One of the most fearsome and intractable wheat diseases in recent decades is wheat blast. First sighted in Brazil in 1985, blast is widespread in South American wheat fields, affecting as much as 3 million hectares [about 7.4 million acres] in the early 1990s and seriously limiting the potential for wheat cropping on the region's vast savannas. Currently, most [wheat] varieties being planted are susceptible and fungicides have not been effective in controlling the disease.
Experts had feared 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 confirmed the truth of these predictions. The consequences of a wider outbreak in South Asia could be devastating to a region of 300 million people who consume over 100 million tons of wheat each year.
For more detail regarding wheat blast disease, suggested control measures, and links to selected scientific literature, click here.
Wheat blast (or "brusone" in South America) is caused by the fungus _Pyricularia oryzae_ (synonym _Magnaporthe oryzae_, previously classified as a strain of _M. grisea_). Although the fungus is currently classified as the same species as the one that causes rice blast, the wheat blast pathogen is a distinct population (referred as _P. oryzae_ Triticum population) and does not cause disease in rice.
Over 50 species of grasses and sedges can be affected by related fungal strains which appear to be highly variable favouring the emergence of new strains. Further work is needed regarding genotypic differentiation related to host range, including differences between the wheat and rice pathovars. Wheat blast is now considered an emerging disease and a threat to global food security.
Blast symptoms on wheat (and barley) may be confused with fusarium head blight (see previous ProMED-mail posts in the archives and link below) and include bleaching of ears, shriveled kernels, and no seed production at all for severe infections. Yield losses seem to average 40 to 50 per cent, but cases of 100 per cent losses have also been reported, 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 material (including seed), mechanical means (including insect activity), water and wind, and it is likely that the wheat pathogen is spread in similar ways. Current chemicals appear not to be very effective against the fungus. Available wheat cultivars lack resistance to wheat blast and only limited tolerance can be found. Resistance breeding programmes are being established in the Americas. More information is needed to forecast epidemics and protect other regions and cropping systems from infection.
Rice blast is one of the most destructive diseases of rice worldwide.
Initially, wheat blast was thought to be caused by a fungal strain which crossed from rice to wheat, but it is now considered more likely to originate from local wild grasses. For example, wheat blast strains reported in the US (ProMED-mail post http://promedmail.org/post/20120430.1117921) were found to be genetically much more similar to a regional pathovar causing grey leaf spot of ryegrass than to wheat blast from South America. This would suggest that genetic events leading to a change in host range from grasses to wheat have occurred separately in the different locations and, therefore, may potentially occur again in other regions.
Sequencing studies will be required to clarify a possible origin of wheat blast strains in South Asia.