Date of Graduation

8-2023

Document Type

Thesis

Degree Name

Master of Science in Horticulture (MS)

Degree Level

Graduate

Department

Horticulture

Advisor/Mentor

Shi, Ainong

Committee Member

Dickson, Ryan W.

Second Committee Member

Joshi, Neelendra K.

Third Committee Member

Rojas, Alejandro

Keywords

Tomatoes

Abstract

This thesis comprises two studies that focused on screening tomato genotypes for bacterial wilt resistance and a genome-wide association study and genomic prediction for yield-related traits in sugarcane with the aim of contributing to the breeding programs. In the first chapter, 40 tomato accessions from the USDA collection were evaluated for their resistance to Ralstonia solanacearum isolate P822 under controlled greenhouse conditions. The plants were inoculated, and disease symptoms were visually assessed. Disease severity was rated on a scale of 0 to 4, where 0 represented no wilting and 4 indicated complete leaf wilting. Among the tested accessions, five (PI 645370, PI 647306, PI 600993, PI 355110, and PI 270210) exhibited resistance to BW. The heritability of BW resistance was estimated to be 59.9% based on the 0-4 disease incidence scale and 42.8% based on the disease severity index. Furthermore, the 40 accessions were divided into two distinct clusters, with three and two BW-resistant accessions in each cluster, respectively. This suggests that the five resistant accessions possess different genetic mechanisms for resistance. The incorporation of these resistant accessions into tomato breeding programs holds significant potential for developing new BW-resistant cultivars. The second study focuses on identifying molecular markers associated with yield-related traits in sugarcane (Saccharum spp.) through a genome-wide association study (GWAS). The study involved a self-progeny mapping population of the LCP85-384 cultivar, and data on plant height, stalk number, stalk diameter, and stalk weight were collected from plant cane and first ratoon crops in two replicated field plots. The mapping population was genotyped using amplified fragment length polymorphisms (AFLPs), simple sequence repeats (SSRs), and target region amplification polymorphism (TRAP) markers. GWAS was conducted using mixed linear models (MLM), generalized linear models (GLM), and single marker regression (SMR) approaches. The analysis identified a total of 11 markers associated with stalk number, 9 markers associated with stalk weight, 21 markers associated with stalk diameter, and 5 markers associated with plant height. Additionally, the performance of these yield-related traits was predicted using genomic prediction (GP). Five models: ridge regression best linear unbiased prediction (rrBLUP), Bayesian ridge regression (BRR), Bayesian A (BA), Bayesian B (BB), and Bayesian least absolute shrinkage and selection operator (BL) were used to predict prediction accuracy. The prediction accuracy (r-value) achieved was 0.40 for cane height, 0.36 for the number of stalks, 0.44 for stalk diameter, and 0.54 for stalk weight. Once validated, these yield trait-associated molecular markers will serve as valuable tools in sugarcane improvement programs to aid in the selection of desirable traits. In summary, the identification of BW-resistant tomato accessions and the discovery of yield trait-associated molecular markers in sugarcane provide promising ways for developing disease-resistant tomato cultivars and improving yield-related traits in sugarcane breeding programs, respectively. These findings contribute to the advancement of agricultural practices aimed at combating crop diseases and enhancing crop productivity.

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