Solani AG-1 IA Research Articles

Genetic Structure of Populations of Rhizoctonia solani Anastomosis Group-1 IA from Soybean in Brazil

The Basidiomycete fungus Rhizoctonia solani anastomosis group (AG)-1 IA is a major pathogen of soybean in Brazil, where the average yield losses have reached 30 to 60% in some states in Northern Brazil. No information is currently available concerning levels of genetic diversity and population structure for this pathogen in Brazil. A total of 232 isolates of R. solani AG1 IA were collected from five soybean fields in the most important soybean production areas in central-western, northern, and northeastern Brazil. These isolates were genotyped using 10 microsatellite loci. Most of the multilocus genotypes (MLGTs) were site-specific, with few MLGTs shared among populations. Significant population subdivision was evident. High levels of admixture were observed for populations from Mato Grosso and Tocantins. After removing admixed genotypes, three out of five field populations (Maranhao, Mato Grosso, and Tocantins), were in Hardy-Weinberg (HW) equilibrium, consistent with sexual recombination. HW and gametic disequilibrium were found for the remaining soybean-infecting populations. The findings of low genotypic diversity, departures from HW equilibrium, gametic disequilibrium, and high degree of population subdivision in these R. solani AG-1 IA populations from Brazil are consistent with predominantly asexual reproduction, short-distance dispersal of vegetative propagules (mycelium or sclerotia), and limited long-distance dispersal, possibly via contaminated seed. None of the soybean-infecting populations showed a reduction in population size (bottleneck effect). We detected asymmetric historical migration among the soybean-infecting populations, which could explain the observed levels of subdivision.

Detection and Quantification of Rhizoctonia solani AG-1 IA, the Rice Sheath Blight Pathogen, in Rice Using Real-Time PCR.

Rhizoctonia solani Kühn is the causal organism of sheath blight, a major rice disease worldwide that severely impairs yield and quality. It is difficult to identify the pathogen in the early phase of the infection and to accurately quantify the fungal development based on visual inspection. Therefore, a rapid and reliable method is advantageous for the detection and quantification of the pathogen causing this important rice disease. In this study, a real-time, quantitative polymerase chain reaction (QPCR) assay was developed to detect and quantify R. solani AG-1 IA DNA from infected rice plants. A specific primer pair was designed based on the internal transcribed spacer region of the fungal ribosomal DNA. The specific detection of R. solani DNA was successful with quantities as low as 1 pg. The QPCR assay could be used for detecting the rice sheath blight pathogen, quantifying fungal aggressiveness, and evaluating the resistance level of rice cultivars.

Incompatibilidade somática em Rhizoctonia solani AG-1 IA da soja

O fungo Rhizoctonia solani AG-1 IA é um dos patógenos mais importantes que afeta a cultura da soja no Brasil, causando a mela ou queima foliar. A doença está associada com a fase teleomórfica de R. solani, o basidiomiceto Thanatephorus cucumeris. Neste estudo, baseando em conhecimento prévio sobre a biologia de R. solani AG-1 IA, duas hipóteses foram testadas. Na primeira hipótese postulou-se a ocorrência de incompatibilidade somática em populações de R. solani AG-1 IA. A segunda hipótese testada foi de que esta população de R. solani AG-1 IA da soja apresenta indicações de estrutura sexual clonal. Duas amostras de isolados de R. solani AG-1 IA da soja obtidas no Maranhão e no Mato Grosso foram utilizadas. Na primeira amostra, foram selecionados isolados apresentando diferentes perfis de RAPD (Random Amplified Polymorphic DNA), procurando maximizar a diversidade dos isolados, e evitando a introdução de possíveis clones no teste. Os isolados foram pareados em todas as combinações possíveis em meio de BDA mais carvão ativado e examinados quanto às interações somáticas resultantes. Seis grupos de incompatibilidade somática (GCS) foram detectados entre 24 isolados do AG-1 IA. Entretanto, análises microscópicas dos pareamentos entre isolados indicaram maior freqüência de incompatibilidade somática, impossibilitando o grupamento em GCS. No geral, a metodologia de avaliação das interações somáticas macroscópicas em meio BDA + carvão ativado, não se mostrou totalmente apropriada para discriminação das categorias de reações de compatibilidade entre isolados de R. solani AG-1 IA. Com a segunda amostra procurou-se determinar a ocorrência de clones na população do patógeno, ou seja, isolados que compartilham o mesmo padrão fenotípico de RAPD e somaticamente compatíveis. No caso de R. solani AG 1 IA da soja, a gama de interações somáticas entre pareamentos de isolados e, principalmente, os desvios na associação estrita entre os GCS detectados neste trabalho, conjuntamente com os perfis de RAPD observados anteriormente por Fenille (11) e Meyer (20), são consistentes com recombinação. Entretanto, o patógeno ainda apresenta um componente clonal expressivo na população. De um total de 43 isolados, os exemplos de prováveis clones na população do patógeno totalizaram 16 isolados.

