Doctor of Philosophy (PhD)
Plant Pathology and Crop Physiology
Cercospora leaf blight (CLB) caused by Cercospora kikuchii, has become a troublesome disease in the southern United States. C. kikuchii produces a non-hostspecific phytotoxin and a pathogenicity factor known as cercosporin during infection of soybean. A quantitative real-time PCR assay was developed for detection and quantification of C. kikuchii. The sensitivity of detection is 1 pg of genomic DNA. The assay detected the presence of C. kikuchii in soybean leaves long before the appearance of disease symptoms. C. kikuchii DNA levels in soybean leaves increased slowly during early soybean development, followed by a quick increase at late reproductive stages. Results from three year field studies of soybean plants with various fungicide treatments showed that multiple fungicide applications beginning from late vegetative stages until late reproductive stages can reduce C. kikuchii growth and CLB symptom development. However, different fungicides vary in their effectiveness. In order to identify genes involved in cercosporin biosynthesis, proteins were isolated from C. kikuchii and compared between cultures grown under light (promotes toxin production up to 6 fold) and dark conditions through proteomics. Six proteins were up-regulated and two were down-regulated in C. kikuchii grown under light. Two of the up-regulated proteins [hydroxynaphthalene reductase (HNR) and adenosylhomocysteinase (AHCY)] were further studied through gene disruption. The resulting mutants showed reduced cercosporin production in vitro and virulence on soybean leaves. C. kikuchii secreted proteins from culture were also examined to identify proteins involved in the infection of soybean. Two of them showed high homology to glucan beta 1,3-glucosidase and EAP30 family proteins and identity of several proteins remains unknown. The function of these proteins in infecting soybean remains to be determined. Two small portions of AHCY gene also were inserted into a Bean Pod Mottle Virus (BPMV) derived vectors and introduced into soybean to explore the possibility of using host-induced gene silencing (HIGS) in controlling C. kikuchii infection of soybean, The target gene expression was reduced by 3.5 to 6.6-fold, and C. kikuchii growth was reduced by 16 to18-fold in the HIGS treated soybean compared to vector control plants, indicating a possible new approach to control CLB in soybean.
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Chanda, Ashok Kumar, "Molecular Approaches to Detect and Control Cercospora kikuchii in Soybeans" (2012). LSU Doctoral Dissertations. 3002.