Genetic variation and pathogenicity of anastomosis group 2 isolates of Rhizoctonia solani in Australia

Metatranscriptomic Sequencing of Sheath Blight-Associated Isolates of Rhizoctonia solani Revealed Multi-Infection by Diverse Groups of RNA Viruses

Rice sheath blight, caused by the soil-borne fungus Rhizoctonia solani (teleomorph: Thanatephorus cucumeris, Basidiomycota), is one of the most devastating phytopathogenic fungal diseases and causes yield loss. Here, we report on a very high prevalence (100%) of potential virus-associated double-stranded RNA (dsRNA) elements for a collection of 39 fungal strains of R. solani from the rice sheath blight samples from at least four major rice-growing areas in the Philippines and a reference isolate from the International Rice Research Institute, showing different colony phenotypes. Their dsRNA profiles suggested the presence of multiple viral infections among these Philippine R. solani populations. Using next-generation sequencing, the viral sequences of the three representative R. solani strains (Ilo-Rs-6, Tar-Rs-3, and Tar-Rs-5) from different rice-growing areas revealed the presence of at least 36 viruses or virus-like agents, with the Tar-Rs-3 strain harboring the largest number of viruses (at least 20 in total). These mycoviruses or their candidates are believed to have single-stranded RNA or dsRNA genomes and they belong to or are associated with the orders Martellivirales, Hepelivirales, Durnavirales, Cryppavirales, Ourlivirales, and Ghabrivirales based on their coding-complete RNA-dependent RNA polymerase sequences. The complete genome sequences of two novel RNA viruses belonging to the proposed family Phlegiviridae and family Mitoviridae were determined.

Global distribution, traditional and modern detection, diagnostic, and management approaches of Rhizoctonia solani associated with legume crops

Sustainable development relies heavily on a food system that is both safe and secure. Several approaches may lead to sustainability and food safety. An increase in the cultivation of legume crops is one of the approaches for enhancing agricultural viability and ensuring adequate food supply. Legumes may increase daily intake of fiber, folate, and protein as substitutes for meat and dairy. They are also crucial in various intercropping systems worldwide. However, legume production has been hampered by Rhizoctonia solani due to its destructive lifestyle. R. solani causes blights, damping off, and rotting diseases in legume crops. Our knowledge of the global distribution of R. solani associated with legume crops (alfalfa, soybean, chickpea, pea, lentil, common bean, and peanut), detection, diagnosis, and management of legume crops diseases caused by R. solani is limited. Traditional approaches rely on the incubation of R. solani, visual examination of symptoms on host legume crops, and microscopy identification. However, these approaches are time-consuming, require technical expertise, fail to detect a minimal amount of inoculum, and are unreliable. Biochemical and molecular-based approaches have been used with great success recently because of their excellent sensitivity and specificity. Along with conventional PCR, nested PCR, multiplex PCR, real-time PCR, magnetic-capture hybridization PCR, and loop-mediated isothermal amplification have been widely used to detect and diagnose R. solani. In the future, Next-generation sequencing will likely be used to a greater extent to detect R. solani. This review outlines global distribution, survival, infection and disease cycle, traditional, biochemical, molecular, and next-generation sequencing detection and diagnostic approaches, and an overview of the resistant resources and other management strategies to cope with R. solani.

Surveillance of Pathogenicity of Rhizoctonia solani Japanese Isolates with Varied Anastomosis Groups and Subgroups on Arabidopsis thaliana

