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United States Department of Agriculture

Agricultural Research Service

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Location: Vegetable Research

2009 Annual Report

1a. Objectives (from AD-416)
Objective 1: Elucidate the etiology and epidemiology of Pepino mosaic virus on tomato, Pseudomonas syringae pv. maculicola on vegetable Brassicas, and vine decline pathogens on cucurbits to identify vulnerable areas 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
Greenhouse tomato production in the U.S. has increased significantly in recent years. With an estimate value of $400 million, it has captured nearly 46% of the U.S. fresh tomato market shares. Pepino mosaic virus (PepMV) is an emerging disease in greenhouse tomato. Previously we have identified a resistance source to PepMV in Solanum hybrochaites. In collaboration with a breeder from University of Florida, breeding materials (F1 and F2 lines) were developed and are being used to study the inheritance of resistance. In another study, we also determined the presence of genetic diversity of PepMV in North America. It was shown that PepMV is seed-borne and the virus can be transmitted to seedlings through handling of the contaminated seeds. Various means of seed treatments, including chemo-or thermo-therapy, were shown to be effective to deactivate the virus infectivity. Watermelon production is seriously affected by a number of viruses in watermelon growing areas in the U.S. However, current watermelon cultivars are susceptible to Zucchini yellow mosaic virus (ZYMV) infection. Using a candidate gene approach, a molecular marker closely linked to ZYMV resistance was identified and is being used in marker-assisted selection. Sweetpotato is another important vegetable crop in several southern states. A whitefly transmitted Sweet potato leaf curl virus (SPLCV) was shown to be widespread in some areas and caused serious damage. Field trials showed that SPLCV could cause 30-80% yield loss in various sweetpotato cultivars and meristem tip culture is capable of eliminating the virus. In a host range study, SPLCV is capable of infecting numerous morning-glories, which could potentially serve as virus reservoirs. Bacterial leaf blight of vegetable Brassica causes millions of dollars in loss to growers in South Carolina. It was determined that this disease is caused by two unique bacteria. A source of disease resistance was identified in the national germplasm collections and breeding efforts are underway to generate populations for the study of inheritance of resistance and to incorporate the resistance into more acceptable lines of vegetable Brassica. Field trials were set up to test the new resistant germplasm in combination with commercial plant growth promoters and resistance activators to develop an integrated approach to control the disease. Because little is known about the behavior of these bacteria, we have developed molecular-based markers for tracking and identifying the pathogens. These markers are being used to determine if the pathogens are seed-borne and how they are being spread to other fields. We also have identified two other bacteria that appear to have biocontrol properties in inhibiting the growth of both of the pathogens involved in the leaf blight of Brassica. Bacterial fruit blotch of watermelon and melon has caused devastating crop losses over the past 20 years. There are no pesticides available to control this disease. We have screened 400 accessions of melon germplasm and identified a number of lines with resistance. We are confirming these results and selecting for the most resistant lines.

4. Accomplishments
1. Development of an effective seed treatment system for an emerging seed-borne virus (Pepino mosaic virus) on tomato. With increasing global seed trade activities, the seed-borne nature of Pepino mosaic virus allowed it to become widespread around the world. In this study, it was determined that the Pepino mosaic virus is a seed-borne pathogen and its virus particles are located on the surface of the seed coat. Thus, the virus is sensitive to various seed treatments with chemo- and thermo-therapies. Deactivating the virus infectivity could potentially save seed company millions of dollars each year. Planting tested virus-free seed should be considered as a first priority in the management of this seed-borne disease.

2. Development of a molecular marker associated with Zucchini yellow mosaic virus resistance in watermelon. Zucchini yellow mosaic virus is an important virus on cucurbits, including watermelon. The source of virus resistance has been identified, but selection for plants with disease resistance is complicated due to the recessive nature of the resistance. In this study, we identified a potential virus-resistance gene in watermelon and developed a molecular marker that could be used in marker-assisted selection by breeders. Currently, an ARS collaborator is using this technology to select virus-resistant plants in advanced breeding materials. This technology will enable seed company breeders to apply marker assisted selection in developing new watermelon cultivars. The U.S. watermelon growers will be benefited from growing virus-resistant watermelon.

Review Publications
Ling, K. 2008. Pepino Mosaic Virus on Tomato Seed: Virus Location and Mechanical Transmission. Plant Disease. 92:1701-1705.

Ling, K., Wintermantel, W.M., Bledsoe, M. 2008. Genetic Composition of Pepino mosaic virus Population in North American Greenhouse Tomatoes. Plant Disease. 92:1683-1688.

El Salamouny, S., Shapiro, M., Ling, K., Shepard, B.M. 2009. Black Tea and Lignin as Ultraviolet Protectants for the Beet Armyworm Nucleopolyhedrovirus. Journal of Entomological Science. 44(1):50-58

Ha, Y., Fessehaie, A., Ling, K., Wechter, W.P., Keinath, A.P., Walcott, R.R. 2009. Simultaneous Detection of Acidovorax Avenae Subsp. Citrulli and Didymella Bryoniae in Cucurbit Seedlots Using Magnetic Capture Hybridization and Real-Time Polymerase Chain Reaction. Phytopathology. 99:666-678.

Last Modified: 2/23/2016
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