Rhizoctonia Meeting Abstracts

Integration of microbiome-based approaches for biological control of sclerotia. Tomislav Cernava¹, Pascal Mülner^1,2, Philipp Wagner¹, Alessandro Bergna^1,3, Kristin Dietel², Rita Grosch⁴, and Gabriele Berg^{1 1}Institute of Environmental Biotechnology, Graz University of Technology, Graz, Austria. ²ABiTEP GmbH, Berlin, Germany.³Austrian Centre of Industrial Biotechnology GmbH, Graz, Austria.⁴Leibniz Institute of Vegetable and Ornamental Crops Großbeeren/Erfurt e.V., Großbeeren, Germany. Corresponding and presenting author: tomislav.cernava@tugraz.at

Plant pathogens with a wide host range such as Rhizoctonia solani Kühn and Sclerotinia sclerotiorum (Lib.) de Bary cause extensive crop damage across the globe. Both pathogens form resilient sclerotia during their life cycle that can persist in soil for several years. This is aggravated by the inefficiency of conventional fungicides against these survival structures. We explored sclerotia-associated microbial communities on Solanum tuberosum L. to identify prevalent bacteria. Amplicon data generated by 16S rRNA gene fragment sequencing indicated that the ‘sclerotiome’ of R. solani is highly similar to the microbiome of surrounding soil. In contrast, microbial communities of the unaffected tuber surface showed significant differences in their structure and composition. We found that distinctive bacterial lineages were associated with both healthy and infected areas. Members of the Flavobacteriaceae and Caulobacteraceae were primarily detected in unaffected areas, while Phyllobacteriaceae and Bradyrhizobiaceae were associated with the presence of sclerotia. A complementary approach aimed at isolating natural antagonists of S. sclerotiorum delivered promising candidates for extended interaction studies. We selected isolates that produce bioactive volatiles that substantially increases their inhibition range. The most efficient antagonists among Bacillus, Buttiauxella, Enterobacter, and Pseudomonas isolates were shown to produce various alkylpyrazines. Subsequent experiments with pure compounds confirmed their fungicidal properties. These findings provide a basis for further optimizations of promising biological control agents. We envisage a combination of highly active microorganisms and natural ‘boosters’ that will result in higher efficacy of biopesticides against fungal diseases.

Detection and management of Rhizoctonia root rot of sugar beet. Ashok K. Chanda¹ and Ian V. MacRae^{2 1}Department of Plant Pathology, University of Minnesota, St. Paul, MN, USA. ²Department of Entomology, University of Minnesota, St. Paul, MN, USA. Corresponding and presenting author: achanda@umn.edu

Diseases caused by Rhizoctonia solani impact sugar beet production in Minnesota and North Dakota. We utilized a qPCR assay for quantifying inoculum of R. solani AG 2-2 in field soils. From each field, soil cores were taken at a depth of 0-5, 5-10 and 10-15 cm representing approximately a 0.4 ha area. There was a significant correlation between Rhizoctonia root rot index (RRI) and root rot ratings (r=0.59 and r²=0.34), RRI and root rot incidence (r=0.56 and r²=0.32), and RRI and R. solani soil DNA concentration (r=0.24; r²=0.06). Symptoms of root rot typically begin at 8 weeks after planting, continue to develop until harvest, and often are challenging to diagnose based on foliage symptoms. Consequently, advances in remote sensing have regenerated interest in aerial mapping and detection of Rhizoctonia root rot of sugar beet. Hyperspectral reflectance measurements of individual leaves on control and inoculated plants were obtained using an Ocean Optics Flame hyperspectral radiometer. Disease ratings were taken from sampled plants at the same time as reflectance data were measured. Relative likelihood analysis indicates that the wavelengths most closely associated with root rot occur in a narrow band of approximately 20 nm, centered around 689 nm. A field trial was established to evaluate best management strategies (resistant varieties, succinate dehydrogenase inhibitor fungicide seed treatments and post-emergence application of azoxystrobin at the 4- or 8-leaf growth stage) for managing sugar beet Rhizoctonia diseases. Results suggest an integrated approach utilizing a seed treatment followed by a post-emergence application of azoxystrobin is optimal for managing mid- to late-season disease.