Rhizoctonia solani is a necrotrophic plant pathogen with a wide host range. R. solani is a species complex consisting of thirteen anastomosis groups (AGs) defined by compatibility of hyphal fusion reaction and subgroups based on cultural morphology. The relationship between such classifications and host specificity remains elusive. Here, we investigated the pathogenicity of seventeen R. solani isolates (AG-1 to 7) in Japan towards Arabidopsis thaliana using leaf and soil inoculations. The tested AGs, except AG-3 and AG-6, induced symptoms in both methods with variations in pathogenicity. The virulence levels differed even within the same AG and subgroup. Some isolates showed tissue-specific infection behavior. Thus, the AGs and their subgroups are suggested to be not enough to define the virulence (host and tissue specificity) of R. solani. We also evaluated the virulence of the isolates on Arabidopsis plants pretreated with salicylic acid, jasmonic acid, and ethylene. No obvious effects were detected on the symptom formation by the virulence isolates, but ethylene and salicylic acid slightly enhanced the susceptibility to the weak and nonvirulent isolates. R. solani seems to be able to overcome the induced defense by these phytohormones in the infection to Arabidopsis.

A longitudinal study on morpho-genetic diversity of pathogenic Rhizoctonia solani from sugar beet and dry beans of western Nebraska

Background

Root and stem rot caused by Rhizoctonia solani is a serious fungal disease of sugar beet and dry bean production in Nebraska. Rhizoctonia root rot and crown rot in sugar beet and dry bean have reduced the yield significantly and has also created problems in storage. The objective of this study was to analyze morpho-genetic diversity of 38 Rhizoctonia solani isolates from sugar beet and dry bean fields in western Nebraska collected over 10 years. Morphological features and ISSR-based DNA markers were used to study the morphogenetic diversity.

Results

Fungal colonies were morphologically diverse in shapes, aerial hyphae formation, colony, and sclerotia color. Marker analysis using 19 polymorphic ISSR markers showed polymorphic bands ranged from 15 to 28 with molecular weight of 100 bp to 3 kb. Polymorphic loci ranged from 43.26–92.88%. Nei genetic distance within the population ranged from 0.03–0.09 and Shannon diversity index varied from 0.24–0.28. AMOVA analysis based on ΦPT values showed 87% variation within and 13% among the population with statistical significance (p < 0.05). Majority of the isolates from sugar beet showed nearby association within the population. A significant number of isolates showed similarity with isolates of both the crops suggesting their broad pathogenicity. Isolates were grouped into three different clusters in UPGMA based cluster analysis using marker information. Interestingly, there was no geographical correlation among the isolates. Principal component analysis showed randomized distribution of isolates from the same geographical origin. Identities of the isolates were confirmed by both ITS-rDNA sequences and pathogenicity tests.

Conclusion

Identification and categorization of the pathogen will be helpful in designing integrated disease management guidelines for sugar beet and dry beans of mid western America.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-020-02026-9.

Biological Characteristics and Genetic Diversity of Phomopsis asparagi, Causal Agent of Asparagus Stem Blight

Asparagus stem blight is a regional disease. In the present study, we compared strains of Phomopsis asparagi from six different provinces to determine their biological characteristics and genetic diversity, differences in the pycnidium and conidium production, pathogenicity, and growth rate. Considerable differences were established in the pycnidium and conidium production among the P. asparagi strains from the six studied provinces. The largest pycnidium and conidium production had the strains from Fujian, followed by those from Hainan. The virulence of P. asparagi strains was significantly different but without a correlation with the geographical source of the strain. FJ2 had the highest virulence, followed by HN2, SD4, and SD5, whereas SD5 had the lowest virulence. The colony diameter and dry weight of the strains of asparagus stem blight fungus from the six provinces were substantially different. The colonies of HN1-5 had the largest diameters, whereas those of XT1-5, LT1-3, FJ1-5, and SX6 had smaller diameters. Four primers with good repeatability and strong specificity were selected from 100 intersimple sequence repeat (ISSR) primers. ISSR-PCR amplification was performed on 36 strains of asparagus stem blight fungus, and a large number of repeatable DNA fingerprints were obtained. Most of the amplified fragments were within 300 to 500 bp. In all, 69 total points, 64 multiple points, and 92.75% polymorphism points were established. The number of ISSR gene sites detected by four primers ranged from 14 to 20, with an average of 16 multiple sites. The copolymerization was divided into three groups: XT1-5, LT1-3, and FJ1-5, which were clustered into the first group; SD1-6, SX1-6, and HB1-6, clustered into the second group; and HN1-5 in the third group. The results of the cluster analysis revealed that the strains of the neighboring provinces had a nearer phylogenetic relationship than that between distant ones. Therefore, the system evolution of P. asparagi is related to the geographical distribution of its strains.

