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Research Project: Genomics and Genetic Improvement of Disease Resistance and Horticultural Characteristics of Watermelon, Broccoli, and Leafy Brassicas

Location: Vegetable Research

Title: QTL mapping and pyramiding resistance to Fusarium oxysporum f. sp. niveum (races 1 and 2) and potyviruses in watermelon

item BRANHAM, SANDRA - Clemson University
item Wechter, William - Pat
item Ling, Kai-Shu
item Katuuramu, Dennis
item Levi, Amnon

Submitted to: Eucarpia Cucurbitaceae Symposium Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 5/21/2021
Publication Date: 5/25/2021
Citation: Branham, S.E., Wechter, W.P., Ling, K., Katuuramu, D.N., Levi, A. 2021. QTL mapping and pyramiding resistance to Fusarium oxysporum f. sp. niveum (races 1 and 2) and potyviruses in watermelon. Eucarpia Cucurbitaceae Symposium Proceedings.

Interpretive Summary: Cultivated sweet watermelon (Citrullus lanatus) is an important vegetable crop for millions of people around the world. There are limited sources of resistance to economically important diseases within C. lanatus, whereas Citrullus amarus has a reservoir of traits that can be exploited to improve C. lanatus. Downy mildew, caused by Pseudoperonospora cubensis, is an emerging threat to watermelon production. We screened 122 C. amarus accessions for resistance to downy mildew over two tests (environments). The accessions were genotyped with 2,126,759 single nucleotide polymorphic (SNP) markers. A genome-wide association study approach was deployed to uncover marker-trait associations and identify candidate genes underlying resistance to downy mildew. Our results indicate the presence of wide phenotypic variability (1.1 - 57.8%) for leaf area infection, representing a 50.7-fold variation for downy mildew resistance across the C. amarus diversity panel. Broad-sense heritability estimate was 55%, implying the presence of moderate genetic effect for resistance to downy mildew. The peak SNP markers associated with resistance to P. cubensis were located on chromosomes Ca03, Ca05, Ca07, and Ca11. The significant SNP markers accounted for up-to 30% of the phenotypic variation and were associated with candidate genes including disease resistance proteins. This information will be useful in understanding the genetic architecture of the P. cubensis-Citrullus spp. patho-system as well as development of resources for genomics-assisted breeding for resistance to downy mildew in watermelon.

Technical Abstract: The USDA Citrullus spp. germplasm collection is a valuable resource for enhancing modern watermelon cultivars (Citrullus lanatus) with resistance to fungal and viral diseases. We have generated genetic populations segregating for resistance to important watermelon diseases: Fusarium wilt (FW) (caused by the fungus Fusarium oxysporum f. sp. niveum; Fon races 1 and 2) and potyviruses, including Zucchini yellow mosaic virus (ZYMV) and Papaya ringspot virus-watermelon strain (PRSV-W). QTL mapping of Fon races 1 and 2-resistance identified several significant quantitative trait loci (QTL). A single QTL was associated with resistance to ZYMV and PRSV-W, adhering to expectations of a previous study which indicated a single-recessive gene inheritance in watermelon. We have been developing kompetitive allele specific PCR (KASP) markers tightly linked to resistance loci. We are using the KASP markers together with phenotyping assays to pyramid FW, PRSV and ZYMV-resistance loci into watermelon cultivars. In addition, KASP markers are being developed for powdery mildew (PM) resistance loci and will be used in the development of multi resistance watermelon lines. This study is in part supported by the “National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2020-51181-32139” (CucCAP1 and CucCAP2).