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ORCID

Document Type

Open Access Thesis

Degree Program

Plant & Soil Sciences

Degree Type

Master of Science (M.S.)

Year Degree Awarded

2020

Month Degree Awarded

February

Abstract

Dollar spot, caused by the causal agent Clarireedia spp. (formerly, Sclerotinia homoeocarpa), is one of the most economically challenging turfgrass diseases in North America. To maintain acceptable quality of amenity turfgrasses, multiple fungicide applications are required. Since the launching of boscalid in 2003, succinate dehydrogenase inhibitors (SDHI) have been frequently used, becoming one of the most important fungicide classes not only for dollar spot control but for other plant pathogenic fungal diseases. However, repeated application of fungicides often lead to fungicide resistance. SDHI fungicide resistance has proven to be more complex than the resistance of other fungicide classes, with differential patterns of cross-resistance to five SDHI ingredients dependent on specific mutations to the succinate dehydrogenase (SDH) enzyme. Since 2016, our lab has received samples from several golf courses and one university research plot that experienced SDHI field failure against dollar spot. Through in vitro assays and DNA sequencing, our previous studies identified and profiled four mutations conferring differential SDHI sensitivity in Clarireedia spp.; an amino acid substitution H267Y and a silent mutation (CTT to CTC) at amino acid position 181 in SDHB subunit, and amino acid substitution G91R and G150R in SDHC subunit. In this project, through in vitro assays and DNA sequencing, we identified and profiled two additional mutations conferring differential sensitivity; H267R in SDHB subunit and P80L in SDHC subunit. However, in vitro sensitivity assays alone can present numerous challenges and can sometimes provide inconclusive results. Therefore, in order to fully understand the complicated mechanisms of SDHI resistance, it is important to understand the association between in vitro assays and field efficacy. Further, the ability to quickly detect SDHI resistance using molecular tools could prove useful for providing fast and accurate recommendations for resistance management to practitioners. The first objective of this research was the development of a molecular detection system for SDHI resistance using molecular markers. Using cleaved amplified polymorphic sequence (CAPS) and derived CAPS (dCAPS) molecular markers, different types of mutations in SDHI-resistant isolates were clearly identified. The second objective was an investigation of the association between in vitro SDHI sensitivity and field efficacy. Following inoculation of turf research plots with the identified SDHI-resistant isolates, similar patterns of differential sensitivity that had already been profiled via in vitro assays were validated. In summary, it is important to monitor the distribution of resistance to SDHIs using both in vitro assay and molecular markers, to understand cross-resistance relationship among SDHIs including new chemistries to be registered, and to better understand the resistance mechanism for development of SDHI resistance management strategies.

First Advisor

Geunhwa Jung

Second Advisor

Li-Jun Ma

Third Advisor

Robert Wick

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