Genotypes and Genomic Regions Associated With Rhizoctonia solani Resistance in Common Bean

Rhizoctonia solani Kühn (teleomorph Thanatephorus cucumeris) is an important root rot pathogen of common bean (Phaseolus vulgaris L.). To uncover genetic factors associated with resistance to the pathogen, the Andean (ADP; n = 273) and Middle American (MDP; n = 279) diversity panels, which represent much of the genetic diversity known in cultivated common bean, were screened in the greenhouse using R. solani anastomosis group 2-2. Repeatability of the assay was confirmed by the response of five control genotypes. The phenotypic data for both panels were normally distributed. The resistance responses of ∼10% of the ADP (n = 28) and ∼6% of the MDP (n = 18) genotypes were similar or higher than that of the resistant control line VAX 3. A genome-wide association study (GWAS) was performed using ∼200k single nucleotide polymorphisms to discover genomic regions associated with resistance in each panel, For GWAS, the raw phenotypic score, and polynomial and binary transformation of the scores, were individually used as the input data. A major QTL peak was observed on Pv02 in the ADP, while a major QTL was observed on Pv01 with the MDP. These regions were associated with clusters of TIR-NB_ARC-LRR (TNL) gene models encoding proteins similar to known disease resistance genes. Other QTL, unique to each panel, were mapped within or adjacent to a gene model or cluster of related genes associated with disease resistance. This is a first case study that provides evidence for major as well as minor genes involved in resistance to R. solani in common bean. This information will be useful to integrate more durable root rot resistance in common bean breeding programs and to study the genetic mechanisms associated with root diseases in this important societal legume.

Genome analysis provides insight about pathogenesis of Indian strains of Rhizoctonia solani in rice

The Rhizoctonia solani species complex is comprised of strains belonging to different anastomosis groups and causes diseases in several economically important crops, including rice. However, individuals within same anastomosis group exhibit distinct morphological and pathological differences on the same host. In this study, we have sequenced the genome of two aggressive Indian strains (BRS11 and BRS13) belonging to AG1-IA anastomosis group and compared them with the available genome of R. solani AG1-IA. We identified several SNPs and Indels in both of these genomes, in comparison to the AG1-IA genome. Furthermore, we observed expansion and emergence of orthogroups in these Indian strains and identified those potentially associated with pathogenesis. Amongst them, transposable elements, cell wall degrading enzymes, transcription factors, and oxalate decarboxylase were noteworthy. The current study unravels genetic variations and identifies genes that might account for pathogenicity variations amongst R. solani strains.

Anastomosis Groups of Rhizoctonia solani associated with tomato foot rot in Pothohar Region of Pakistan