Fungicide sensitivity of Rhizoctonia zeae from soybean and corn in Nebraska. Nikita Gambhir, Srikanth Kodati, Anthony O. Adesemoye, and Sydney E. Everhart, Department of Plant Pathology, University of Nebraska, Lincoln, NE, USA. Corresponding and presenting author: nikitagambhir03@gmail.com

Soybean and corn are major commodity crops in Nebraska and seedling diseases reduce stand establishment, causing yield losses up to 100 million bushels. Rhizoctonia sp. is among the top four causal pathogens of corn and soybean soilborne diseases. Since the genus Rhizoctonia consists of phylogenetically distinct species, the occurrence of prevalent species need to be characterized for effective disease management. Soil and symptomatic plants were collected from 54 soybean and 31 corn fields in Nebraska from 2015 through 2017. A total of 179 Rhizoctonia spp. were isolated and identified. The most abundant species recovered were R. solani AG 4 HG-II (38 isolates) and R. zeae (110 isolates). Fungicide seed treatments are commonly used to manage seedling diseases, but few studies have examined fungicide sensitivity of Rhizoctonia fungi sampled from soybean and corn, and there are no studies that have characterized sensitivity of populations in Nebraska. Fungicide sensitivity of 110 R. zeae was determined using a plate dilution method to four seed treatment fungicides: azoxystrobin (QoI), prothioconazole (DeMethylation Inhibitor), fludioxonil (Phenylpyrroles), and sedaxane (Succinate DeHydrogenase Inhibitor). Effective concentration for 50% inhibition (EC₅₀) was estimated from the dose-response curve. Preliminary results showed average EC₅₀ was more than 100 ppm for azoxystrobin, 0.17 ppm for prothioconazole, 0.1 ppm for fludioxonil, and 0.07 ppm for sedaxane. Insensitivity to azoxystrobin might be an artifact of the unblocked alternative oxidase pathway and needs in planta investigation. Results will establish sensitivity of Rhizoctonia zeae in Nebraska and can be used for monitoring sensitivity shifts in future.

Using dual RNA-seq analysis to decipher Rhizoctonia solani host plant interactions. Rita Grosch¹, Bart Verwaaijen^1,3, Franziska Genzel², Alfred Pühler³, Andreas Schlüter³, and Rita, Zrenner¹. ¹Leibniz-Institute for Vegetable and Ornamental Crops, GroSsbeeren, Germany. ²Institute of Bio- and Geosciences, Plant Sciences, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. ³Center for Biotechnology, Bielefeld University, 33615 Bielefeld, Germany. Corresponding and presenting author: Grosch@igzev.de

The soilborne pathogenic fungus Rhizoctonia solani is capable of infecting a wide range of economically important crop plants including lettuce and potato. Isolates of R. solani AG1-IB are responsible for bottom rot on lettuce, whereas AG3PT are responsible for black scurf on potato. The genomes of R. solani AG1-IB (isolate 7/3/14) and AG3PT (Ben3) were recently established. At present, little information regarding the molecular responses in R. solani during the interaction with its host plants is available. Therefore, the transcriptomes of R. solani AG1-IB and AG3 were studied during pathogenesis with lettuce and potato. Transcriptome data were analyzed for highly and differentially expressed genes during the fungus-host plant interaction. Specific patterns of gene expression patterns were observed with a high number of previously undescribed R. solani protein coding transcripts and genes of unknown function were differentially expressed in both pathosystems, particularly at the early phases of interaction. It is hypothesized that these expressed genes contain R. solani effectors that support host colonization by repressing host immunity. Genes with known functions are related to formation of infection structures, suppression of plant defense response at early time point of interaction and induction of necrosis. Some identified genes appeared to have predicted roles in the life cycle of respective isolates of R. solani.

