POPULATION GENETICS, INTRASPECIFIC GROUPS, AND HISTOPATHOLOGY OF RHIZOCTONIA SOLANI AG 2-2 ON SUGAR BEET

Sugar beet is grown in temperate regions around the world for the sucrose that accumulates in its root tissues and accounts for about 12% of global sugar production. In the United States, sugar beet accounts for roughly half of domestic sugar production with an estimated value of around $1.9 billion annually. However, Rhizoctonia root and crown rot is a persistent problem in growing regions around the world and is one of the most important soil-borne diseases of sugar beets. Losses to Rhizoctonia root and crown rot in the U.S. are estimated to exceed $38 million annually affecting harvestability, processing quality, and storability. The causal agent of Rhizoctonia root and crown rot is Rhizoctonia solani AG 2 2 (Kühn), a soil-borne fungus in the Basidiomycota. Strains of R. solani AG 2 2 that affect sugar beet have traditionally been separated into intraspecific groups (ISGs), known as AG 2 2IIIB and AG 2 2IV, that commonly are classified by their ability to grow at 35°C. It has been evident for some time now that, based on ITS sequences, these subgroups are polyphyletic. In the current study, a multigene phylogenetic analysis was used to clarify the relationship between the subgroups and the results indicated that the subgroups 2 2IIIB and 2-2IV are indeed artificial. Therefore, the subgroups of AG 2 2 were redefined to represent a more natural classification. These new subgroups, referred to as AG 2 2BR and AG 2 2PR in the current work, each consisted of two genetic clusters that all have unique genetic characteristics. To examine the characteristics of these newly identified genetic clusters and to address some open questions regarding the population biology of R. solani AG 2 2, a set of microsatellite markers was developed and utilized to genotype 164 isolates from eight growing regions around the world. Sexual reproduction in AG 2-2 has been controversial, but evidence provided by the microsatellite analysis supports sexual reproduction occurring in natural populations, although, it is likely restricted to members of one genetic cluster within AG 2 2BR. In addition, evidence of hybridization between the subgroups 2-2BR and 2-2PR is presented and it appears this hybridization can occur in natural populations. These life-cycle processes have important implications in the generation of genetic diversity in populations. Population studies using the newly developed set of microsatellite markers also revealed evidence of long-distance dispersal that appears to occur across continents and across oceans. These observations highlight the importance of sanitation in managing Rhizoctonia root and crown rot to limit or prevent the movement of inoculum on equipment, crop residues, or personal apparel. The current research project also examines the infection process for Rhizoctonia root and crown rot of sugar beet. Observations provide evidence for the involvement of cell wall degrading enzymes, including lignin degrading enzymes, pectin lyase, and polygalacturonase/ polygalacturonase-inhibiting proteins, in the invasion and colonization of sugar beet root tissue. The involvement of these enzymes has been previously reported in sugar beet, but they have not received a lot of attention since their original reporting. It is anticipated that this work may renew interest in the enzymes involved in the invasion of sugar beet roots and the development of Rhizoctonia root and crown rot and provide additional targets for resistance breeding. Additionally, this dissertation provides a novel perspective on the generation of genetic diversity in R. solani AG 2 2 which is expected to inspire innovative hypotheses regarding strategies of resistance breeding in sugar beet.