Rhizoctonia solani Kühn (teleomorph = Thanatephorus cucumeris (Frank) Donk) is one of the important soil-borne fungal pathogen, which infects tomato with typical symptoms of seedling damping-off and foot rot. During surveys (2014 and 2015 crop season) of nine tomato growing areas in Pothohar region of Pakistan, symptoms of foot rot were noted on approximately 33.4% of the plants observed at soil line level of the stem. Lesions on infected plant stems were irregular in shape, water-soaked, brown in colour manifesting sunken appearance. Fungal colonies isolated from stem portions of the diseased plants on malt extract agar medium were light grey to brown in colour with abundant mycelial growth and branched hyphae. A septum was always present in the branch of hyphae near the originating point with a slight constriction at the branch. No conidia or conidiophores were observed. All isolates were multinucleate when subjected to DAPI (4',6-diamidino-2-phenylindole) stain. Based on morphological characteristics of fungal hyphae, isolates were identified as R. solani. Restriction analysis of PCR-amplified ribosomal DNA with four discriminant enzymes (MseI, AvaII, HincII, and MunI) and hyphal interactions with known tester strains confirmed these isolates belong to AG-3-PT (64.2%), AG-2-1 (14.2%), AG-2-2 (9.5%), AG-5 (7.1%) and AG-4-HGI (4.7%). AG-3-PT was widely distributed to major tomato growing areas while other groups were confined to distinct locations. Internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced which had 99-100% identity with the corresponding gene sequences of respective R. solani AGs. To confirm Koch's postulates, four week old tomato plants were transplanted into 1.5 L plastic pots containing sterilized potting mixture i.e. sand: clay: farmyard manure, at the rate of 1:1:1. Soil inoculum containing 10 g of barley grains colonized with each isolate of R. solani for 14 days was mixed in the upper 2 cm layer of soil (Taheri and Tarighi, 2012). A set of uninoculated plants was used as a control. Ambient conditions were provided under the greenhouse. 21 days after inoculation, water-soaked greyish to brown lesions similar to the symptoms of the previous infection were observed on stem portions of all inoculated plants while control plants remained symptomless. Fungus re-isolated from infections was confirmed as R. solani by microscopic appearance of the hyphae. Present study is the first report of AG composition of R. solani infecting tomato in Pakistan which will be useful to breeding programs working on varietal evaluation.

The Rhizoctonia solani AG1-IB (isolate 7/3/14) transcriptome during interaction with the host plant lettuce (Lactuca sativa L.)

The necrotrophic pathogen Rhizoctonia solani is one of the most economically important soil-borne pathogens of crop plants. Isolates of R. solani AG1-IB are the major pathogens responsible for bottom-rot of lettuce (Lactuca sativa L.) and are also responsible for diseases in other plant species. Currently, there is lack of information regarding the molecular responses in R. solani during the pathogenic interaction between the necrotrophic soil-borne pathogen and its host plant. The genome of R. solani AG1-IB (isolate 7/3/14) was recently established to obtain insights into its putative pathogenicity determinants. In this study, the transcriptional activity of R. solani AG1-IB was followed during the course of its pathogenic interaction with the host plant lettuce under controlled conditions. Based on visual observations, three distinct pathogen-host interaction zones on lettuce leaves were defined which covered different phases of disease progression on tissue inoculated with the AG1-IB (isolate 7/3/14). The zones were defined as: Zone 1-symptomless, Zone 2-light brown discoloration, and Zone 3-dark brown, necrotic lesions. Differences in R. solani hyphae structure in these three zones were investigated by microscopic observation. Transcriptional activity within these three interaction zones was used to represent the course of R. solani disease progression applying high-throughput RNA sequencing (RNA-Seq) analysis of samples collected from each Zone. The resulting three transcriptome data sets were analyzed for their highest expressed genes and for differentially transcribed genes between the respective interaction zones. Among the highest expressed genes was a group of not previously described genes which were transcribed exclusively during early stages of interaction, in Zones 1 and 2. Previously described importance of up-regulation in R. solani agglutinin genes during disease progression could be further confirmed; here, the corresponding genes exhibited extremely high transcription levels. Most differentially higher expressed transcripts were found within Zone 2. In Zone 3, the zone with the strongest degree of interaction, gene transcripts indicative of apoptotic activity were highly abundant. The transcriptome data presented in this work support previous models of the disease and interaction cycle of R. solani and lettuce and may influence effective techniques for control of this pathogen.

