Theses and Dissertations

Issuing Body

Mississippi State University


Tomaso-Peterson, Maria

Committee Member

Allen, Thomas W., Jr.

Committee Member

Aboughanem, Nina

Committee Member

Sabanadzovic, Sead

Date of Degree


Document Type

Graduate Thesis - Open Access


Plant Pathology

Degree Name

Master of Science


College of Agriculture and Life Sciences


Department of Biochemistry, Molecular Biology, Entomology and Plant Pathology


Frogeye leaf spot, caused by Cercospora sojina Hara, is a foliar disease affecting soybean (Glycine max (L.) Merr.), often managed by applications of quinone outside inhibitor (QoI) fungicides. In 2013 and 2014, symptomatic leaf samples were collected from Mississippi soybean fields leading to the collection of 634 mono-conidial C. sojina isolates. In vitro bioassays were performed to evaluate the sensitivity of 14 isolates plus a baseline. Resistant and sensitive isolates were characterized by determining the effective fungicide concentrations at which 50% of conidial germination was inhibited (EC50). Additionally, the molecular mechanism of resistance was determined for all 634 isolates. Greater than 93% of C. sojina isolates collected in Mississippi carried the G143A amino acid substitution indicating a shift to a QoI-resistant population throughout Mississippi soybean fields. Greenhouse studies confirmed that due to this amino acid substitution, symptoms caused by QoI-resistant isolates developed in spite of a QoI fungicide application.