| Maize is mainly produced in Northerneast China which is one of the most important crops in China. Due to the modern growth manner, corn sheath blight is becoming the mainly serious disease on maize, which could be caused by Rhizoctonia spp. So in this study, we carried out the systematic studies on the anastomosis groups identification of Rhizoctonia spp and did further investigation on the genetic diversity of the AG1-IA groups. The main results are summarized as follows:1. Two hundred and eighty-six isolates were obtained from corn sheath blight samples in Northeastern China (including Heilongjiang, Jilin and Liaoning Provinces). Anastomosis group identification showed that the isolates belonged to multinucleate Rhizoctonia AG1-IA, AG1-IB, AG1-IC, AG4-HG-I, AG4-HG-III, AG-5, WAG-Z and binucleate Rhizoctonia AG-Ba. Of these, AG-1-IA was the major anastomosis group (AG) (38.46% of total isolates), followed by WAG-Z (26.92%) and AG-5 (24.83%). AG4-HG-III was isolated for the first time from maize in China.2. Phylogenetic analysis among isolates belonging to different AGs was studied based upon 5.8S rDNA-internal transcribed spacer (ITS) sequence. The results revealed that the isolates of the same AGs (or sub-AG) had a high sequence similarity (98-100%) and different AGs had only 72-89% sequence similarity. So using 5.8S rDNA-ITS analysis, we could easily identify the AGs of unknown isolate. Based on the phylogenetic tree which was constructed with 31 representative measured isolates and 10 isolates belonging to different fusion groups from GenBank, we found that the 41 isolates could be divided into 3 large branches. The 29 isolates which belong to 6 different sub-AG (AG4-HG-III, AG4-HG-I, AG-5, AG1-IC, AG1-IB and AG1-IA) constituted the first branch; the second branch included 5 AG-Ba isolates; the 7 WAG-Z isolates constituted the third branch. From the further analysis of the genetic relationship among the three branches, we could find that binucleate Rhizoctonia AG-Ba isolates and R.solani isolates have close genetic relationship, but multinucleate Rhizoctonia WAG-Z isolates and other isolates have the relatively far relationship.3. The orthogonal design was used to optimize ISSR-PCR amplification system on the AG1-IA groups. Using UBC809 as a primer and L9 (34) orthogonal table, the concentration that the optimum concentration for ISSR-PCR were 2.2 mmol·L-1 Mg2+, 0.15 mmol·L-1 dNTPs, 0.5 U Taq DNA polymerase, 0.32μmol·L-1 primers, 0.5μL templates DNA in 25μl reaction system.4. Genetic variation of the 28 AG1-IA isolates from Jilin, Liaoning, Shandong and Jiangsu provinces was analyzed by ISSR-PCR enlargement reaction technique. Eleven oligonucleotide primers were selected from 35 ISSR primers. A total of 97 bands were amplified, and 74 were polymorphic bands. The rate of polymorphism of the isolates was 76.3%. The dendrogram derived from ISSR data by UPGMA suggested that the 28 isolates could be divided into 4 ISSR groups (IG) with a genetic distance of 0.73. IG1 included 1 isolate from Jiangsu province. IG2 included 5 isolates from Jilin, Liaoning provinces. IG3 included 1 isolate from Liaoning province and 1 isolate from Jiangsu province. IG4 which included the remaining 20 isolates could be further divided into 4 sub-groups at the level of 0.77. The first sub-group included 10 isolates of which 7 isolates from Jilin, Liaoning provinces and 3 isolates from Shandong province; the second sub-group included 3 isolates from Shandong province; the third sub-group included 1 isolate from Liaoning province and 1 isolates from Shandong province; the forth sub-group included 5 isolates from Jiangsu province. These results indicated that the AG1-IA isolates from different parts had certain degree of genetic diversity and also had great degree of similarity. The isolates from Jilin province had a higher degree of genetic similarity with the isolates from Liaoning province. The isolates from Northerneast also had high similarity with the isolates from Shandong province, but they had low similarity with the isolates from Jiangsu province. It proves that the genetic variations of AG1-IA have significant relations to the environment they lived.5. Genetic variation of 28 AG1-IA isolates whose pathogenicity were different was analyzed by ISSR-PCR enlargement reaction technique. Eleven oligonucleotide primers were selected from 35 ISSR primers. A total of 79 bands were amplified, and 55 were polymorphic bands. The rate of polymorphism of the isolates was 69.62%. The dendrogram derived from ISSR data by UPGMA suggested that the 28 isolates could be divided into 3 ISSR groups (IG) with a genetic distance of 0.75. IG1 included 7 strong virulence isolates; IG2 included 1 medium virulence isolates and 2 weak virulence isolates; IG3 included the remaining 18 isolates which were mainly medium virulence and weak virulence isolates. The results showed that some correlation was found between ISSR groups and the isolates'pathogenicity.
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