New disease wheat blast on wheat and barley found in South America
May. 24, 2010
Wheat blast or brusone is a new wheat disease. It is responsible for 5 to 100 percent of wheat yield loss in regions of South America and has the potential to spread. It was identified for the 1st time in 1985 in southern Brazil, from where it quickly spread to neighboring countries. In 1989 it caused serious damage in the wheat fields of Paraguay. In the lowlands of Bolivia, it was responsible for a loss of 90 000 hectares [222 000 acres] of wheat between 1997 and 2000. In 2007, the disease was seen in summer-sown experimental wheat trials in Chaco, Argentina, and although researchers in Uruguay have not observed the disease in wheat, they have found the fungus on barley. A 2009 outbreak cut Brazilian wheat production by up to 30 percent.
Of great concern is that chemical control of wheat blast may not be working. To date, there is a lack of cultivars resistant to wheat blast, and only limited tolerance can be found. Climate change is adding to the problem. Wheat blast needs high temperatures of about 24 to 28 deg C [75.2-82.4 deg F] and long periods of rain. Changes in rainfall may create conditions favorable to wheat blast in other parts of the world such as South Asia or Africa.
What is wheat blast?
Wheat blast is an emerging disease in South America caused by a fungus closely related to the rice blast pathogen Magnaporthe grisea. Initially, it was thought to be a fungal strain, which crossed from rice to wheat, but it is now considered more likely to be a different species originating from local wild grasses. A re-classification of species has been suggested with the wheat pathogen to be named M. grisea and the new name M. oryzae assigned to the rice blast fungus. Wheat blast is now considered a new disease caused by a distinctly different pathogen and sequencing studies are being carried out to determine exact genetic differences between the wheat and rice fungi.
Blast symptoms on wheat and barley include bleaching of ears, shrivelled kernels, and no seed production at all for severe infections. Yield losses are reported to average 40 to 50 percent and wheat production in some affected areas has ceased. Humid and warm conditions favour disease development, but the life cycle of the fungus is still unknown. Spread of the rice pathogen occurs with infected plant debris, mechanical means (including insect activity), water, and wind, and it is likely that the wheat pathogen is spread in similar ways. Although wheat blast can be controlled with fungicides, correct application requires training and is often too expensive for smallholders in developing countries. Resistant cultivars are not yet available, but breeding programmes are being established in the Americas.
More than 50 species of grasses and sedges can be affected by these related fungi. Furthermore, they appear to be highly variable and this favours the emergence of new strains with increased virulence and extended host range. Since rice blast is one of the most destructive diseases of rice worldwide, potential epidemics of wheat blast in subtropical regions of Southeast Asia, Africa, or the Mediterranean are considered a threat to global food security.
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