Wheat blast is one of the most fearsome and intractable wheat diseases in recent decades. It is caused by the fungus Magnaporthe oryzae .
- Blast directly strikes the wheat ear and can shrivel and deform the grain in less than a week from first symptoms, leaving farmers no time to act.
- Fungicides typically provide only a partial defense. They are also often hard to obtain or use in the regions where blast occurs, and must be applied well before any symptoms appear—a prohibitive expense for many farmers.
- Blast appears sporadically on wheat and grows well on numerous other plants and crops, so rotations do not control it. The irregular frequency of outbreaks also makes it hard to understand or predict the precise conditions for disease development, or to methodically select resistant wheat lines.
- The fungus is physiologically and genetically complex, so even after more than three decades, scientists do not fully understand how it interacts with wheat or which genes in wheat confer durable resistance.
First sighted in Brazil in 1985, blast is widespread in South American wheat fields, affecting as much as 3 million hectares in the early 1990s and seriously limiting the potential for wheat cropping on the region’s vast savannas. Experts had foretold the possible spread of blast from Latin America to regions of Africa and Asia where conditions are similar. A severe outbreak of blast in key wheat districts of southwestern Bangladesh in early 2016 has validated that prediction. The spread of wheat blast could be devastating to South Asia, which is home to 300 million undernourished people and whose inhabitants consume over 100 million tons of wheat each year.
The pathogen can be spread by seed and also survives on crop residues. Most wheat varieties currently grown are susceptible to blast and fungicides have not been effective in controlling the disease.
The origin of the Bangladesh wheat blast isolate is being studied to determine if it infects only wheat or other grasses as well, including rice. The M. oryzae genome is well-studied but major gaps remain in knowledge about its epidemiology.
Wheat blast requires concurrent heat and humidity to develop and is presently confined to areas that feature those conditions. But there are many examples of crop fungi that evolve and adapt to new conditions. For example, yellow rust of wheat was historically endemic mainly in colder regions but has quickly adapted to warmer climates and now poses a major threat for wheat crops in regions where it was unknown only a few decades ago. If blast shows a similar capacity, it could spread to other hot and humid wheat-growing regions in South Asia and beyond, including China, which would threaten the food security and livelihoods of hundreds of millions of disadvantaged farmers and consumers.
Photo: Kansas State University
Wheat blast disease can be controlled through a combination of the following measures:
- Improved wheat varieties that carry genetic resistance to M. oryzae.
- Global monitoring of disease appearances, movement, and evolution, in coordination with local governments and research agencies, as well as predictive models.
- Advanced studies on potentially effective, safe, and affordable chemical control measures.
- Genetic and epidemiological research to strengthen knowledge of the fungus and its interactions with wheat and other host plants.
Thirty years of research in Latin America has resulted in some wheat cultivars that are tolerant to blast, but little tolerance has been observed in currently-grown Bangladeshi wheat varieties. With the arrival of wheat blast in South Asia, wheat researchers are challenged with new, high-priority tasks: to identify sources of resistance and develop resistant varieties, to elucidate the epidemiology of the disease, and to find optimal control practices.
To put these integrated solutions into practice, in 2011 CIMMYT led the launch of a global Wheat Blast Consortium comprising 13 institutions from Europe and the Americas.
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Blast can infect many grasses, including rice, wheat, barley, lolium, but specific isolates of Magnaporthe oryzae generally infect only a single species; that is, wheat isolates infect only wheat.
CIMMYT Global Wheat Program
CGIAR Research Program WHEAT
h.braun @ cgiar.org
To access a print pdf briefing on wheat blast, click here.