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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Research

2011 Annual Report

1a. Objectives (from AD-416)
Determine the genetic control of resistance to root-knot nematode (Meloidogyne incognita) and Fusarium wilt (Fon race 2) in watermelon, identify and map DNA-based markers closely linked to identified resistance loci, and release resistant watermelon lines. Utilize genomic tools to develop genetic linkage maps for watermelon and diagnostic DNA-based markers for host-plant resistance to viruses and key watermelon fruit traits. Develop inbred broccoli lines with tolerance to high temperature stress and elucidate the underlying genetic control of the tolerance. Breed high yielding, self-compatible inbred broccoli lines with high productivity and high levels of health-promoting compounds. Elucidate the genetic control of bacterial leaf spot resistance in leafy green Brassicas (B. juncea and B. rapa) and release resistant breeding lines resulting from the research.

1b. Approach (from AD-416)
Select parental lines of watermelon, broccoli, or leafy green brasscias based on phenotypic expression of resistance, tolerance, or quality traits under study. Use the selected parents to construct conventional (i.e., F2, BC1, recombinant inbred) and doubled haploid (for broccoli only) populations that segregate for the traits of interest, and then employ those populations in studies to determine mode of inheritance of each character. Utilize PCR-based markers and other genomic technologies to identify sequences linked to the studied characters and to locate controlling genes on linkage maps. Use particular markers (i.e., SSR, SRAP, SNPs, or SCARs) closely associated with traits of interest to develop tools for marker-assisted selection. Based on knowledge gained through above studies, devise breeding strategies, and applications of marker technologies to use in the further development of horticulturally enhanced lines or hybrids that express resistances and other traits of interest and that also produce high quality vegetables. Make enhanced lines available through public releases or commercial licensing. Continue ongoing searches for new resistances and tolerances among watermelon and vegetable Brassica accessions for the U.S. PI and other collections.

3. Progress Report
For the watermelon portion of this project, the genome of the watermelon heirloom cultivar Charleston Gray has been sequenced using 454 technology. Using the watermelon genome sequencing data we have developed a new type of Deoxyribonucleic acid (DNA) marker for watermelon. This type of marker is called a high frequency oligonucleotide-targeting active genes, and it has proven to be highly useful in the construction and comparative mapping of the watermelon genome. Three Citrullus lanatus var. citroides plant introductions (PIs) that exhibited resistance to fusarium wilt (FW) have been used to develop germplasm lines containing resistance to this important disease. Specifically, selected plants from the most resistant PIs have been self-pollinated several generations to create pure lines that will be used for the development of genetic populations segregating for FW resistance, and for identifying putative genes associated with the resistance(s). In other studies, genetic populations derived by crossing wild watermelon accessions with resistant and susceptible responses to root knot nematodes (RKNs) that segregate for these different responses, have been created and then evaluated for specific reaction to infection with RKNs. Inheritance of RKN resistance is to be determined based on these results. In addition to the above, three DNA markers linked to a gene associated with resistance to zucchini yellow mosaic virus (ZYMV) have been used in a marker-assisted selection (MAS) scheme. Selected breeding lines showing tolerance to a ZYMV Florida strain have been developed and six lines showing superior fruit quality were selected. For the broccoli and leafy Brassica portion of this project, an additional cycle of breeding broccoli for tolerance to high temperature stress was completed, and new tolerant selections were identified and advanced another generation. Replicated trials in the summer at Charleston are helping to identify the most tolerant broccoli inbreds and hybrids for possible germplasm releases. In addition, two cooperating public scientists are testing three experimental hybrids from the Charleston program for summer adapation in New York and Maine. A large replicated evaluation of a doubled haploid population of broccoli that segregates for adaptation to summer has been conducted and initial results appear to indicate quantitative inheritance of this trait. In work aimed at developing leafy green Brassicas like mustard and turnip greens resistant to bacterial leaf spot disease, resistant accessions found to date were tested in a fall trial and results confirmed strong host plant resistance in the best accessions grown under a heavy field infection. A large replicated field evaluation of over 80 collard landraces obtained from seed savers in the coastal Carolinas was conducted in the fall to identify accessions with economic value. Additionally, pilot-scale cage productions of hybrid collard seed were conducted to test the feasibility of commercializing collard varieties developed by the Vegetable Laboratory program; negotiations with a seed company to cooperate in this effort are ongoing.

4. Accomplishments
1. Development of broccoli hybrids adapted to high-temperature summer environments of the Eastern United States. Broccoli production in the Eastern United States is limited by the fact that the crop is sensitive to warm temperatures near harvest, which induce flaws that include incomplete head development, uneven bud size, leafiness of heads, and rough head surface. In recent studies conducted by scientists at the U.S. Vegetable Laboratory in cooperation with scientists at Cornell University, selected experimental hybrids that were bred for adaptation to summer conditions in Charleston, South Carolina were compared to some commonly raised commercial broccoli hybrids in field trials. The investigators found that all commercial broccoli hybrids tested produced good quality heads in the fall when cool temperatures prevail, but nonmarketable heads in summer environments when conditions are hot. On the contrary, the experimental hybrids developed by ARS produced similar good quality heads in both fall and summer. The unique ARS broccolis that are adapted to summer conditions will be of great interest to plant breeders working to develop broccoli adapted to high temperatures and they may ultimately be used by growers to produce a crop in environments previously deemed too hot.

Review Publications
Farnham, M.W., Bjorkman, T. 2011. Evaluation of Experimental Broccoli Hybrids Developed for Summer Production in the Eastern United States. HortScience. 46:858-863.

Levi, A., Wechter, W.P., Massey, L.M., Carter, L., Hopkins, D. 2011. Genetic Linkage Map of Citrullus lanatus var. Citroides Chromosomal Segments Introgressed into the Watermelon Cultivar Crimson Sweet (Citrullus lanatus var. lanatus) Genome. American Journal of Plant Sciences. 2:93-110.

Quiros, C.F., Farnham, M.W. 2011. The Genetics of Brassica oleracea. In: Schmidt, R., Bancroft, I., editors. Genetics and Genomics of the Brassicaceae. New York, NY:Springer. p. 261-290.

Levi, A., Thies, J.A., Simmons, A.M., Harrison Jr, H.F., Hassell, R., Keinath, A. USVL-220, A Novel Watermelon Breeding Line. HortScience. 46(1):135–138. 2011.

Levi, A., Wechter, W.P., Harris-Shultz, K.R., Davis, A.R., Fie, Z. 2010. High-frequency Oligonucleotides in Watermelon Expressed Sequenced Tag-unigenes Are Useful in Producing Polymorphic Polymerase Chain Reaction Markers among Watermelon Genotypes. Journal of the American Society for Horticultural Science. 135(4):369-378.

Levi, A., Harris-Shultz, K.R., Wechter, W.P., Kousik, C.S., Thies, J.A. 2010. DNA markers and pollen morphology reveal that Praecitrullus fistulosus is more closely related to Benincasa hispida than to Citrullus spp. Genetic Resources and Crop Evolution. 57:1191-1205.

Last Modified: 06/26/2017
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