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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