Metabolomics of plant-Rhizoctonia interactions: more than meets the eye. Suha Jabaji, Plant Science Department, Faculty of Agricultural and Environmental Sciences, McGill University, Montreal, Canada. Corresponding and presenting author: suha.jabaji@mcgill.ca

Metabolomics offer insight into complex biochemical responses in microbial systems and during plant-microbe interactions. Metabolomics is complementary to genomics and proteomics approaches. Using advanced methods of analysis such as nuclear magnetic resonance spectroscopy (NMR) and/or mass spectrometry (MS), and multivariate data analyses, we present newly developed approaches to: (i) mine data for novel bioactive compounds from sclerotia of Rhizoctonia solani exhibiting antibacterial, fungitoxic and phytotoxic activities (ii) understand global metabolism regulation in response to Rhizoctonia solani infection in soybean-R. solani AG-4; potato-R. solani AG-3, and soybean-R.solani AG1-1A pathosystems: (iii) apply integrated transcriptomics-metabolomics approach, to study regulation of plant primary metabolism and oxidative stress tolerance in response to Rhizoctonia infection. Key elements in our approach are the construction of a comprehensive metabolite library for target organisms that accelerates metabolite identification and biological interpretation of results, and bioinformatics tools for analysis and visualization of its metabolome. Unraveling the biochemical mechanism(s) operating during plant-Rhizoctonia interactions in different pathosystems, in addition to its significance towards understanding the plant’s metabolism regulation under biotic stress, provides valuable insights with potential for applications in biotechnology, crop breeding, and agrochemical and food industries.

Diversity and pathogenicity of Waitea circinata on row crops. Srikanth Kodati and Anthony O. Adesemoye, University of Nebraska-Lincoln, West Central Research and Extension Center, North Platte, NE, USA. Corresponding and presenting author: skodati@huskers.unl.edu

Rhizoctonia zeae (teleomorph = Waitea circinata var. zeae) is a species with multinucleate hyphal cells and reported as a major soilborne pathogen in turfgrass, but is not well studied in row crops. While there is a scant report on corn, there is almost no information for soybean or wheat. In a statewide survey of Rhizoctonia spp. in row crops in Nebraska from 2016-2017, a large number of isolates of W. circinata was recovered. The objective of this study was to determine the taxonomic diversity of W. circinata isolates and to examine their pathogenicity on corn, soybean and wheat. Sixty-one isolates of W. circinata were recovered from 17 fields in nine Nebraska counties in 2016 and 2017. Isolates were identified based on morphology and sequence analysis of the internal transcribed spacer (ITS) and β-tubulin regions. Pathogenicity tests were conducted by inoculating each isolate onto soybean using a repeated rolled towel and greenhouse assay methods. Visual rating of symptoms on root and crown was conducted to determine disease severity. Strains virulent on soybean were also screened for pathogenicity on corn and wheat. Of 61 isolates of W. circinata collected in 2016 and 2017, two were identified as W. circinata var. oryzae, two as W. circinata var. circinata and the remaining 57 isolates were R. zeae. Eighty percent of the R. zeae isolates were pathogenic on soybean in the rolled towel assay. Eleven virulent isolates of R. zeae showed significantly (p = 0.05) high disease severity and virulence on all three row crops compared to the non-inoculated control. Additional studies are being conducted to further understand R. zeae pathogenesis.

Seasonal dynamics on the diversity of pathogens causing brown patch of tall fescue and implications for fungicide management.  Alyssa M. Koehler and H. David Shew, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh, NC, USA. Corresponding and presenting author: amkoehle@ncsu.edu

Brown patch, caused by the Rhizoctonia solani complex, is the most severe disease of tall fescue in home lawns across the southeast US. Understanding population genetic diversity of pathogens present is an important consideration for management of this disease. Home lawns were surveyed in central North Carolina from 2013-2015 to determine organisms present during typical epidemics of brown patch in tall fescue. Isolates of Rhizoctonia and Rhizoctonia-like fungi were obtained by sampling 147 locations over July 2013 and May and July 2014. In addition, 11 sites were sampled once a week for 12 consecutive weeks from late May to the end of July 2015. Isolates were identified to species and anastomosis group by ITS sequence analysis. Isolations from brown patch lesions in May 2014 predominately yielded Ceratobasidium cereale (77% of organisms recovered), whereas the organisms recovered in July 2013 and 2014 were R. solani AG 2-2-IIIB (44%), R. solani AG 1-1B (37%), and R. zeae (14%). In 2015, Ceratobasidium cereale was isolated from all 11 locations in May, but was replaced by Rhizoctonia species in June and July. Sensitivity of May 2014 isolates to multiple concentrations of the fungicides azoxystrobin, flutolanil, fluxapyroxad, and propiconazole was compared to sensitivity of isolates sampled in 2003, and historic isolates with no fungicide exposure, to determine whether multiple years of exposure to fungicides applied for brown patch control had altered fungicide sensitivity. Mean EC₅₀ values varied across fungicides and species, but no resistance was observed, and there was no apparent shift in sensitivity over the years.