Effect of doses of fungicides and plant resistance activators on the control of Rhizoctonia foliar blight of soybean, and on Rhizoctonia solani AG1–IA in vitro development

Rhizoctonia foliar blight (RFB) of soybean [ Glycine max (L.) Merrill] occurs in many tropical and subtropical regions, causing yield reductions of up to 70% and in Brazil, up to 60%. The disease is caused by Rhizoctonia solani AG1–IA and AG1–IB, and by AG2–3 in Japan. RFB occurs in the North, Northeast and Mid-west regions of Brazil. Chemical control remains the only effective method of controlling RFB, but its efficiency depends upon environmental conditions. In this study, 18 fungicides, salicylic acid (SA) and acibenzolar- s-methyl (ASM) were evaluated on R. solani AG1–IA in vitro, by mycelial growth rating and estimating effective concentration for 50% (EC 50) and 90% (EC 90) inhibition of mycelial growth, and in vivo by reduction of disease severity on soybean plants in greenhouse conditions. Mycelial growth was strongly inhibited by the fungicides pyraclostrobin+boscalid and fludioxonil. Preventive fungicide applications were the most effective. Strobilurins were more efficient both in preventive and curative applications. Best results with plant resistance activators were obtained with SA (2.5 mM) sprayed at 20 d before inoculation and with ASM (12.5 mg a.i. l −1) 10 d before inoculation.

Intraspecific Evolution of Rhizoctonia solani AG-1 IA Associated with Soybean and Rice in Brazil based on Polymorphisms at the ITS-5.8S rDNA Operon

Rhizoctonia solani AG-1 IA causes leaf blight on soybean and rice. Despite the fact that R. solani AG-1 IA is a major pathogen affecting soybean and rice in Brazil and elsewhere in the world, little information is available on its genetic diversity and evolution. This study was an attempt to reveal the origin, and the patterns of movement and amplification of epidemiologically significant genotypes of R. solani AG-1 IA from soybean and rice in Brazil. For inferring intraspecific evolution of R. solani AG-1 IA sampled from soybean and rice, networks of ITS-5.8S rDNA sequencing haplotypes were built using the statistical parsimony algorithm from Clement et al. (2000) Molecular Ecology 9: 1657-1660. Higher haplotype diversity (Nei M 1987, Molecular Evolutionary Genetics Columbia University Press, New york: 512p.) was observed for the Brazilian soybean sample of R. solani AG-1 IA (0.827) in comparison with the rest of the world sample (0.431). Within the south-central American clade (3-2), four haplotypes of R. solani AG-1 IA from Mato Grosso, one from Tocantins, one from Maranhao, and one from Cuba occupied the tips of the network, indicating recent origin. The putative ancestral haplotypes had probably originated either from Mato Grosso or Maranhao States. While 16 distinct haplotypes were found in a sample of 32 soybean isolates of the pathogen, the entire rice sample (n=20) was represented by a single haplotype (haplotype 5), with a worldwide distribution. The results from nested-cladistic analysis indicated restricted gene flow with isolation by distance (or restricted dispersal by distance in nonsexual species) for the south-central American clade (3-2), mainly composed by soybean haplotypes.

