fields (27.60%), exhibiting bacterial mosaic symptoms. The Fars, Khuzestan, Kerman, and Bushehr provinces had disease incidences of 35.7% (30 infected farms out of 84 sampled farms), 30.55%, 22.7%, and 7.1%, respectively, indicating the relatively widespread occurrence of the disease in these provinces. To our knowledge, the wheat bacterial mosaic is reported for the first time in the provinces of Khuzestan, Kerman, and Bushehr, and its occurrence in other parts of the country would not be unexpected.