Rhizoctonia contributes to poor establishment of sunflower seedlings in South Africa. Sandra C. Lamprecht, Thabo J. Phasoana, Chris F.J. Spies, and Yared T. Tewoldemedhin Agricultural Research Council, Plant Health and Protection, Private Bag X5017, Stellenbosch, South Africa. Corresponding and presenting author: LamprechtS@arc.agric.za

Sustainable production of field crops depends on proper establishment of seedlings. In South Africa, poor establishment of sunflower seedlings is a production constraint. The role and importance of soilborne pathogens in poor seedling establishment of sunflower was investigated in surveys during 2014 and 2015 in the major sunflower production areas (Free State, North West and Limpopo provinces) of South Africa. Many potential pathogens, including Rhizoctonia were recovered. Eighty-four isolates of Rhizoctonia were identified by sequence analysis of the rDNA internal transcribed spacer (ITS) regions. Of these, 44 % were multinucleate and 56 % binucleate. The multinucleate anastomosis groups (AGs) included 2-2LP, 3TB, 4-HGI, 4-HGIII, R. zeae, and an unidentified multinucleate AG (SB-3) which was the dominant AG. The binucleate AGs included A (the dominant binucleate AG), F, K, Q and four unidentified binucleate AGs (SB-1, SB-2, SB-4 and UNR-1). Pathogenicity studies showed that survival of seedlings were significantly reduced by AGs 2-2LP, 4-HGI, 4-HGIII and SB-3, but not by the binucleate AGs. The most virulent AGs were 2-2LP, 4-HGI and SB-3. AG 2-2LP were obtained only from Limpopo and 4-HGI only from North West province, whereas SB-3 were obtained from all three provinces. Seed treatment with a compound with active ingredients fludioxonil, mefenoxam and thiamethoxam significantly improved survival of seedlings and can contribute to better establishment of seedlings and sustainable sunflower production.

Composition and expression of Carbohydrate-Active Enzymes in Rhizoctonia cerealis transcriptome. Wei Li, Xiangrong Ren, Haiyan Sun, Ning Wang, and Huaigu, Chen. Institute of Plant Protection, Jiangsu Academy of Agricultural Sciences, Nanjing, China. Corresponding and presenting author: lw0501@hotmail.com

The soilborne fungus Rhizoctonia cerealis, which belongs to binucleate Rhizoctonia anastomosis group AG-DI, is the pathogen of wheat sharp eyespot in temperate regions of the world with significant impacts on grain yield and quality. Until now, several genome sequences of R. solani isolates that belong to multinucleate Rhizoctonia, have been published. However, it has rarely been reported in binucleate Rhizoctonia spp. isolates and never described in R. cerealis. In this study, we sequenced the transcriptome and secretome of R. cerealis strain R0301 using Illumina Genome Analyzer sequencing technology, and analyzed the composition and expression of Carbohydrate-Active Enzymes in this fungus. Results indicate that among 21,021 unigene entries of R. cerealis transcriptome, 465 (2.2%) matched with one CAZyme family. These putative CAZymes include 44 families of carbohydrate esterases, 255 families of glycoside hydrolases, 146 families of glycosyltransferases, and 19 families of polysaccharide lyases. The species and quantities of CAZymes in R. cerealis were similar with CAZymes in R. solani AG1-IA and AG8 genomes, which are pathogens of monocotyledon plants. We further compared CAZyme expression patterns in R. cerealis under three different conditions, results suggest that many CAZyme genes were significantly up-regulated during infection of wheat, which indicated that some CAZymes play important roles in the infection process. This study will serve as a resource for future studies to understand the infection mechanism(s) of R. cerealis to wheat.

