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United States Department of Agriculture

Agricultural Research Service

Research Project: GENETIC AND BIOLOGICALLY-BASED MANAGEMENT OF VEGETABLE CROP DISEASES
2011 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
This research relates to in house objective 1: Determine the effect of production location, maturity, curing, seed moisture, composition, processing and relevant interactions to improve flavor and flavor consistency in peanut varieties and breeding lines.

Several emerging diseases have caused serious concerns for the $400 million U.S. greenhouse tomato industry. In addition to the continuing epidemic by the Pepino mosaic virus infection, the emerging viroid disease caused by several pospiviroids and a bacterial disease caused by ‘Candidatus Liberibacter solanacearum’ have been identified in various greenhouses in the U.S. and Mexico. With the identification of the causal agents in the disease complex, the appropriate disease management strategies may be developed for their control. Such efforts have been greatly enhanced this year with additional funding support for a postdoctoral research associate from a USDA-specialty crop research initiative project, in collaboration with industrial and academic partnerships led by the Ohio State University. With an increasing trend in the internationalization of seed production and trading, accurate, sensitive and timely detections of seed-borne pathogens are very important steps in maintaining seed quality control. Significant progress have been made in the development of sensitive seed health assays based on real-time Polymerase Chain Reaction (PCR) technology for a reliable detection of two important seed borne pathogens: Squash mosaic virus and Didymella bryoniae (the causal agent of gummy stem blight of cucurbits). In the study to determine the experimental host range and natural reservoir for Sweet potato leaf curl virus, we showed that this virus is generally limited to Ipomoea species in nature. In collaboration with Alcorn State University, we were able to identify a new begomovirus in sweetpotato which is derived from a natural recombination between two viruses, Sweet potato leaf curl virus and Sweet potato leaf curl Georgia virus. Bacterial fruit blotch, caused by Acidovorax avenae subsp. citrulli, is a serious seed-borne disease that threatens most cucurbit crops. Although, limited resistance has been found in a small number of Plant Introductions (PI) in watermelon, there are no reports of high levels of resistance in germplasm lines of cantaloupe. We screened 332 cantaloupe PI for resistance to bacterial fruit blotch using a newly developed seed vacuum-infusion assay. Although, the majority of lines were found to be extremely susceptible to the disease, we identified several PI with moderate levels of resistance. A total of 16 PIs were selected for additional evaluation. Five of these were found to have significantly greater levels of resistance than susceptible control cultivars or other PI in two independent spray inoculation tests. Germplasm lines developed from these PI may be useful as sources of resistance to bacterial fruit blotch in cantaloupe breeding programs. In collaboration with others, a new gene-based random amplification system (High-frequency oligonucleotides PCR) for identifying genetic polymorphism in watermelon was developed.


4.Accomplishments
1. Molecular characterization, insect transmission and host range of Sweet potato leaf curl virus. In the study to determine the experimental host range and natural reservoir for Sweet potato leaf curl virus, we showed that this virus is generally limited to Ipomoea species in nature. In collaboration with Alcorn State University, we were able to identify a new begomovirus in sweetpotato. We determined this new virus is derived from a natural recombination between the two originally recognized viruses, Sweet potato leaf curl virus and Sweet potato leaf curl Georgia virus. The greater genetic diversity of sweetpotato begomoviruses has caused greater concerns for the sweetpotato industry and the scientific community, as it will add complexity to achieve accurate virus detection as well as in breeding for durable disease resistance. In our previous field studies, we showed that this disease can result in 20-80% yield reductions to various U.S. sweetpotato cultivars making it a significant problem for growers.


Review Publications
Ling, K., Wechter, W.P., Somai, B.M., Walcott, R.R., Keinath, A.P. 2010. An Improved Real-Time PCR System for Broad-Spectrum Detection of Didymella bryoniae, the Causal Agent of Gummy Stem Blight of Cucurbits. Seed Science and Technology. 38:692-703

Ling, K., Sfetcu, D. 2010. First report of natural infection of greenhouse tomatoes by potato spindle tuber viroid in the United States. Plant Disease. 94(11):1376.

Ling, K., Lin, H., Lewis Ivey, M.L., Zhang, W., Miller, S. 2011. First Report of 'Candidatus Liberibacter Solanacearum' Naturally Infecting Tomatoes in the State of Mexico, Mexico. Plant Disease. 95:1026.

Ling, K., Zhang, W. 2011. First Report of Pepino Mosaic Virus Infecting Tomato in Mexico. Plant Disease. 95:1035.

Ling, K., Harrison Jr, H.F., Simmons, A.M., Zhang, S., Jackson, D.M. 2011. Experimental Host Range and Natural Reservoir of Sweet Potato Leaf Curl Virus in the United States. Crop Protection. 30:1055-1062.

Zhang, S., Ling, K. 2011. Genetic diversity of Sweet potato begomoviruses in the United States and identification of a natural recombinant between Sweet potato leaf curl virus and Sweet potato leaf curl Georgia virus. Archives of Virology. 156(6):955-968.

Wechter, W.P., Levi, A., Ling, K., Kousik, C.S., Block, C.C. 2011. Identification of resistance to Acidovorax avenae subsp. citrulli among melon (Cucumis spp.) Plant Introductions. HortScience. 46(2):207-212.

Last Modified: 10/21/2014
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