Population structure of the rice sheath blight pathogen Rhizoctonia solani AG-1 IA from India

The population structure of Rhizoctonia solani AG-1 IA causing rice sheath blight from India was evaluated for 96 isolates using seven RFLP loci. Nineteen of the isolates did not hybridise to R. solani AG-1 IA RFLP probes and rDNA analyses subsequently confirmed that they were either Ceratobasidium oryzae-sativae isolates or another Rhizoctonia sp. The population structure of the remaining 77 R. solani AG-1 IA Indian isolates was similar to that of a previously characterized Texas population. Clonal dispersal of R. solani AG-1 IA in India was moderate within fields and no clones were shared among field populations. Low levels of population subdivision and small genetic distances among populations were consistent with high levels of gene flow. Frequent sexual reproduction was indicated by the fact that most populations were in Hardy–Weinberg equilibrium (HWE). The two loci (R68 and R111) that deviated significantly from HWE showed an excess of heterozygosity. Although Texas and Indian populations were geographically very distant, they exhibited only moderate population subdivision, with an FST value of 0.193.

Synergistic Antifungal Activity of the Culture Filtrates of Serratia marcescens Strain B2 and Chemical Fungicides against the Sclerotial Viability of the Rice Sheath Blight Pathogen, Rhizoctonia solani

Serratia marcescens strain B2 is an antagonistic bacterium that inhibits the fungal growth of Rhizoctonia solani Kuhn AG-1 IA and suppresses the rice sheath blight caused by R. solani AG-1 IA. The sclerotial germination of R. solani AG-1 IA decreased when the sclerotia were incubated in the culture filtrate of the strain B2. The antifungal activity of the culture filtrate was enhanced by the treatment in combination with low concentrations of the chemical fungicides, flutolanil, pencycuron and validamycin. There was a synergistic antifungal activity produced by the culture filtrate of the strain B2 and chemical fungicides on the sclerotial viability of R. solani AG-1 IA.

Characterization of Rhizoctonia solani Associated with Soybean in Brazil

Rhizoctonia solani causes pre- and post-emergence damping-off, root and hypocotyl rot and foliar blight in soybean. Foliar blight has resulted in yield losses of 31–60% in north and northeast Brazil. The aim of this study was to characterize isolates of R. solani associated with soybean in Brazil. Among 73 Rhizoctonia isolates examined, six were binucleate and 67 were multinucleate. The multinucleate iso1ates were characterized according to hyphal anastomosis reaction, mycelial growth rate, thiamine requirement, sclerotia production, and RAPD molecular markers. Four isolates that caused hypocotyl rot belonged to AG-4 and using RAPD analysis they grouped together with the HGI subgroup. Another isolate that caused root and hypocotyl rots was thiamine auxotrophic, grew at 35°C, and belonged to AG-2-2 IIIB. All 62 isolates that caused foliar blight belonged to AG-1 IA. RAPD analysis of R. solani AG-1 IA soybean isolates showed high genetic similarity to a tester strain of AG-1 IA, confirming their classification. The teleomorph of R. solani, Thanatephorus cucumeris was produced in vitro by one AG-1 IA isolate from soybean. The AG-4 and AG-2-2 IIIB isolates caused damping-off and root and hypocotyl rots of soybean seedlings cv. ‘FT-Cristalina’, under greenhouse conditions. The AG-2-2 IIIB isolate caused large lesions on the cortex tissue, that was distinct from the symptoms caused by AG-4 isolates. The AG-1 IA isolates caused foliar blight in adult soybean plants cv. ‘Xingu’ under the greenhouse and also in a detached-leaf assay.

