Degree

Doctor of Philosophy (PhD)

Department

Department of Plant Pathology & Crop Physiology

Document Type

Dissertation

Abstract

Cercospora leaf blight (CLB) is a major disease of soybean across the Gulf South and was responsible for an estimated annual loss of 60.5 million bushels in the mid-South during 2022. Cercospora kikuchii has long been considered the sole causal agent of CLB and seeds were thought to be the primary source of inoculum for the disease. But the identification of the generalist pathogens Cercospora cf. flagellaris and Cercospora cf. sigesbeckiae as the predominant causal agents suggest airborne conidia may be critical in CLB epidemiology. The role of airborne inoculum in CLB epidemics is supported by changes in Cercospora community composition throughout the growing season and the persistence of disease after seed disinfestation. The aim of this study was to investigate the temporal and spatial dynamics of airborne conidia associated with the causal agents of CLB and determine if this information can be leveraged to improve the efficacy of chemical management. Airborne particles were sampled in Winnsboro, Baton Rouge, and Alexandria, Louisiana from June-November 2019 through 2022, using a combination of active (Burkard and garden motor) and passive (windvane and windsock) spore traps. The utility of the different types of spore traps were evaluated by comparing the diversity of fungi captured as estimated from amplifying and sequencing the nuclear ribosomal internal transcribed spacer 2 (nrITS-2) region. While the nrITS region did not provide species resolution for CLB pathogens (or other important plant pathogens), linking nrITS sequences with species identified through multilocus phylogenetics coupled with field survey data expanded the utility of these sequences. Burkard and windvane traps captured significantly greater diversity than the windsock trap. To study the contribution of individual CLB pathogens to airborne inoculum, species-specific quantitative polymerase chain reaction (qPCR) assays were developed using comparative genomics that were sensitive and reproducible. Based on weekly spore sampling, conidia of all three CLB pathogen species were detected by qPCR prior to symptoms in every year and location. Consistent with previous field surveys, the dominant pathogen based on spore sampling was C. cf. flagellaris. Efforts to target conidial peaks with fungicide applications showed positive numerical trends in 2021 but were ineffective in 2022, suggesting application timing needs greater optimization. This study utilized metabarcoding as a tool to study aerobiota, investigated the temporal and spatial dynamics of airborne conidia associated with the CLB disease cycle, and took the first steps in targeting fungicide applications for CLB management.

Date

4-3-2024

Committee Chair

Thomas-Sharma, Sara

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