وقوع و پراکنش بیماری موزاییک باکتریایی گندم در جنوب ایران

بیماری موزاییک باکتریایی گندم ناشی از clavibacter tessellarius، یکی از بیماری‌های بذرزاد گندم است که علایمی نظیر موزاییک و لکه‌های کلروز در سطح برگ ایجاد می‌کند. به منظور بررسی وقوع و تعیین پراکنش این بیماری در سال‌های زراعی 99-1398 از مزارع گندم چهار استان جنوبی کشور شامل فارس، خوزستان، کرمان و بوشهر بازدید و تعداد 192 نمونه‌ ی گیاهی با علایم شبیه به این بیماری جمع‌آوری و جداسازی باکتری از آن‌ها انجام شد. در مجموع، 400 جدایه‌ی باکتریایی جداسازی شد که براساس نتایج مثبت pcr با آغازگرهای اختصاصی جنس کلاویباکتر (cmr16f1/cmr16r1)، آزمون بیماری‌زایی روی گندم و ویژگی‌های فنوتیپی و بیوشیمیایی، 61 جدایه به‌عنوان‌ c. tessellarius تشخیص داده شدند. ده جدایه‌ی نماینده انتخاب و با آغازگرهای 2f/4r در آزمون  pcr قسمتی از ژن gyrb در آن‌ها تکثیر شد. تعیین توالی قطعات تکثیر شده در pcr و جستجوی بلاست با استفاده از ترادف‌های ژن gyrb در ژن بانک تایید کرد که هر ده جدایه بیش‌ترین شباهت (98.8 تا 99.5 درصد) را در ترادف این ژن با جدایه‌های c. tessellarius داشتند و در درخت تبارزایی نیز در یک گروه مجزا از سایر گونه‌هایclavibacter  قرار گرفتند. در مجموع، از 192 مزرعه‌ی نمونه‌برداری شده، از 53 مزرعه (27.60 درصد) باکتری c. tessellarius جداسازی شد. وقوع بیماری، براساس درصد مزارعِ آلوده، در استان‌های فارس، خوزستان، کرمان و بوشهر به ترتیب 35.7، 30.55، 22.7 و 7.1 درصد بود. این بیماری برای اولین بار از استان‌های خوزستان، کرمان و بوشهر گزارش می‌شود و وقوع آن در سایر استان‌های کشور دور از انتظار نیست.

Occurrence and distribution of wheat bacterial mosaic in southern Iran

Background and ObjectivesThe bacterial mosaic induced by Clavibacter tessellarius is one of the wheat seedborne 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 wheatgrowing 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 noninfectious wheat diseases. This study aimed to determine the occurrence and distribution of wheat bacterial mosaic in wheat fields in southern Iran.Materials and MethodsBetween 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 semiselective media, and Grampositive isolates were selected using Gram staining and KOH solubility tests. The isolates were identified using a polymerase chain reaction (PCR) with Clavibacterspecific 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 neighborjoining method.ResultsIn 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.DiscussionC. 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.