Identification and Characterization of Rhizoctonia Species Associated with Soybean Seedling Disease

In an effort to identify the Rhizoctonia spp. associated with seedling diseases of soybean, Rhizoctonia isolates were recovered from soybean seedlings with damping off and root and hypocotyl rot symptoms from Arkansas, Illinois, Kansas, Michigan, Minnesota, and the Canadian province of Ontario between 2012 and 2014. Based on cultural morphology, polymerase chain reaction restriction fragment length polymorphism, and phylogenetic analysis of the internal transcribed spacer (ITS) region of the ribosomal RNA genes, 80 isolates were confirmed to be Rhizoctonia solani, 24 were binucleate Rhizoctonia spp., and 10 were R. zeae. Of the 80 R. solani isolates, one belonged to anastomosis group (AG) 2-1, 52 belonged to AG-2-2IIIB, five belonged to AG-3 PT, three belonged to AG-4 HGI, two belonged to AG-4 HGIII, nine belonged to AG-7, and eight belonged to AG-11. Bayesian inference of phylogeny using the ITS region revealed two clades of R. solani AG-7 that possibly correspond to different AG-7 subgroups. Phylogenetic analysis also provided evidence for genetic relatedness between certain binucleate Rhizoctonia and some R. solani isolates. On 'Williams 82' soybean, isolates of AG-2-2IIIB were the most aggressive, followed by isolates of AG-7, AG-4, and AG-11. On 'Jubilee', a sweet corn cultivar, AG-2-2IIIB and AG-4 isolates caused significant stunting and root damage, whereas the damage caused by the AG-11 isolates was mostly restricted to the mesocotyl. Isolates of R. zeae and the binucleate Rhizoctonia spp. were not pathogenic on soybean or corn. Our results indicate that soybean and corn are hosts to the predominant and aggressive AG of R. solani, implying that rotation between these two crops may not be an effective management practice.

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

Stunted Patches in Onion Bulb Crops in Oregon and Washington: Etiology and Yield Loss

Onion stunting caused by Rhizoctonia spp. is an important soilborne disease on very sandy soils in the Columbia Basin of Oregon and Washington. From 2010 to 2013, 251 isolates of Rhizoctonia or Rhizoctonia-like spp. were obtained from soil and onion plant samples collected from inside and outside patches of stunted plants in 29 onion fields in the Columbia Basin. Sequence analysis of the internal transcribed spacer (ITS) region was used to identify the isolates, with 13 anastomosis groups (AGs) or subspecies detected. The most frequent was Waitea circinata var. circinata (25%), followed by Rhizoctonia solani AG 3 (17%), R. solani AG 4 (14%), Ceratobasidium sp. AG A (10%), R. solani AG 8 (7%), Ceratobasidium sp. AG K (6%), R. solani AG 2-1 (6%), W. circinata var. zeae (6%), R. solani AG 5 (4%), Ceratobasidium sp. AG G (2%), R. solani AG 11 (2%), and R. solani AG 1-1B and AG 10 (each <1%). However, the distribution of AGs and subspecies varied depending on whether soil or onion plants samples were collected within or adjacent to patches of stunted onion plants. In an attempt to predict the risk of onion stunting for a field prior to planting, DNA concentrations of AG 2-1, AG 3, AG 4, and AG 8 were quantified from bulk soil samples collected from each of nine growers' fields approximately 1 month before onion sowing in 2012. The preplant DNA concentrations did not show a significant association with the amount of stunting observed in the fields during the growing season. In contrast, the frequency of isolation and DNA concentration of R. solani AG 8 detected in soil samples collected during the growing season were greater from inside patches of stunted onion plants than from adjacent healthy areas of an onion crop sampled in 2012, but not for soil samples collected similarly from an onion crop in 2013. AG 2-1, AG 3, and AG 4 DNA concentrations did not differ significantly in soil sampled inside versus outside stunted patches in the fields sampled in 2012 and 2013. Relationships between the number of bulbs harvested or bulb weight versus severity of stunting were defined using correlation and regression analyses for six onion cultivars grown in seven fields surveyed in 2012 and 2013. Onion stunting reduced the average marketable bulb yield by 25 to 60% within stunted patches of the six cultivars. Stunting did not reduce onion plant stand but consistently reduced the size of bulbs, and yield reduction increased with increasing disease severity.

