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Title: Population genetic structure, gene flow and recombination of Cochliobolus miyabeanus on cultivated wildrice (Zizania palustris)

item Castell-Miller, Claudia
item Samac, Deborah - Debby

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2011
Publication Date: 1/19/2012
Citation: Castell-Miller, C.V., Samac, D.A. 2012. Population genetic structure, gene flow and recombination of Cochliobolus miyabeanus on cultivated wildrice (Zizania palustris). Plant Pathology. 61(5):903-914.

Interpretive Summary: Wildrice is a native aquatic plant from Minnesota, Wisconsin, and Michigan as well as from southern Canada. It was a staple for Native Americans who harvest the grain directly from plants growing in shallow lakes and rivers. Today wildrice is still harvested from natural stands, but most production occurs in cultivated paddies. The combined grain yield in 2008 and 2009 reached more than 19 million pounds. Intensive cultivation increases threats to production from diseases. One of the most prevalent diseases is fungal brown spot. To manage the disease, improved cultivars, sanitation, and fungicides are in place. However, for these measures to be efficient and sustainable the pathogen population (not just one individual) needs to be evaluated in terms of diversity. Even though populations are dynamic and may change over time, the current knowledge of this pathogen as a whole can indicate how fast it will evolve under pressure from fungicides and genetic resistance. The more variable the pathogen population is, the more likely it will overcome the control methods. We collected strains of the pathogen from the wildrice growing areas in Minnesota and used molecular markers to understand variability of the pathogen and its distribution in different areas. The population showed moderate to high variability, depending on collection sites. Movement of the pathogen between collection areas was detected. Also, the two genes required for sexual recombination were found. Thus, it appears that the pathogen has the potential to evolve rapidly under pressure from fungicides and genetic resistance. This information is key to developing integrated disease management methods to retain fungicide effectiveness and durable resistant cultivars.

Technical Abstract: Cochliobolus miyabeanus (Bipolaris oryzae) is the causal agent of fungal brown spot (FBS) in wildrice (Zizania palustris), an aquatic grass endemic to Minnesota, Wisconsin, Michigan, and parts of Canada. Grain yield losses can reach up to 74% when the disease starts at the boot stage and continues until grain maturity. In Minnesota, management of FBS in paddies is mostly by application of fungicides and sanitation. Breeding for resistance is in progress. Knowledge of the amount and distribution of fungal diversity is required for implementing an integrated disease management system for FBS, but this information is lacking. A collection of 168 C. miyabeanus strains was made from cultivated wildrice paddies in Polk, Aitkin, Beltrami, Itasca, and Lake of the Woods counties during 2007 and 2008. Analysis of 26 polymorphic amplified fragment length polymorphism (AFLP) markers generated with three primer-pair combinations indicated an average gene diversity of 0.23 and significant population subdivision by collection area (FST = 0.29, P = 0.0). Gene diversity within collection areas varied. Seventy haplotypes were present among 168 strains. Genotypic diversity and haplotype numbers were high in all collection areas with the exception of an Itasca paddy. Although indirect tests for random mating within collection areas were rejected, sexual recombination may be possible in areas where mating type idiomorphs, MAT1-1 and MAT1-2, were found. Indirect measurement of gene flow, Nm, was on average 1.57, but higher in central-eastern areas that also exhibited population admixture. No spatial autocorrelation was found. A virulence test with strains representing three geographically disparate areas indicated that the level of virulence among tested strains was similar. These results indicate that the population of C. miyabeanus in Minnesota is genetically diverse and may evolve rapidly under pressure from fungicides and genetic resistance.