Molecular characterization of the pathogen and symbiotic fungal community of common European potato varieties grown in conventionally treated field soil. Kaire Loit¹, Liina Soonvald¹, Alar Astover¹, and Leho Tedersoo^2 1Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, Tartu 51014, Estonia. ²Natural History Museum, University of Tartu, 14a Ravila, Tartu, Estonia. Corresponding and presenting author: kaire.loit@emu.ee

Potato (Solanum tuberosum) has a heavy demand for fertilizer and pesticide use among field and vegetable crops. As Arbuscular Mycorrhizal (AM) fungi are naturally present in soil, proper management of AM fungi may provide a sustainable alternative to reduce high inputs of fertilizer and pesticides. In this study, communities of indigenous soilborne fungal pathogens and symbiotic AM fungi were studied in a conventionally treated potato field by Illumina MiSeq sequencing of ITS rDNA region. In total, 315 root and rhizosphere soil samples were collected at different phenological growth stages of 21 potato cultivars. Results showed slight differences in residual richness of AM and pathogenic fungi throughout the growing season. Taxonomic classification of OTU-representative sequences to species level highlighted that the abundance and richness of the potato pathogen Rhizoctonia solani were mainly influenced by cultivar and less by plant growth stage. Despite applied conventional disease management regimes, plant roots were frequently colonized by pathogenic fungi. In contrast, colonization of plant roots by AM fungi was relatively low and insufficient to suppress pathogenic organisms. The study indicated that potato cultivar and plant growth stage were essential factors that altered pathogenic fungal abundance and richness.

Genetic analysis of the contribution of bacterial phenyl acetic acid production to virulence of Rhizoctonia solani AG 2-2-IIIB. Ken Obasa¹, Zhao Peng¹, Anna Block², and Frank White¹. ¹Department of Plant Pathology, University of Florida, Gainesville, FL, USA.²Center for Medical, Agricultural and Veterinary Entomology, Agricultural Research Service, United States Department of Agriculture, Gainesville, FL. Corresponding and presenting author: obasa@ufl.edu

Rhizoctonia solani is species complex of genetically distinct subgroups described as anastomosis groups (AGs), and strains of fungus that infect many economically important crops, resulting in significant yield losses. We have demonstrated that an endosymbiont bacterium in a strain of AG 2-2-IIIB contributes to pathogenicity on creeping bentgrass. Loss of the bacterium, Enterobacter sp. EnCren, was shown to affect production of phenylacetic acid (PAA), which is considered a virulence factor of the fungus. PAA production was significantly reduced in the bacteria-free fungus. The correlation between PAA biosynthesis by the bacterium and virulence of the host fungus in sugarbeet was examined. An indole-3-pyruvate decarboxylase (IpdC) bacteria mutant (EnCren-IpdC), which in the free-living state showed reduced PAA levels, was reintroduced into a bacteria-free strain of the fungus to generate Restored-IpdC-RsCren. An IpdC-complemented strain of EnCren-IpdC, designated as EnCren+IpdC, was also reintroduced into the bacteria-free fungus to generate Restored+IpdC-RsCren. Disease assays of Restored-IpdC-RsCren and Restored+IpdC-RsCren on sugarbeet seedlings under growth-chamber conditions indicate reduced virulence in the Restored-IpdC-RsCren strain. Findings from this study could lead to better understanding of the underlying mechanism(s) of bacterial-endosymbiont-mediated pathogenicity in R. solani AG 2-2-IIIB and development of novel disease management strategies.

