Degree

Doctor of Philosophy (PhD)

Department

Plant Pathology & Crop Physiology

Document Type

Dissertation

Abstract

Cercospora kikuchii was historically considered the sole pathogen responsible for Cercospora leaf blight (CLB) of soybean, with pathogen identity determined based on colony morphology and host association. However, phylogenetic studies have revealed that several Cercospora species, including C. cf. flagellaris, C. cf. sigesbeckiae, and C. iranica, also contribute to CLB. The presence of these species, which are impossible to distinguish based solely on morphological traits, increases the complexity of the etiology of CLB. To better understand CLB etiology, we developed standardized methods for inducing sporulation and reproducing CLB symptoms in controlled environments. We fulfilled Koch’s postulates, establishing the roles of the Cercospora spp. Comparative studies demonstrated that C. kikuchii was the most aggressive. In contrast, C. cf. flagellaris exhibited a high secondary inoculum load, which may enhance its persistence in soybean fields and contribute to its despite its lower aggressiveness. Heat-induced stress significantly shortened the latent period across species, suggesting that abiotic factors influence the biotrophic-to-necrotrophic transition. Cercospora was recovered from both symptomatic and asymptomatic seeds under controlled conditions, highlighting its role as a systemic endophyte. A population genetics approach was used to investigate the genetic structure of C. cf. flagellaris across in the southern USA. Population differentiation was measured through Nei’s genetic differentiation values, and phylogenetic relationships were inferred using multilocus sequence analysis of six loci. These analyses revealed high gene flow among subpopulations, indicating a lack of geographical structure. STRUCTURE analysis indicated that genetic differentiation is driven by factors other than geography. To further assess whether C. cf. flagellaris represents a single species or a complex of cryptic taxa, we performed a multilocus phylogenetic analysis. The isolates did not cluster within a monophyletic clade, suggesting that C. cf. flagellaris may comprise multiple lineages. This highlights the need for additional genetic markers to fully delineate species within this group. Additionally, the index of pattern suggests the potential for sexual recombination. Despite the lack of an observed teleomorphic structure, C. cf. flagellaris may not reproduce strictly clonally but instead undergoes cryptic sexual reproduction. Taken together, these findings underscore the necessity of integrated pest management strategies for CLB.

Date

4-24-2025

Committee Chair

Vinson Doyle

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Available for download on Monday, May 01, 2028

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