Distribution of Vegetatively Compatible Populations of Rhizoctonia solani AG1-IA in a Field Planted with Different Host Species

To understand the distribution pattern and divergence of Rhizoctonia solani in a field over a 4-year period, R. solani AG1-IA isolates were collected from diseased tissues of several crops. Pairing tests between isolates to detect hyphal anastomosis and vegetatively compatible population (VCP) groupings were done on 2% water agar and potato dextrose agar. A single VCP of R. solani AG1-IA dominated a large upland crop field at the Institute of Plant Breeding, University of the Philippines at Los Banos. The VCP changed more slowly and at a lower frequency as compared to other reports.

Efficacy of Hypovirulent Binucleate Rhizoctonia sp. to Control Banded Leaf and Sheath Blight in Corn

Water agar, artificially infested soil and leaf sheath inoculation methods were used to assess the suitable time of application, varietal host response and persistence of Rhv7, a hypovirulent, binucleate Rhizoctonia collected from soil in the Philippines, to effectively control virulent Rhizoctonia solani AG1-IA isolate RS35 on corn. With the water agar method, prior inoculation (2 to 3 days) with the biocontrol agent is essential to achieve maximum control of the pathogen. Disease protection was increased from 23 to 70% as the pre-incubation time of Rhv7 prior to the challenge inoculation with virulent isolate was lengthened from 0 to 3 days. Disease severity and incidence of banded leaf and sheath blight (BLSB) on the three corn hybrids were also suppressed in artificially infested soil. In leaf sheath inoculation, the suppressive ability of Rhv7 against BLSB on corn persisted during lesion expansion. This suppression was expressed as slower disease progress in plants with Rhv7 than in plants without Rhv7. Early and timely brace root formation that detached infected sheaths, also aided in reducing the number of diseased plants in Rhv7-treated plots at the final observation. Mycelial growth activity of RS35 was reduced when corn plants were pre-inoculated with Rhv7 before challenge even if there was no contact detected between Rhv7 and RS35, which suggests that Rhv7 protects corn against BLSB by induced resistance. The hypovirulent, binucleate Rhizoctonia Rhv7 strain effectively controlled R. solani AG1-IA isolate RS-35 in corn.

High Levels of Gene Flow and Heterozygote Excess Characterize Rhizoctonia solani AG-1 IA (Thanatephorus cucumeris) from Texas

To date, much of the genetics of the basidiomycete Thanatephorus cucumeris (anamorph=Rhizoctonia solani) remains unknown. Here, we present a population genetics study using codominant markers to augment laboratory analyses. Seven single-copy nuclear RFLP markers were used to examine 182 isolates of Rhizoctonia solani AG-1 IA collected from six commercial rice fields in Texas. Thirty-six multilocus RFLP genotypes were identified. Population subdivision analyses indicated a high degree of gene flow/migration between the six geographic populations. Tests for Hardy–Weinberg equilibrium (HWE) among the 36 multilocus RFLP genotypes revealed that four of the seven loci did not significantly differ from HWE. Subsequent analysis demonstrated that departures from HWE at the three remaining loci were due to an excess of heterozygotes. Data presented here suggest that R. solani AG-1 IA is actively outbreeding (heterothallic). Possible explanations for heterozygote excess, which was observed at all seven RFLP loci, are discussed.

Identification of Rhizoctonia solani AG-UNK from Rice and Rice Fields in Texas as AG-11.

