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Tracking down a notorious cereal killer
 Rice blast — a name that triggers alarm among rice farmers and rice scientists worldwide — is a widespread threat to food security, destroying over 150 million tons of cultivated rice each year, enough rice to feed 60 million people worldwide.
The disease is found in 85 countries and costs farmers a loss of nearly $5 billion a year. Not surprisingly, it accounts for the world’s largest fungicide market; the Japanese market alone for blast fungicides is estimated at US$400 million per year!
Now, rice blast has become quite well known in the world of plant science, after the publication in July 2002 of the draft genome sequence of the fungus Magnaporthe grisea that causes blast. Scientists considered the blast fungus and its plant host as a model system for understanding fungal-plant interactions and therefore, an ideal candidate for genome sequencing.
It is hoped that the availability of the M. grisea draft sequence on public databases will help scientists to have a more comprehensive understanding of the host-pathogen interaction and find ways that would enable rice plants to better resist the blast fungus, or hinder the ability of the fungus to infect rice plants.
In general, blast is controlled with fungicides or through the use of blast-resistant varieties. However, neither strategy has been very effective, because the blast fungus is increasingly becoming resistant to fungicides and has learned to assume more virulent forms to attack resistant rice varieties.
In fact, the blast fungus is so variable, especially under intensive, large-scale monoculture conditions, that it can turn resistant rice varieties vulnerable within 2 or 3 years.
Mapping blast in West and Central Africa
Although present in all rice ecosystems of West and Central Africa (WCA), rice blast is most important in upland and lowland rainfed systems. Many of the traditional upland rice varieties grown in WCA have been found to be sufficiently resistant to prevent massive outbreaks. A West African upland rice variety called Moroberekan, is considered to confer durable resistance to rice blast.
In WCA, rice blast causes significant losses in farmers’ fields mainly when farmers try to improve their traditional system by using higher-yielding but susceptible rice varieties.
Researchers from WARDA and their partners have found that an integrated pest management (IPM) approach, including the use of resistant rice varieties, cultural practices and rational chemical control, remains the best way to tackle blast in farmers’ fields in WCA.
“WARDA's research emphasizes the control of blast through stable varietal resistance,” said Dr Y Sere, WARDA Plant Pathologist. “Apart from routine breeding and selection for blast resistance, new breeding methods for durable blast resistance are being sought on the basis of a broad molecular and biological characterization of the blast pathogen's genetic diversity in West Africa.”
As part of a 4-year multi-institutional project comprising WARDA, the Horticultural Research International (HRI) in UK, and national agricultural research system (NARS) in Ghana, researchers have been studying the blast pathogen diversity in WCA.
The final outputs of this project funded by the Department for International Development, Crop Protection Program (DFID/CPP), were discussed at a workshop on Strategic Characterization of Blast Lineages and Tools for Long-term Monitoring by WARDA/NARS in March 2003. These outputs serve as important contributions to the global atlas of blast pathogen diversity.
A major output reported by the project is the establishment and characterization of a baseline collection of over 350 blast pathogen isolates from key screening sites and surrounding farms in Côte d’Ivoire, Ghana, Burkina Faso and Nigeria.
Common lineages (genetic groups) of these isolates were identified in the four countries, revealing nine distinct West African blast lineages. Nearly half of the 40 rice varieties provided by WARDA and NARS that are being screened under controlled conditions, have shown resistance to dominant representative blast lineages of West Africa.
WARDA researchers have been able to develop a strategy to identify rice material with durable resistance to blast, which can be adopted directly by farmers and also used in breeding programs.
Building on the project outputs, the following activities are planned:
• The promotion of a molecular tool kit for monitoring blast pathogen diversity in WCA through the training of NARS scientists
• Characterization of blast pathogen populations in key countries using molecular tools at WARDA
• Use of characterized populations/sites and host resistance sources to achieve durable blast resistance in
WCA.
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