Date of Award

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant and Environmental Science

Committee Chair/Advisor

Dr. Sandra Branham

Committee Member

Dr. Anthony Keinath

Committee Member

Dr. Ksenija Gasic

Committee Member

Dr. Hehe Wang

Abstract

Bacterial leaf blight is a devastating disease for Brassica crops worldwide and it can cause 40-100% repeated yield losses in Southeastern United States. This disease is known to be caused by a bacterium, Pseudomonas cannabina pv. alisalensis (Pcal). No effective chemical options are available to manage this pathogen, and all commercially available cultivars of turnip greens (B. rapa) are highly susceptible. Previously, 446 accessions were screened for their response to Pcal, but no significantly resistant accession was identified. Therefore, I screened 275 additional B. rapa accessions through a series of greenhouse trials. Here, I report three B. rapa accessions (Ames 7572, PI 649171, and PI 649183) with significant resistance to Pcal strain T3C. Among these three, the Ames 7572 was tested twice, followed by repeated selfing and selection to develop a B. rapa resistant breeding line (r1000). The r1000 was later crossed with a susceptible breeding line r1297 to develop a mapping population of 184 F2 individuals. Genotyping by Sequencing (GBS) was used to genotype the F2 individuals, and at least 16 plants per F2:3 family were phenotyped through artificial inoculations with Pcal strain T3C in a growth chamber. Using R/qtl, a single major QTL (qPcal-10) on chromosome A10 was identified as the bacterial leaf blight resistance loci in r1000.

During recent bacterial leaf blight outbreaks, representative necrotic leaf tissues were collected from three commercials farms in South Carolina. BOX-PCR for the newly collected isolates indicated a different banding profile than Pcal. In order to identify the causal Pseudomonas sp., whole genome sequencing was used. Using two taxonomic classification methods, digital DNA-DNA hybridization (dDDH%) and average nucleotide identity (ANI%), the seven isolates were identified as Pseudomonas capsici, with five of them exhibiting copper tolerance. Comparison with genomes of the publicly available P. capsici isolates revealed minimal divergence even when separated by different hosts and geographical origin. Through this research, I identified a separate pathogen, P. capsici, which can cause bacterial leaf blight and/or spot in turnip greens (B. rapa); mustard greens (B. juncea); swiss chard (B. vulgaris var. cicla); spinach (Spinacia oleracea); melon (Cucumis melo); green beans (Phaseolus vulgaris) and six different Brassica oleracea morphotypes including collard (B. oleracea var. viridis), broccoli (B. oleracea var. italica), kale (B. oleracea var. sabellica), cauliflower (B. oleracea var. botrytis), kohlrabi (B. oleracea var. gongylodes), and brussels sprout (B. oleraceavar. gemmifera).

Additionally, a Brassica oleracea leafy greens diversity panel of 238 accessions from USDA NPGS and 45 accessions from Seed Savers Exchange (SSE) was developed. The panel included collard accessions (B. oleracea var. viridis); 15 curly kale accessions (Brassica oleracea var. sabellica); one lacinato kale accession (B. oleracea var.palmifolia); 34 kale accessions (B. oleracea var. acephala); 12 Portuguese kale accessions (B. oleracea var. costata); 28 cauliflower (B. oleracea var. botrytis) accessions; one B. oleracea oleracea accession; 60 non-heading cabbage (B. oleracea var. capitata) accessions. The accessions were self- pollinated for two generations and then evaluated during two field seasons for horticulturally important traits including, leaf color, leaf shape, leaf margin, leaf incision, leaf variegation, leaf blistering, plant growth habit, plant uniformity, and fresh weight. Moreover, a genome-wide association study was conducted for 74 collard accessions and 39 significant associations across eight horticulturally desirable traits were identified. Thereafter, functionally relevant candidate genes were identified in the Linkage Disequilibrium (LD) blocks of the trait associations.

Collectively, this research identified three novel sources of Pcal resistance in turnip greens, characterized an emerging pathogen species capable of causing bacterial leaf blight and generated genomic resources to facilitate breeding for improved collard green cultivars and Pcal resistant Brassica rapa cultivars.

Author ORCID Identifier

0000-0002-3491-5634

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