Phylogenetic Diversity of Rhizoctonia solani Associated with Canola and Wheat in Alberta, Manitoba, and Saskatchewan

Rhizoctonia solani is a damaging soilborne pathogen, which affects most field crops in the Canadian provinces of Alberta, Manitoba, and Saskatchewan. The objective of this study was to conduct a phylogenetic comparison of isolates of R. solani collected from a previous survey in the major canola- and wheat-growing regions of western Canada. A total of 128 multinucleate isolates from a previous survey were identified by internal transcribed spacer (ITS) sequence and compared to anastomosis group (AG) results. The multinucleate isolates of R. solani were grouped into eight distinct clades. Each clade corresponded to a specific AG with the exception of two distinct clades that were observed for isolates classified as AG 2-1 by anastomosis testing. While most isolates of AG 5 clustered together according to ITS sequences, three isolates classified by anastomosis grouping as AG 5 grouped with AG 2-1, AG 4, and a binucleate Rhizoctonia sp. in the phylogenetic analysis. In most instances, the results from AG tests were consistent with ITS sequence, but there were still several cases where isolates were inconsistently classified or failed to undergo anastomosis with any of the tester strains used in this study. This provides support for the use of the ITS region as a valuable tool for rapid identification of R. solani isolates to their respective AGs.

Molecular characterization and screening for sheath blight resistance using Malaysian isolates of Rhizoctonia solani

Two field isolates of Rhizoctonia solani were isolated from infected paddy plants in Malaysia. These isolates were verified via ITS-rDNA analysis that yielded ~720 bp products of the ITS1-5.8S-ITS4 region, respectively. The sequenced products showed insertion and substitution incidences which may result in strain diversity and possible variation in disease severity. These strains showed some regional and host-specific relatedness via Maximum Likelihood and further phylogenetic analysis via Maximum Parsimony showed that these strains were closely related to R. solani AG1-1A (with 99-100% identity). Subsequent to strain verification and analysis, these isolates were used in the screening of twenty rice varieties for tolerance or resistance to sheath blight via mycelial plug method where both isolates (1801 and 1802) showed resistance or moderate resistance to Teqing, TETEP, and Jasmine 85. Isolate 1802 was more virulent based on the disease severity index values. This study also showed that the mycelial plug techniques were efficient in providing uniform inoculum and humidity for screening. In addition this study shows that the disease severity index is a better mode of scoring for resistance compared to lesion length. These findings will provide a solid basis for our future breeding and screening activities at the institution.

Transcriptome analysis of the phytopathogenic fungus Rhizoctonia solani AG1-IB 7/3/14 applying high-throughput sequencing of expressed sequence tags (ESTs)

Rhizoctonia solani is a soil-borne plant pathogenic fungus of the phylum Basidiomycota. It affects a wide range of agriculturally important crops and hence is responsible for economically relevant crop losses. Transcriptome analysis of the bottom rot pathogen R. solani AG1-1B (isolate 7/3/14) by applying high-throughput sequencing and bioinformatics methods addressing Expressed Sequence Tag (EST) data interpretation provided new insights in expressed genes of this fungus. Two normalized cDNA libraries representing different cultivation conditions of the fungus were sequenced on the 454 FLX (Roche) system. Subsequent to cDNA sequence assembly and quality control, ESTs were analysed applying advanced bioinformatics methods. More than 14 000 transcript isoforms originating from approximately 10 000 predictable R. solani AG1-IB 7/3/14 genes are represented in each dataset. Comparative analyses revealed several differentially expressed genes depending on the growth conditions applied. Determinants with predicted functions in recognition processes between the fungus and the host plant were identified. Moreover, many R. solani AG1-IB ESTs were predicted to encode putative cellulose, pectin, and lignin degrading enzymes. Furthermore, genes playing a possible role in mitogen-activated protein (MAP) kinase cascades, 4-aminobutyric acid (GABA) metabolism, melanin synthesis, plant defence antagonism, phytotoxin, and mycotoxin synthesis were detected.

