Degree

Doctor of Philosophy (PhD)

Department

Plant Pathology and Crop Physiology

Document Type

Dissertation

Abstract

Cercospora leaf blight (CLB) of soybean (Glycine max L. Merr.), an important disease in Louisiana, is primarily caused by Cercospora cf. flagellaris. The pathogen produces cercosporin, a toxin necessary for fungal virulence. So far, there are few commercial cultivars of soybean with CLB resistance and few effective fungicides to manage this disease. Consequently, alternative management approaches are needed. With that in mind, the first objective was to develop a screening method to compare the effectiveness of bacterially-expressed double stranded (ds) RNAs corresponding to genes associated with cercosporin accumulation (Avr4, CTB1, CTB8, AY-1 and HN-2) on suppressing the toxin via targeted gene silencing in liquid culture. The second objective was to investigate the effect of select dsRNAs applied topically on soybean leaves on CLB suppression under field conditions. The third objective of the study was to elucidate the effects of host factors, such as differences in the sugar content and the endophytic bacteria between the R2 (full flower) and R5 (beginning seed) soybean leaves on C. cf. flagellaris growth and cercosporin production. All tested dsRNAs showed varying degrees of effectiveness in reducing cercosporin in liquid culture, with Avr4 dsRNA having the lowest EC50 value of 10.4 µg/mL. Targeted gene silencing was detected in cultures treated with dsRNAs from Avr4, CTB8 and CTB1. However, topical application of dsRNAs targeting Avr4, CTB1, CTB8, as well as CYTB and CYP51, involved in cellular respiration and ergosterol biosynthesis, respectively, on soybean leaves under field conditions did not show suppression of CLB development. This could be due to unsuccessful dsRNA penetration into the plants, or degradation before penetration. It was found that C. cf. flagellaris produced more cercosporin in R5, than in R2 leaf extracts and that R5 leaf extracts also had higher simple sugar levels compared to the R2. Moreover, fructose levels were significantly higher in R5 than in R2 soybean leaves. In addition, bacterial endophytes from R5 leaf extracts and Paenibacillus polymyxa, isolated from R2 leaves, significantly reduced the growth of C. cf. flagellaris. The findings from this work implicated soybean leaf fructose in CLB development and highlighted the potential of using bacterial endophytes for CLB management.

Date

7-6-2020

Committee Chair

Chen, Zhi-Yuan

Available for download on Tuesday, August 01, 2023

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