Date of Graduation

8-2024

Document Type

Thesis

Degree Name

Master of Science in Plant Pathology (MS)

Degree Level

Graduate

Department

Entomology and Plant Pathology

Advisor/Mentor

Spurlock, Terry N.

Committee Member

Rojas, Alejandro

Second Committee Member

Faske, Travis

Third Committee Member

Ross, Jeremy

Keywords

Cotton; Fungicide resistance; Soybean; Target spot

Abstract

Target spot, caused by the fungus Corynespora cassiicola, has reemerged since 2016 as an economically important disease on soybean and cotton across the Southern U.S. The quinone outside inhibitors (QoI) and succinate dehydrogenase inhibitors (SDHI) are important fungicide classes used to control foliar diseases in soybean and cotton. The efficacy of these products is rapidly decreasing, impacting control, and causing significant yield losses. The overall objective of this study was to determine the risks associated with chemical control by screening fungicide sensitivity, evaluating target site alterations, and analysis of disease control efficacy and yield response of fungicides for soybean in Arkansas. During the seasons 2022 and 2023, 159 isolates of C. cassiicola were collected from 16 Arkansas counties from soybean and cotton fields. First, the QoI sensitivity of C. cassiicola was evaluated by establishing EC50 (effective concentration that reduces 50% growth) ranging from 0.0001 to 27.7315 mg/L (ppm). Isolates that exhibited EC50 values greater than 1.0 mg/L were sequenced and subjected to a PCR-RFLP analysis to establish the presence of mutations that could result in target site alterations associated with QoI- resistance. In parallel, the sensitivity of C. cassiicola isolates to SDHI fungicides was also established. Similarly, C. cassiicola isolates were analyzed using EC50, ranging from 0.00428 to 6.7863 mg/L. The resulting values for a subset of the population were used to determine a discriminatory dose. The remaining isolates were screened with 0.1 mg/L and 1 mg/L doses, and those with growth beyond 1 mg/Lwere further analyzed by sequencing. Subunits SdhB and SdhC were sequenced to confirm the presence of mutations previously reported. Based on the mutations observed, a KASP genotyping assay was developed for the most frequent mutations in both subunits. Across these 16 counties, the frequency of isolates containing the mutation conferring resistant to QoI fungicides (G143A) were 66.66%, and for SDHI, 33.96% and 2.51% for B-H278Y and C-N75S mutations, respectively. Finally, field trials were established from 2020 to 2023 to analyze the efficacy of different modes of action to control foliar disease in soybeans. Trials indicated a decrease in efficacy of the products containing QoI often producing less yield compared with the treatment containing DMI and SDHI as the active ingredients. This demonstrates that large portions of the C. cassiicola population in Arkansas is resistant to QoI and SDHI fungicides. Therefore, continued monitoring using diagnostic assays and field trial efficacy will be necessary to modify disease management and provide the necessary level of crop protection to reduce yield losses to target spot in cotton and soybean.

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