Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Cell & Molecular Biology (PhD)

Degree Level



Biological Sciences


Ainong Shi

Committee Member

James C. Correll

Second Committee Member

Vibha Srivastava

Third Committee Member

Margaret L. Worthington


Disease Resistance, Downy mildew, Molecular Breeding, Plant Breeding, Spinach


Spinach (Spinacia oleracea) is an important cool-season leafy vegetable crop in the United States (US). Downy mildew, caused by the obligate oomycete Peronospora effusa, is the most economically important disease of spinach. A total of 17 races of P. effusa have been reported on spinach, and many of these races (>10) have emerged in the last three decades. The new races of the pathogen are continually overcoming the genetic resistances used in the newly released cultivars.

A detached leaf inoculation assay was evaluated, standardized, and validated as a new method to differentiate resistant and susceptible spinach genotypes. Disease response on detached leaves was compared to the response of corresponding cultivars in the standard whole-plant assay, and a complete correspondence was found between the two inoculation assays. The detached leaf assay will facilitate advances in breeding for P. effusa resistance and the analysis of pathogen epidemiology.

The segregating F1 progeny population derived from a cross between the hybrid cultivars Whale and Lazio were inoculated with race 16 (isolate UA201519B) of P. effusa and genotyped using genotyping by sequencing (GBS). Association analysis identified six significant SNP markers mapped to a 0.57 Mb segment of chromosome 3 that includes four disease resistance candidates genes: Spo12736, Spo12784, Spo12908, and Spo1282.

In another study, the F1 progeny from several crosses (Polka x either Swan, Squirrel, Tonga, and T-Bird; Whale x Swan; and Whale x Squirrel) were inoculated with race 13 of P. effusa (isolate UA0510C). The inoculated seedlings were scored as either resistant or susceptible. Association analysis using GBS markers identified significant SNPs in 0.39, 0.98, and 1.2 Mb of chromosome 3 within 0.98-6.88 Kb of the disease resistance genes Spo12719, Spo12905, and Spo12821.

The USDA spinach germplasm collection and commercial cultivars were evaluated for downy mildew disease resistance in the field condition under natural inoculum pressure. Genomewide association analysis identified 19 SNP markers associated with reduced disease ratings under field conditions in all the tested models. The findings from this dissertation research provide new resources to improve genetic resistance against the downy mildew pathogen and in developing durably resistant cultivars.