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United States Department of Agriculture

Agricultural Research Service

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Location: Vegetable Research

2010 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
The U.S. greenhouse tomato production increased significantly in recent years and is supplying nearly half of the demands for the U.S. fresh tomato markets. The intensive production and cultivation system and the favorable environmental conditions resulted in a series of unique disease problems, especially mechanically transmitted viral diseases. In addition to pepino mosaic which is now becoming an endemic disease problem in many greenhouse tomato facilities in the U.S., an emerging chlorotic stunting disease caused by several viroids was identified in 2009 in North America (Canada, Mexico and the U.S.). Our earlier studies have identified a source of resistance to Pepino mosaic virus (PepMV) in Solanum hybrochaites. In collaboration with a breeder from University of Florida, several breeding lines (F1 and F2) were developed, which are being used to study the inheritance of resistance. This project is now supported by an international firm through a cooperative research and development agreement (CRADA). In recent years, the U.S. watermelon industry has developed an interest in planting grafted watermelon. One of the promising rootstock for watermelon grafting is bottle gourd (Lagenaria siceraria). We have selected and further developed several bottle gourd lines that possess multiple virus resistance and also shown to be effective as rootstocks for watermelon grafting. A whitefly transmitted Sweet potato leaf curl virus (SPLCV) was shown to be widespread in South Carolina and caused serious yield loss (30-80%) on various sweetpotato cultivars. In collaboration with Alcorn State University, we have sequenced more than 10 isolates collected from Mississippi and South Carolina and discovered the presence of genetic diversity and genome recombination. Bacterial leaf blight, caused by a pathogenic bacterium, of vegetable Brassica causes millions of dollars in loss to growers in South Carolina, California, and other major Brassica leafy greens producing states. A source of disease resistance was identified in the national germplasm collections and breeding efforts are underway to study the inheritance of resistance and to incorporate the resistance into new cultivars. We have generated F1, F2 and backcross populations for such purposes. We have developed molecular-based markers for tracking and identifying the pathogens to determine if these bacteria are seed-borne and to quick diagnose the disease from field samples. We have identified a biocontrol bacterium that reduces leaf blight disease in greenhouse studies. These studies include optimizing application and timing of application for the best control using this biological control agent. A new system has been developed to look at the modulation of genes involved in root colonization from the nematode –killing biological control bacterium BG33R in a more “natural” soil environment compared to studies that use agar-based systems. We are currently looking at a subset of genes at different time-points in the colonization process. In addition, we have identified a unique compound that is involved in the nematode egg-kill activity in the biological control agent BG33R.

4. Accomplishments
1. Development of an efficient real-time polymerase chain reaction (PCR) system for sensitive detection of a panel of viruses (14) infecting crop plants. Serological method (Enzyme Linked ImmunoSorbent Assay or ELISA) is the most commonly used technique in plant virus detection. Real-time polymerase chain reaction (real-time PCR) is quickly gaining ground in plant virus detection due to its sensitivity. However, two major factors limit its practical application in routine virus detection, the slow sample processing and primer specificity. The immunocapture real-time PCR technology developed in the present study will allow us to efficiently process large number of samples for simultaneous for virus detection. Accordingly, the industry partner has developed various testing kits based on this technology and is now offering a new line of products in 2010. The success of this technology will allow us to provide U.S. growers timely and accurate information of the virus infection status in their crop plants. Thus the appropriate disease management measures may be deployed either to prevent the on-set of the diseases or to effectively manage such diseases.

Review Publications
Ling, K. 2010. Effectiveness of chemo- and thermo-therapeutic treatments on Pepino mosaic virus in tomato seed. Plant Disease. 94:325-328.

Ling, K., Harris-Shultz, K.R., Meyer, J.D., Levi, A., Guner, N., Wehner, T.C., Bendahmane, A., Havey, M.J. 2009. Non-synonymous single nucleotide polymorphisms in the watermelon eIF4E gene are closely associated with resistance to Zucchini yellow mosaic virus. Theoretical and Applied Genetics. 120:191-200.

Ling, K., Zhang, W. 2009. First report of a natural infection by Mexican papita viroid and tomato chlorotic dwarf viroid on greenhouse tomatoes in Mexico. Plant Disease. 93:1216.

Ling, K., Verhoeven, J.J., Singh, R.P., Brown, J.K. 2009. First Report of Tomato Chlorotic Dwarf Viroid in Greenhouse Tomatoes in Arizona. Plant Disease. 93:1075.

Wechter, W.P., Keinath, A.P., Farnham, M.W., Smith, J.P. 2010. First Report of Bacterial Leaf Blight on Broccoli and Cabbage Caused by Pseudomonas syringae pv. alisalensis in South Carolina. Plant Disease. 94:132.

Ling, K., Jackson, D.M., Harrison Jr, H.F., Simmons, A.M., Pesic-Valesbroeck, Z. 2010. Field evaluation of yield effects on the U.S.A. heirloom sweet potato cultivars infected by sweet potato leaf curl virus. Crop Protection. 29:757-765.

Ling, K., Bledsoe, M. 2009. First Report of Mexican Papita Viroid Infecting Greenhouse Tomato in Canada. Plant Disease. 93:839.

Last Modified: 2/23/2016
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