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Using Biotechnology Tools to Battle Deadly Tomato Virus Diseases in Mali

The Tomato Magazine
October 2007

Severe diseases, caused by viruses transmitted by whitefl ies, are emerging as a major constraint to the production of tomatoes, peppers, beans, cotton and cucurbits in developing and less developed
countries. Plants infected with these viruses show symptoms including severely stunted growth; twisted, curled and discolored leaves; and greatly reduced yields.

Dr. Robert Gilbertson, a plant pathologist at the University of California-Davis, is part of a team put together to address a viral disease epidemic in tomatoes in Mali and other West African countries (e.g., Benin, Burkina Faso, Ghana, Niger, Senegal and Togo). The long-term goal of this project is to develop strategies to increase regional tomato production, which has been greatly reduced by
these diseases.

USAID missions in Mali and West Africa have partnered with two USAID Washington projects, IPM-CRSP and ABSP (Application of Biotechnology Support Project) II, and scientists from the Asian Vegetable Research Development Center (AVRDC), the Institut D’Economie Rurale (IER-Mali), and the U.S. universities U.C. Davis, Cornell, Purdue and Virginia Tech.

Three Major Goals
Gilbertson and others on the team have been working to accomplish three goals: (1) identify the specifi c viruses involved; (2) identify improved tomato varieties resistant or tolerant to the viruses and horticulturally acceptable to West African farmers and consumers; and (3) develop and initiate an overall program for tomato production that includes integrated management of the tomato virus diseases.

In their search to identify the viruses involved with the disease, it was established that most of the samples were infected with one or more whitefl y-transmitted geminiviruses (begomoviruses), consistent
with the presence of whiteflies in the areas, Gilbertson noted. Further testing identified three new whitefl y-transmitted begomovirus species. These were named after the host plant they were first detected in, the predominant disease symptom type and/or the country in which the fi rst identification was made (Mali). The names of these viruses are: Tomato leaf curl Mali virus (ToLCMLV), Tomato yellow leaf curl Mali virus (TYLCMLV) and Tomato yellow leaf crumple virus (ToYLCrV).

As each of these had not been previously identified in the literature, their sequences were deposited in the GenBank and are available to the scientific community, Gilbertson reported. The cloned DNA of these viruses and their genetic sequences were then used to develop additional biotechnology tools, specific to these viruses, for studying their distribution and biology (e.g., types of plants infected).
ToLCMLV and ToYLCrV proved to be the most prevalent and were detected in diseased tomatoes throughout West Africa, Gilbertson observed. Furthermore, the viruses were detected primarily in tomatoes and peppers but not in weeds and other crops, including the widespread African eggplant and okra.

The researchers also put together a test (based upon the highly sensitive and specific PCR method) that will allow detection of these viruses in whiteflies. The next step for the team was to select a
test area (Baguineda, Mali) to evaluate various IPM strategies to combat the viruses. They patterned their approach after a successful IPM program implemented in the Dominican Republic for managing Tomato yellow leaf curl virus.

Program Basics
Keys to the program, Gilbertson said, are: (1) a two-month host (tomato/pepper)-free period imposed area wide in Baguineda with the help of local extension personnel; (2) extensive sanitation efforts to remove and destroy tomato and pepper plants after harvest; (3) screening and evaluation of new improved varieties; and (4) monthly monitoring of whitefl y populations and virus incidence to understand when the disease is most severe as well as the impact of these practices on the target problem.

The whitefly monitoring and collection, viral disease evaluation, local varietal screening and assessment of the implementation of the host-free period are being conducted as part of the Ph.D. program of Moussa Noussourou at the University of Bamako. The PCR detection of geminiviruses in whiteflies is being carried out by Gilbertson and other scientists at U.C. Davis.

In addition to the screening of varieties in small test plots, seeds of selected early maturing disease tolerant hybrid varieties were donated by Campbell and Heinz seed companies for Baguineda farmers to plant as part of the overall program to increase tomato production in the area. The months selected for the host-free period were July and August, and a vigorous education program was initiated to inform farmers why this was needed.