Mycorrhizal fungi in Vanilla: challenges and new perspectives in the study of Rhizoctonia. Andrea Porras-Alfaro¹, Ma del Carmen A. Gonzalez-Chavez², and Lynnaun Johnson^3 1Western Illinois University, Macomb, IL, USA. ²Colegio de Postgraduados, Campus Montecillo. Carr. México-Texcoco, 56230, Montecillo, Mexico State, Mexico. ³Northwestern Illinois University, Plant Biology and Conservation- Chicago Botanic Garden Program, Evanston, IL, USA. Corresponding and presenting author: A-Porras-alfaro@wiu.edu

Vanilla is an orchid of commercial importance with epiphytic and terrestrial roots. Rhizoctonia-like fungi have been described as the main endomycorrhizal fungi in previous studies, however use of direct sequencing methods revealed complex communities of endomycorrhizal fungi. Samples were collected from multiple farms in Mexico. Characteristic root colonization by pelotons was observed and sequencing revealed colonization by a variety of ectomycorrhizal fungi, as well as Rhizoctonia. Fungi within the Ceratobasidiaceae were present in terrestrial and epiphytic roots, and represented the third most common mycorrhizal fungi detected in terrestrial roots and were the most common fungi in epiphytic roots. Rhizoctonia fungi were more diverse and abundant in farms with little management with respect to highly managed crops. Isolation of these fungi in culture was challenging and direct amplification of pelotons revealed Scleroderma in contrast to previous studies in Puerto Rico and Costa Rica in which Rhizoctonia was the main colonizer detected in pelotons. Germination and plant growth experiments using isolates from Puerto Rico and Costa Rica showed a variety of interactions from mutualistic to highly virulent. Our study reveals a high level of complexity of orchid mycorrhizal fungi in Vanilla and shows that farming practices impact diversity of Rhizoctonia in roots.

Untangling microbial networks for insights into disease suppressive microbial consortia. Daniel Schlatter and Timothy Paulitz. USDA-ARS, Wheat Health, Genetics and Quality Research Unit, Pullman, WA, USA. Corresponding and presenting author: timothy.paulitz@ars.usda.gov

Next-generation sequencing offers a powerful tool to decipher the communities and consortia responsible for natural suppression of Rhizoctonia (bare-patch of wheat), caused by R. solani AG-8. In a long-term field trial in Washington State, the frequency of bare patches decreased after 5 years of continuous cereals. Bulk and rhizosphere soil were sampled from diseased plants in patches, and from patches that had disappeared and healthy areas of the field. Bacteria populations were characterized by pyrosequencing of the bacterial 16S rRNA gene. Using traditional ANOVA methods and general linear models, we identified members of Bacteroidetes and Oxalobacteraceae as being more frequent on diseased roots in active patches. The same trend was reproduced in the greenhouse by cycling field soil amended with R. solani. We isolated a species of Chryseobacterium (family Flavobacteriaceae) and demonstrated that it had biocontrol activity in greenhouse experiments. However, network analysis offers a more powerful tool to examine relationships among bacterial taxa and identify microbial consortia with potential roles in plant disease. Using network analyses, we observed three large modules of co-occurring taxa. One module was associated with diseased roots and contained copiotrophic families such as Enterobacteriaceae, Pseudomonadaceae, Chitinophagaceae, and Flavobacteriaceae. Another large module was associated with bulk soil and healthy state- oligotrophic bacteria Gemmatimonadaceae, Acidobacteriaceae and Actinobacteria families Koribacteraceae, Gaiellaceae and Solirubrobacteraceae. These two modules were negatively linked to each other. We also identified three other unlinked modules. Network analyses provide powerful tools to decipher complex relationships among microbial taxa. However, although often used as exploratory analyses, further research is needed to examine the specific roles of individual taxa within microbial networks and relationships between network structures and plant health.

