Location: Vegetable ResearchTitle: Genetics of resistance to powdery mildew in watermelon line USVL608-PMR Author
|Kousik, Chandrasekar - Shaker|
|Mihir, Mandal - Oak Ridge Institute For Science And Education (ORISE)|
Submitted to: Cucurbitaceae Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 9/30/2018
Publication Date: N/A
Technical Abstract: Powdery mildew (PM) of watermelon (Citrullus lanatus) and other cucurbits caused by Podosphaera xanthii is a major factor limiting production in greenhouses and open fields. In recent years, occurrence of PM has been increasing on watermelon across the United States, and commercial watermelon cultivars with resistance are rare. Four PM resistant germplasm lines with broad resistance to isolates from South Carolina, Georgia, Florida, California and New York were developed from plant introductions and released in 2018 by USDA ARS. All four lines have red-pink flesh and hybridize readily with commercial cultivars and inbred lines. One of these, USVL608-PMR (S6), a red fleshed watermelon line with high levels of resistance to PM was used as the female parent (P1) and crossed with USVL677-PMS which is highly susceptible (P2). The parents, F1, backcrosses to both parents (BC1, BC2) and a large F2 population were inoculated with a local isolate of PM and assessed for disease severity on a 0-10 scale of increasing disease severity. All susceptible parent (USVL677-PMS) plants were rated >7 [mean disease severity (DS) = 94%], whereas most resistant parent (USVL608-PMR) plants were rated as 1 (DS=2.5%). Majority of the BC1 plants were rated =2 and considered as resistant. Of the 466 F2 plants, 221 were rated =2 (DS=3.1%). Of the 76 BC2 plants, 23 were rated =2 (DS=2.9%). Chi-square analyses of the observed segregation of phenotypes for the F2 plants indicated that two genes control PM resistance with a good fit for a 7:9 resistance to susceptibility ratio. The proposed model for this ratio is two genes with one recessive for high resistance and one dominant for high resistance. This is supported by a backcrossing segregation ratio of 1:3. We have observed some highly and moderately resistant plants in the F2 indicating the cumulative effect of the two genes. QTL-seq analysis on the extremes from the F2 populations and RNA-seq analysis of the parents during PM infection are being conducted to identify the chromosomal regions involved in resistance. USVL608-PMR will serve as a useful source to incorporate PM resistance into commercial cultivars.