Downy mildew resistance in B. oleracea

Gomez Gutierrez, Abraham (2022) Downy mildew resistance in B. oleracea. Doctoral thesis, University of East Anglia.

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Downy mildew (Hyaloperonospora brassicae) is one of the most devastating and widely reported diseases on horticultural and agricultural Brassica plants worldwide. Yield losses can be up to 90-100% if environmental conditions are favourable for downy mildew, especially at the early stages of plant development. In recent years, downy mildew incidence has increased in many Brassica growing countries, most likely due to the steady rise of seasonal temperatures in some of these regions of the world caused by cli-mate change, making the disease more widespread. To palliate this, cultural and chemical techniques for controlling downy mildew have been reported to be ineffective due to the systemic nature of the disease. As a result, Brassica cultivars with inherent genetic resistance to this pathogen can provide the best and most cost-effective alternative to avoid yield losses in the future.

To achieve this, in this thesis, I have created a collection of single spore H. brassicae isolates from 9 field isolates collected from distinct geographical locations in the UK and USA (Chapter 2). In Chapter 3, I assessed different sources of plant material at the cotyledon stage and identified new sources of resistance to H. brassicae. The host specificity of all H. brassicae isolates used in this thesis was very high, only being able to infect B. oleracea and a very few B. napus genotypes (Chapter 3). In Chapter 4, a specific F2 cross-segregating for a single dominant downy mildew resistant gene at the seedling and adult plant stage was used for genetic mapping using a bulk segregant analysis approach. The bulk segregant analysis was able to map the downy mildew resistance to an NLR cluster in the B. oleracea chromo-some 2. The NLR cluster was observed to have a high level of copy number variation when compared to other genome assemblies. Therefore, to resolve the NLR cluster, a draft genome assembly of the resistant parental line was created using long-read sequencing. I used the assembly to create diagnostic KASP markers of the resistance and validate them using 466 F2 individuals for marker-assisted selection.

These newly identified sources of downy mildew resistance could provide a solid foundation to improve the management of this disease in the fields of many B. oleracea crops, especially vegetable crucifers. To date, only 4 sources of resistance to downy mildew have been mapped in B. oleracea. The new source of resistance located to the B. oleracea Chromosome 2 and the diagnostic KASP markers created in this thesis can be used in marker-assisted breeding selection to develop new commercial varieties resistant to downy mildew. Finally, the data generated in this thesis of the NLR cluster and sequences of the candidate genes based on the long-read draft genome assembly could be used to clone the gene responsible for the resistance.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Nicola Veasy
Date Deposited: 22 Jun 2023 14:57
Last Modified: 22 Jun 2023 14:57


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