Surveys conducted during this period, over the past three growing seasons, revealed little tomato or pepper production during the host-free period and significant reductions in whitefly populations, Gilbertson said. It was likely that this drop was due to the host-free period and the rainy weather conditions during these months.

In September of 2004 and 2005, immediately after the end of the host-free period, farmers were provided with seeds of the improved varieties for planting. For various reasons, however, the team found it difficult to coordinate farmers’ planting dates. Many waited until October and November to plant. However, by 2006, a smaller number of farmers were involved, allowing for better coordination of planting dates.

In 2004 and 2005, viral diseases increased faster than had been hoped, Gilbertson explained. This was due, in part, to what the surveys of whiteflies and virus incidence revealed. There was a rapid
increase in whitefl ies and virus in whiteflies during the latter part of October and November. However, in the fall of 2006, when tomatoes were planted earlier, the incidence of virus was considerably lower. The good news, the researcher added, was that despite these conditions, the early maturing hybrid varieties still yielded much more than the traditional open-pollinated varieties, Roma and UC 82, in each year of the study. This indicated the potential for this program to work, he said. In addition, a number of varieties with promising levels of resistance were identified in the small plot variety trials conducted in seven West African countries in collaboration with national partners in each country.

Whitefly Population Peak Dates
During their area-wide survey in Baguineda, the scientists were able to document that whitefly populations on tomato crops peaked in November and December and then noticeably declined by the end of December. This resulted in lower whitefly and virus pressure on the second season tomato crop planted from the end of December and early January.

Yields from early maturing obtained during this growing season (January-May) have been very good, Gilbertson said. In fact, in 2006 the tomato production generated with these varieties was the highest in 15 years. The fruit of these varieties also found local market acceptance, Gilbertson reported. It had both higher quality and longer shelf life and commanded a better price.

Local Baguineda farmers have recognized the revival in tomato production is due to imposing the host-free period (cleaning out the virus from the area) and the use of the new varieties. They also have indicated their willingness to continue to participate in the host-free period and the intensive sanitation program.

“This project represents the successful application of biotechnology tools for the development of a sustainable IPM project for a devastating plant disease,” Gilbertson said. “While still in development, this project has the potential to result in a significant increase in tomato production in Mali, and perhaps even facilitate the reopening of the cannery in Baguineda, which closed in part due to losses caused by the virus diseases. With the implementation of the regional ABSP II project aimed at examining this problem throughout West Africa, there is potential for increased tomato production on
a regional level.

Long-term Benefits
“This project also has a number of long-term benefi ts for Mali,” Gilbertson continued. “First, our efforts should lead to the strengthening of disease diagnostics and biotechnology capabilities through training and establishment of an IER biotechnology laboratory. This should fi t in very well with efforts by the IPM-CRSP Global Theme Project on establishment of an International Plan Diagnostic Network (IPDN).

“Second, the successful introduction/evaluation of new improved hybrid tomato varieties will facilitate an examination of seed distribution networks, particularly as farmers seek out seeds of these varieties. It is our experience that farmers in Mali are willing to purchase seed if the yield potential can be established. The challenge will be to identify a means to make the seeds commercially
available.

“Third, the use of the new seeds will bring new technologies to improve tomato production (e.g., drip irrigation), and a handbook of tomato cultivation practices is being planned which will recommend using seeds of improved varieties.

“Finally, it is hoped that this type of collaborative project between USAID, host country agricultural research scientists and scientists from U.S. universities can serve as a model for addressing other crop production problems in Africa. In the case of tomatoes, it is clear that the devastating bacterial wilt disease, caused by Ralstonia solanacearum, is looming as the next potential epidemic disease threatening this important cash crop for African farmers,” Gilbertson said.

© 2007 Columbia Publishing

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