Isolates of an unidentified Rhizoctonia sp. (UN isolates) were obtained from Japanese zoysia grass (Zoysia japonica Steud) that exhibited symptoms of a new sheath rot disease. UN isolates were binucleate and showed hyphal fusion with tester isolates of Rhizoctonia anastomosis group (AG)-D. Those isolates were compared with isolates of subgroups I and II of Rhizoctonia AG-D based on cultural morphology, hyphal growth rate at different temperatures, anastomosis frequency, pathogenicity, and sequence analysis of the internal transcribed spacer (ITS) region of ribosomal DNA genes (rDNA-ITS region). The mycelial color of UN isolates was light yellow which differs from AG-D I but is similar to AG-D II. Sclerotia of UN isolates were dark brown in color and larger in size (1 to 3 mm in diameter) than those of AG-D subgroup I (1 mm in diameter), whereas isolates of AG-D II produced white mycelial clamps 4 to 5 mm in size. Hyphal growth rate of UN isolates was slower than that of two AG-D subgroups at several temperatures, especially 25°C. In pathogenicity tests on Japanese zoysia grass, UN isolates showed moderate disease severity and lower pathogenicity than isolates of AG-D subgroups I and II. Sequences of the rDNA-ITS region within UN isolates were almost homologous, but had lower homology with subgroups AG-D I or II. Phylogenetic trees constructed using ITS sequences showed that UN isolates formed an individual cluster that differed from the clusters of the two subgroups. We propose that UN isolates are a new subgroup of Rhizoctonia AG-D, subgroup III, and the name of the disease is “spring-rot” on Japanese zoysia grass.
Use of single-protoplast isolates in the study of the mating phenomena of Rhizoctonia solani (Thanatephorus cucumeris) AG-1 IC and IA Abstract This study evaluates the effectiveness of using single-protoplast isolates (SPIs) to study the mating phenomena of Rhizoctonia solani AG-1 IC and IA. SPIs obtained from three fi eld isolates (F-1, Rh28, and RO2) of AG-1 IC were paired with representative single-basidiospore isolate (SBI)-M1/-M2 testers, each from their own fi eld isolates, or paired in all possible combinations. Tufts were formed between SPIs and SBI-M1/-M2 testers and between SPIs-M1 and -M2. The separation ratios of SPIs-M1 and -M2 were approximately 1 : 1, which were similar to the results obtained with SBIs. SPIs obtained from three isolates (GNSD, R59, and Tr8) of AG-1 IA, which failed to form basidiospores, were paired in all possible combinations. Although no tufts formed among SPIs from Tr8 and R59, tufts did form between SPIs from GNSD. SPIs from GNSD were separated into homokaryotic (-M1 or -M2) and heterokaryotic isolates, and the separation ratio of -M1 and -M2 was also around 1 : 1. Amplifi ed fragment length polymorphism (AFLP) phenotypes of the tuft isolates formed between GNSD SPIs-M1 and -M2 suggested that these tuft isolates were all heterokaryotic. These results indicate that all three isolates of AG-1 IC and one isolate GNSD of AG-1 IA are heterokaryotic, and that the other two isolates of Tr8 and R59 of AG-1 IA are homokaryotic. Single-protoplast isolates are effective for studies of the mating phenomena of isolates belonging to different AGs of R. solani that could not form a perfect stage.
Single-basidiospore isolates (SBIs) were obtained from field isolates of Thanatephorus cucumeris (Rhizoctonia solani) AG-1 IC and AG-2-2 IV. Formation of distinctive tufts, a recognized feature of heterokaryon synthesis, was observed, and isolates derived from hyphal-tipped tuft hyphae were obtained following pairings between various strains. Three distinctive types of tufts were formed: the fibrous type of mating-compatible homokaryon-homokaryon (Hom-Hom) pairings, the sparse type between heterokaryon-homokaryon (Het-Hom) pairings originating from one parent, and the compact type between Het-Hom pairings originating from different parents. Amplified Fragment Length Polymorphism (AFLP) profile of fingerprints of these tuft isolates verified that they were all heterokaryotic. Because of heterokaryotic vigor, the growth and pathogenicity of the majority of tuft isolates increased compared with their contributing SBIs. New somatic compatibility groups (SCGs) that were different from parental field isolates occurred following heterokaryon formation within T. cucumeris. Tuft isolates produced by Hom-Hom and Het-Hom pairings among isolates of different parents yielded no somatic compatibility with the original parent isolates and a high frequency of new SCGs (62–100%). This was in contrast to those produced by Hom-Hom and Het-Hom pairings among isolates with a common parent that yielded only 12–37% new SCGs. The SCG diversity of R. solani in the field may be attributed to new fitter heterokaryons formed between a heterokaryon of one pair of parents and a homokaryon of another parent pair. This mechanism greatly contributes to genetic diversity in the field and accounts for the failure to recover the expected distribution of SCGs from a field population.
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