ABSTRACT
Pectic zymogram, RFLP and PCR analyses were used to characterize Rhizoctonia solani AG 3 isolates collected from diseased potatoes in South Australia. The pectic zymogram data were compared with those obtained for isolates collected from central Iran. Analyses of bands corresponding to pectin esterase and polygalacturonase revealed three zymogram subgroups (ZG) in AG 3. In addition to the previously reported ZG7 (here renamed ZG7-1), two new zymogram subgroups, ZG7-2 and ZG7-3, were identified. Of the 446 isolates tested, 50% of the South Australian and 46% of the Iranian isolates were ZG7-1. The majority of the isolates originating from stem and root cankers were ZG7-1, whereas most of the isolates designated ZG7-2 and ZG7-3 originated from tuber-borne sclerotia. Pathogenicity tests revealed that ZG7-1 generally produced fewer sclerotia and more severe cankers of underground parts of the potato plants than the other two ZGs. Two random DNA clones, one originating from an AG 3 isolate and the other from an AG 4 isolate, were used as probes for RFLP analyses of Australian isolates. The AG 3 probe, previously identified to be specific to this group, detected a high level of genetic diversity, with 11 genotypes identified amongst 50 isolates analysed. The low-copy AG 4 probe resolved three genotypes amongst 24 isolates. For 23 isolates analysed with both markers, the combined data distinguished a total of six genotypes and similarity analysis resolved the isolates into two main groups with 50% homology. PCR, using primers for the plant intron splice junction region (R1), also revealed variation. No obvious relationship among pectic zymogram groups, RFLP and PCR genotypes was observed.
Subject(s)
Genetic Variation , Rhizoctonia/genetics , Solanum tuberosum/microbiology , Blotting, Southern , Carboxylic Ester Hydrolases/genetics , Carboxylic Ester Hydrolases/isolation & purification , Carboxylic Ester Hydrolases/metabolism , DNA, Bacterial/analysis , DNA, Bacterial/genetics , Genotype , Iran , Pectins/metabolism , Plant Roots/microbiology , Plant Stems/microbiology , Polygalacturonase/genetics , Polygalacturonase/isolation & purification , Polygalacturonase/metabolism , Polymerase Chain Reaction , Polymorphism, Restriction Fragment Length , Rhizoctonia/isolation & purification , South Australia , Species SpecificityABSTRACT
Rhizoctonia disease, caused by Rhizoctonia solani is one of the most important fungal diseases in bean fields in Isfahan, Iran. Bean plants showing stem and root cankers were collected and Rhizoctonia-like fungi obtained from the samples were identified by anastomosis. Pure cultures of bean isolates of R. solani were identified as AG-4. There were also AG-4 isolates from tomato, potato, cucumber, alfalfa and sugar beet in the areas sampled. A total of 163 isolates of R. solani AG-4 originating from stem and root cankers of beans were examined using pectic zymogram electrophoresis. Polygalacturonase (PG) and pectin estrase isozymes were observed in all AG-4 isolates tested. One (PG) and one pectic esterase (PE) band was found in common between all isolates examined. The electrophoretic patterns were grouped into seven zymogram groups (ZGs) according to the diagnostic PG and PE bands. One ZG occurred in a high frequency throughout the areas sampled. A pathogenicity test was conducted and representative isolates of each ZG were used to inoculate healthy bean plants. The results showed that each ZG caused different symptoms with varying severity. Isolates belonging to two ZGs were highly pathogenic causing root, stem and hypocotyl cankers whereas isolates of the other ZGs produced weak or no symptoms.