A survey of Texas rice fields in 1984 and 1985 yielded collections of Rhizoctonia solani AG-1 IA (causal agent of sheath blight of rice), R. oryzae (causal agent of sheath spot of rice), and a collection of eight multinucleate Rhizoctonia solani-like isolates that would interanastomose, but not anastomose, with tester isolates of AG-1 through AG-8 (representing those available at that time). In 1985, the isolates were characterized as R. solani AG-UNK (2). Isolates were recovered as atypical sclerotia from elutriated field soils in rice-soybean and rice-fallow rotations. Isolates also were recovered from late-season stem lesions nearly identical to those associated with sheath spot disease and from rice residues at locations throughout the upper Gulf Coast of Texas but at extremely low frequencies compared with recovery of R. solani AG-1 IA and R. oryzae. Teleomorphs of R. solani AG-UNK were observed during middle to late season on rice sheaths and matched descriptions of Thanatephorus cucumeris. Isolates were pathogenic on rice and soybean foliage in greenhouse trials but caused no significant yield losses when inoculated on adult rice plants (50 days after emergence) in field trials (2). Isolates exhibited mean hyphal diameters of 5.1 μm, averaged 8.3 nuclei per penultimate cell, grew 0.53 mm/h at 28°C on potato dextrose agar, and were negative in phenol tests (2). From samples maintained in storage during the past 15 years, the isolates have now been identified as AG-11 based on positive anastomosis with tester strains of AG-11 (1). This report records the occurrence of AG-11 in Texas, establishes the identity of the AG-UNK group, and expands the known geographic range of AG-11 in the United States.

Soybean (Glycine max) Response to Weed Interference and Rhizoctonia Foliar Blight (Rhizoctonia solani)

Field studies evaluated response of soybean to Rhizoctonia foliar blight (RFB) disease in combination with varying densities of common cocklebur, hemp sesbania, or johnsongrass. Soybean plants at both V10 and R1 growth stages were not inoculated or inoculated with suspensions containing equal concentrations ofRhizoctonia solaniAG-1 IA and IB mycelia. Intensity of RFB was rated weekly beginning at V1 soybean growth stage, and data were used to determine area under disease progress curves. Intensity of RFB was greater in 1993 than in 1994. When averaged across weed species and weed densities, soybean yield in 1993 was reduced 18% in plots inoculated withR. solanicompared with those not inoculated. Intensity of RFB, however, did not differ between inoculated and noninoculated plots in 1994. Interactions betweenR. solaniand weed density for RFB intensity and yield were not significant either year. Soybean yields in 1994, however, were reduced by hemp sesbania and johnsongrass in inoculated plots. Soybean maturity was delayed both years when hemp sesbania was present.

Herbicide Effects onRhizoctonia solaniin Vitro and Rhizoctonia Foliar Blight of Soybean (Glycine max)

Acifluorfen, alachlor, glufosinate, glyphosate, paraquat, and pendimethalin were evaluated for their effects on mycelial growth and sclerotia/microsclerotia production byRhizoctonia solaniAG-1 IA and IB in culture. All of these herbicides except glufosinate and glyphosate were evaluated for effects on severity of Rhizoctonia foliar blight of soybean in the field. In laboratory studies, all herbicides reduced colony radius ofR. solani.Growth reductions for IB were greater than for IA in the presence of pendimethalin, alachlor, and acifluorfen, but glufosinate reduced growth of IA more than IB. Sclerotia production by both isolates was prevented by paraquat, greatly reduced by glufosinate, but markedly less affected by the other herbicides tested. In field studies, all tested herbicides influenced severity of Rhizoctonia foliar blight when disease pressure was low, but only paraquat reduced severity when disease pressure was high.

Characterization and Pathogenicity of Rhizoctonia spp. Isolated from Rice, Soybean, and Other Crops Grown in Rotation with Rice in Texas

Isolates of Rhizoctonia spp. collected from diseased rice sheaths were characterized as R. solani AG-1 IA (sasakii form), R. oryzae, and a previously indescribed collection of isolates of R. solani designated AG-UNK that failed to anastomose with AG tester isolates 1 through 8 of R. solani but anastomosed strongly with each other. R. solani AG-1 IA and AG-1 IB (web blight form) were recovered from diseased foliage of soybean showing characteric signs and symptoms of aerial blight and web blight, respectively