Genetic diversity and fungicide sensitivity of Rhizoctonia solani associated with soybean seedling disease
Rhizoctonia solani, a ubiquitous and genetically diverse fungus, is an important seedling pathogen of soybean in North America. With the dearth of commercial soybean varieties marketed for resistance to members of this species, seed treatment fungicides have become increasingly important as a management option. However, our knowledge of the groups associated with soybean seedlings, the genetic structure of the pathogenic groups, as well as their potential for adaptation to fungicide seed treatments, is very limited. To bridge this knowledge gap, we characterized, using conventional and molecular techniques, Rhizoctonia isolates collected from farmers’ fields in the U.S. and in Canada to identify the predominant and most aggressive groups. Three taxonomic groups were identified: R. solani, R. zeae, and the binucleate Rhizoctonia. The R. solani isolates comprised members of anastomosis groups (AG) 2-2IIIB, 3PT, 4 HGI, 4 HGIII, 7, and 11. Isolates of AG-2-2IIIB were the most frequently recovered and the most aggressive on soybean and corn. Using single nucleotide polymorphism markers identified from genotyping-by-sequencing approach, the genetic structure of the populations of AG-2-2IIIB from Illinois, Ohio, and Ontario was assessed for clues about the pathogen’s reproductive biology and to determine if the pattern of genetic variation within populations is consistent with that of a pathogen that is at a high risk of adapting to repeated fungicide applications. While the Illinois population was mostly clonal, the genetic structure of the AG-2-2IIIB populations from Ontario and Ohio revealed a mixed reproductive mode, suggesting the need for caution when applying fungicides. Our results also presented genotype flow as a predominant force shaping the population genetic structure of this AG. To determine if R. solani populations are becoming less sensitive to the fungicide classes commonly used to manage seedling disease, a fungicide resistance monitoring program was initiated to compare the sensitivities of historical isolates with no prior fungicide exposure to the sensitivities of isolates that have been exposed to fungicides over time. Results from fungicide sensitivity assays showed that sensitivity to the fungicide classes tested has decreased in comparison to the baseline R. solani population, but control of seedling disease caused by R. solani was still achieved regardless of in vitro sensitivity. The appendix chapter of this dissertation presents the results of a separate study evaluating the potential of a three-gene pyramid for improved soybean aphid management . From greenhouse studies evaluating the differential reaction of soybean isolines with different combinations of aphid resistance genes, Rag1, Rag2, and Rag3 to four soybean aphid biotypes, the Rag1/2/3 pyramid was found to be the most effective.
Soybean seedling disease, Rhizoctonia solani, Genetic diversity, Fungicide sensitivity
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign