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United States Department of Agriculture

Agricultural Research Service

Related Topics


Location: Vegetable Research

2008 Annual Report

1a. Objectives (from AD-416)
Objective 1: Elucidate the etiology and epidemiology of Pepino mosaic virus on tomato and Pseudomonas syringae pv. maculicola on vegetable Brassicas to identify vulnerable areas (e.g., seed or alternative host) that provide biologically-based control opportunities. Objective 2: Identify and characterize genetic sources of disease resistance and facilitate the incorporation of these genes into enhanced germplasm of watermelon, tomato, and vegetable Brassicas. Objective 3: Identify and characterize new and existing bacteria antagonistc to phytopathogens and elucidate the factors that affect the potential efficacy of these biological control agents. Objective 4: Evaluate biologically-based control strategies to develop new and effective management practices against root-knot nematodes, pathogenic bacteria, and viruses.

1b. Approach (from AD-416)
Develop sensitive PCR-based detection methods and utilize these techniques to evaluate virus distribution in seed and plant tissues of tomato as well as other alternative crops or weed hosts. Develop molecular-based markers for identification and utilize these markers for environmental tracking of the vegetable Brassica leaf spotting bacterium Pseudomonas syringae pv. maculicola (Psm). Screen tomato germplasm for resistance to PepMV, evaluate the inheritance of resistance to Zucchini yellow mosaic virus (ZYMV) in watermelon, and develop molecular markers linked to the ZYMV resistance locus in watermelon. Screen germplasm from national collections of Brassica rapa and Brassica juncea for resistance to Pseudomonas syringae pv maculicola, and evaluate the genetics of resistance. Identify non-phytopathogenic pseudomonads that inhibit Pseudomonas syringae pv. maculicola and test for efficacy as biological control agents. Identify bacterial genes involved in bacterial-biocontrol colonization of plants using full-genome microarray analysis. Develop an effective seed treatment method for PepMV in tomato seed and generate virus-free materials of heirloom sweetpotato germplasm and breeding materials. Test effectiveness of the nematode-ovicidal bacterium Pseudomonas synxantha BG33R against root-knot nematode on melon in greenhouse and field assays.

3. Progress Report
In recent years, an estimated $400 million greenhouse fresh tomato production in the US has been under a serious threat by Pepino mosaic virus (PepMV) infection. Through extensive screening to the tomato germplasm collection, one accession (LA 1731) was shown to have potential resistance to PepMV. In collaboration with a breeder from University of Florida, breeding materials are under development. Our study has found that PepMV is seed-borne, but not seed-transmitted. The ease of mechanical transmission of the virus from seed to seedlings could make contaminated seed a potential source of initial virus inoculum. A sensitive seed health test (Real-time Reverse Transcription-Polymerase Chain Reaction (RT-PCR) was developed for effective detection of PepMV on tomato seed. A thermotherapy through the use of dry-heat treatment was shown to effectively deactivate the virus infectivity without adverse effect on seed germination. Watermelon production is seriously affected by several aphid-transmitted viruses in watermelon growing areas in the US. Marker-assisted selection would greatly enhance breeder’s abilities to select breeding materials with the trait of interest. Using candidate gene approach, markers closely linked to the Zucchini yellow mosaic virus resistance were identified and shown to be effective for use in marker-assisted selection. Following the discovery last year of the leaf spotting pathogen Pseudomonas syringae pv. maculicola (Psm) as a causal agent of severe disease outbreaks in the major vegetable Brassica (collards, turnip and mustard greens, and kale) growing areas of South Carolina, we also have found a variant of the black rot pathogen Xanthomonas campestris pv. campestris (XCC) which also causes severe leaf spot and blighting of vegetable Brassica. The resistant germplasm will be used to incorporate Psm resistance into breeding lines. In addition, we have begun screening the 700 plant introductions of turnip and mustard greens for resistance to the new leaf spotting variant of XCC. Our laboratory has generated a subtractive hybridization library of unique DNA fragment associated with Pseudomonas syringae pv. maculicola (Psm). These DNA fragments are being tested for use as molecular markers for tracking the pathogen in the environment, as well as identification of the pathogen in diseased plants. A collaborative effort was undertaken in our laboratory, using high-density microarray analysis, to identify genes involved in the development and ripening of watermelon fruit. Three hundred and thirty-five genes were found to be modulated during different stages of watermelon fruit development. Identification of a large number of modulated ethylene-related genes, as well as biochemical analysis of ethylene production, showed that ethylene is quite important in the development and ripening of watermelon fruits. This research contributes to Natinal Program 303 Plant Diseases, with problem statements: 2C-Population dynamics, spread, and epidemiology of pathogens; 3A- Mechanism of plant disease resistance; 3B-Disease resistance in new germplasm and varieties; and 4A-Biological and cultural control technologies.

4. Accomplishments
1. Development of a sensitive Real-time Polymerase Chain Reaction (RT-PCR) detection method for Pepino mosaic virus (PepMV) in tomato seeds. PepMV is a seed-borne virus disease threatening the $400 million U.S. greenhouse tomato industry. Real-time PCR technique was developed to allow sensitive and reliable detection of PepMV in tomato seed or plant tissues. Currently, a plant diagnostic laboratory is interested in further development of this technology into commercial testing kits. Because pepino disease resistance is not available in tomato cultivars, planting certified PepMV-free seeds is the first step to prevent any accidental introduction of this devastating virus from dirty seeds to tomato production greenhouses. This accomplishment addresses the National Program 303 - Plant Diseases with the problem statement 1B: Detection, Identification, Characterization and Classification of Pathogens.

5. Significant Activities that Support Special Target Populations

Review Publications
Ling, K., Kousik, C.S., Keinath, A.P. 2008. First Report of Southern Blight on Bottle Gourd (Lagenaria siceraria) caused by Sclerotium Rolfsii in South Carolina. Plant Disease. 92:656.

Ling, K., Zhu, H., Gonsalves, D. 2008. Resistance to Grapevine Leafroll Associated Virus-2 is Conferred by Post-Transcriptional Gene Silencing in Transgenic Nicotiana benthamiana. Transgenic Research. 17:733-740.

Ling, K., Zhu, H., Petrovic, N., Gonsalves, D. 2007. Serological detection of Grapevine leafroll virus 2 using an antiserum developed against the recombinant coat protein. Journal of Phytopathology. 155:65-69.

Ling, K., Zhu, H., Gonsalves, D. Grapevine leafroll associated viruses. In Rao, G.P., Myrta, A., and Ling, K. (eds). Characterization, Diagnosis and Management of Plant Viruses, Vol 2. Horticultural Crops. Studium press, Houston, Texas, USA, pp 181-199. 2008. (Book Chapter)

Rao, G.P., Myrta, A., Ling, K. (eds). Characterization, Diagnosis & Management of Plant Viruses, Vol. 2. Horticultural Crops. Studium press, Houston, Texas, USA. 302 pp. 2008 (Edited Book)

Wechter, W.P., Keinath, A.P., Smith, J.P., Farnham, M.W. 2008. First Report of Severe Outbreaks of Bacterial Leaf Spot of Leafy Brassica Greens Caused by Xanthomonas Campestris pv. Campestris in South Carolina. Plant Disease. 92:1134.

Wechter, W.P., Levi, A., Harris-Shultz, K.R., Davis, A.R., Fei, Z.J., Katzir, N., Giovannoni, J.J., Salman-Minkov, A., Hernandez, A., Thimmapuram, J., Tadmor, Y., Portnoy, V., Trebitsh, T. 2008. Gene expression in developing watermelon fruit. Biomed Central (BMC) Genomics. 9:275-282.

Last Modified: 2/23/2016
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