Performance of cotton seed treatments under different planting dates and seeding rates. Shelby M. Young. Department of Plant and Soil Science, Texas Tech University, Lubbock, TX, USA. Corresponding and presenting author: shelby.young@ttu.edu

Seedling disease of cotton is capable of affecting cotton seed germination and seedling emergence, survival, and development. Experimental trials were conducted in two fields with a history of seedling disease caused by Rhizoctonia solani in 2017 to evaluate combinations of early season disease management options. Treatments consisting of two planting dates (early and optimal), two seeding rates (2 and 4 seed per foot), and four seed treatments (with increasing levels of early season pest protection) were arranged in a split-split-split plot design with four replications. Overall, the application of additional chemical inputs resulted in mixed trends across other factors. Additional inputs resulted in stand increases in five of eight scenarios. At field location 1, earlier planting held the yield advantage over later planting while field location 2 exhibited an opposite trend; these differences in trends between planting dates at the two locations were most likely due to weather conditions during seedling emergence. To evaluate cost reduction to account for increased use of chemical inputs, seeding rate was examined. Use of seed treatment fungicides, insecticides and nematicides has increased over the past decade as chemical management options have become more readily available. Additional studies determining the risk of early season pest pressure and use of combinations of products are warranted.

The secret life inside Rhizoctonia solani: The contributions of endohyphal bacteria. Peiqi Zhang¹, Ken Obasa¹, Jose C. Huguet-Tapia¹, Zhao Peng¹, Anna Block², and Frank F. White¹. ¹Department of Plant Pathology, University of Florida, Gainesville, FL, USA and ²Center for Medical, Agricultural and Veterinary Entomology, Agricultural Research Service, US Department of Agriculture, Gainesville, FL. Corresponding and presenting author: pqzhang256@ufl.edu

Bacterial endophytes have a profound impact on the physiology and pathogenicity of their host fungi. Several species of endohyphal bacteria have been isolated from fungal pathogen causing brown patch disease in cool season turfgrass, Rhizoctonia solani AG2-2-IIIB, among which include an Enterobacter sp. strain named En-cren. Endophytic En-cren was not only shown to affect virulence of R. solani, but also could be released from damaged hyphae as a free-living bacterium and migrate rapidly along the outside of hyphae. The bacteria can travel by the aerial hyphae of R. solani and presumably expand to new niches in the soil environment, as well as reside within fungal hyphae. The objectives of this study are to identify the genetic nature of hyphal motility of the bacteria and examine in more detail the bacterial production of phytohormones. The motility was not prevented by mutations in the gene for the flagella hook protein (flgE) or type IV pilus secretin (pilQ). Screening for motility mutants and tests of double mutants of both motility apparatus are in progress. En-cren produces phenylacetic acid (PAA) and indole acetic acid (IAA), of which the former has been proposed as a virulence factor of R. solani. Mutation of the key gene for indole pyruvate decarboxylase in En-cren suppressed PAA and IAA production in free-living bacterium cultures. The study will provide greater understanding of the interaction of fungi and endohyphal bacteria in the mycorhizosphere.

Management of sheath blight in organic rice using cultivar resistance and biocontrol agents. Xin Gen Zhou. Texas A&M AgriLife Research Center, Beaumont, TX, USA. Corresponding and presenting author: xzhou@aesrg.tamu.edu

Sheath blight, caused by Rhizoctonia solani AG1-IA, is one of the most important diseases affecting organic rice production in the southern US. Management of sheath blight is a challenge because no synthetic fungicides are allowed for use in organic production systems. In this study, field trials were conducted under organic management at Beaumont, Texas over 3 years to evaluate resistance of 27 rice cultivars and efficacy of eight biological control agents for managing sheath blight. Plots were inoculated with R. solani inoculum at panicle differentiation. For biological control trials, plots were sprayed with biocontrol agents at 2 weeks after inoculation and unsprayed plots served as the controls. Sheath blight severity was rated on a scale of 0 to 9 where 0 represents no symptoms and 9 represents severe damage (plants collapsed) at 1 week before harvest. No cultivars had immune reactions or high levels of resistance to sheath blight. Cultivars CLXL729, Diamond, Rondo, Roy J, XL753 and XP760 showed moderate resistance with the ratings below 5. Cocodrie, CL111, CL151, CL153, Della-2 and Jazzman 2 had highest levels of disease with ratings greater than 7. Application of Neo-Boost (peroxyacetic acid), Serenade Max (Bacillus subtilis) and MBI-600 (B. subtilis) significantly reduced sheath blight severity and numerically increased yield. Use of partially resistant cultivars and biological control agents can reduce the damage caused by sheath blight in organic rice.