Genetic and genomic analysis of Rhizoctonia solani interactions with Arabidopsis; evidence of resistance mediated through NADPH oxidases

Rhizoctonia solani is an important soil-borne necrotrophic fungal pathogen, with a broad host range and little effective resistance in crop plants. Arabidopsis is resistant to R. solani AG8 but susceptible to R. solani AG2-1. A screen of 36 Arabidopsis ecotypes and mutants affected in the auxin, camalexin, salicylic acid, abscisic acid and ethylene/jasmonic acid pathways did not reveal any variation in response to R. solani and demonstrated that resistance to AG8 was independent of these defense pathways. The Arabidopsis Affymetrix ATH1 Genome array was used to assess global gene expression changes in plants infected with AG8 and AG2-1 at seven days post-infection. While there was considerable overlap in the response, some gene families were differentially affected by AG8 or AG2-1 and included those involved in oxidative stress, cell wall associated proteins, transcription factors and heat shock protein genes. Since a substantial proportion of the gene expression changes were associated with oxidative stress responses, we analysed the role of NADPH oxidases in resistance. While single NADPH oxidase mutants had no effect, a NADPH oxidase double mutant atrbohf atrbohd resulted in an almost complete loss of resistance to AG8, suggesting that reactive oxidative species play an important role in Arabidopsis's resistance to R. solani.

Characterization and Pathogenicity of Rhizoctonia solani Isolates Affecting Pisum sativum in North Dakota

Acreage of dry field pea (Pisum sativum) in North Dakota has increased approximately eightfold from the late 1990s to the late 2000s to over 200,000 ha annually. A coincidental increase in losses to root rots has also been observed. Root rot in dry field pea is commonly caused by a complex of pathogens which included Fusarium spp. and Rhizoctonia solani. R. solani isolates were obtained from roots sampled at the three- to five-node growth stage from North Dakota pea fields and from symptomatic samples received at the Plant Diagnostic Lab at North Dakota State University in 2008 and 2009. Using Bayesian inference and maximum likelihood analysis of the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA), 17 R. solani pea isolates were determined to belong to anastomosis group (AG)-4 homogenous group (HG)-II and two isolates to AG-5. Pathogenicity of select pea isolates was determined on field pea and two rotation hosts, soybean and dry bean. All isolates caused disease on all hosts; however, the median disease ratings were higher on green pea, dry bean, and soybean cultivars when inoculated with pea isolate AG-4 HG-II. Identification of R. solani AGs and subgroups on field pea and determination of relative pathogenicity on rotational hosts is important for effective resistance breeding and appropriate rotation strategies.

Genetic and Virulence Analysis of Rhizoctonia spp. Associated with Sugar Beet Root and Crown Rot in the Northeast Region of Iran

Rhizoctonia spp. are the main causal agents of root and crown rot on sugar beet. In this study, isolates of Rhizoctonia spp. were obtained from diseased sugar beet in Iran over 2 years. Of 68 isolates, 61 were R. solani and 7 were R. cerealis. The anastomosis group (AG) of all isolates was determined on glass slides against the testers. Characterization of intraspecific groups (ISGs) of R. solani isolates revealed that, of 61 isolates, 43 were AG2-2 IIIB and 18 were AG2-2 IV. Amplified fragment length polymorphism (AFLP) analyses were used to investigate genetic structure of Rhizoctonia populations. Principal coordinate plots and cluster analysis differentiated R. solani from R. cerealis isolates and separated the R. solani isolates belonging to different ISGs. AFLP data indicated that the R. solani and R. cerealis populations are not clonal. Analysis of molecular variance in AG2-2 IIIB isolates showed that geographic region was the main factor determining genetic structure of the populations. Sampling year had no significant effect on the genotypes. Pathogenicity tests on Beta vulgaris 'FD0432' revealed that R. solani AG2-2 IIIB and AG2-2 IV isolates were more virulent than R. cerealis.