Wheat Blast: Past, Present, and Future

Abstract

The devastating wheat blast disease first emerged in Brazil in 1985. The disease was restricted to South America until 2016, when a series of grain imports from Brazil led to a wheat blast outbreak in Bangladesh. Wheat blast is caused by Pyricularia graminis-tritici ( Pygt), a species genetically distinct from the Pyricularia oryzae species that causes rice blast. Pygt has high genetic and phenotypic diversity and a broad host range that enables it to move back and forth between wheat and other grass hosts. Recombination is thought to occur mainly on the other grass hosts, giving rise to the highly diverse Pygt population observed in wheat fields. This review brings together past and current knowledge about the history, etiology, epidemiology, physiology, and genetics of wheat blast and discusses the future need for integrated management strategies. The most urgent current need is to strengthen quarantine and biosafety regulations to avoid additional spread of the pathogen to disease-free countries. International breeding efforts will be needed to develop wheat varieties with more durable resistance.

Wheat blast: from its origins in South America to its emergence as a global threat

“…The recognition of Pygt as a distinct species has important implications for quarantine regulations worldwide (Castroagudín et al ., ). A lack of recognition that the formerly geographically restricted wheat blast pathogen Pygt was not the same species as the globally distributed rice blast pathogen P. oryzae may explain in part the lack of phytosanitation screens that led to the introduction of Pygt into Bangladesh (Government of Brazil, ; Ceresini et al ., ). It is possible that local officials assumed that the wheat blast pathogen Pygt was the same species as the rice blast pathogen P. oryzae , which was already a well‐established pathogen in Bangladesh (Government of the People’s Republic of Bangladesh, , ; Khan et al ., ).…”

“…One of the important findings on the population genetics of Pygt was the discovery of a super‐race, named virulence group A, which caused blast on the entire tested panel of wheat and barley cultivars (Castroagudín et al ., ; Ceresini et al ., ). Virulence group A was not a clone; this phenotype was found in many different Pygt genetic backgrounds.…”

“…This suggests that the virulence group A phenotype may represent the loss of a major avirulence gene that is now segregating in many different genetic backgrounds as a result of recombination. It was detected at a higher frequency on Brazilian wheat, but was also found on several grass species invading wheat fields, including Avena sativa , Cenchrus echinatus , Chloris distichophylla , Cynodon spp., Echinochloa crusgalli , Digitaria insularis , D. sanguinalis , Eleusine indica , Eragrostis plana , Panicum maximum , Rhynchelytrum repens , Sorghum sudanense and U. brizantha (Castroagudín et al ., ; Ceresini et al ., ). Other common virulence groups were also shared between the grass‐ and wheat‐infecting Pygt populations, providing additional evidence for the movement of Pygt between wheat fields and nearby grasses.…”

“…In Brazil, wheat seed fungicide treatments were mandated by law (Mehta, ), but, since the 1970s, the legislation has been relaxed and seed health testing has been neglected (Mehta, ). This decreased vigilance facilitated the dispersal of seed‐borne Pygt inoculum across Brazil’s wheat cropping areas, as well as into Bolivia and Paraguay, through frequent imports of Brazilian wheat seed used for sowing (Ceresini et al ., ).…”

“…When used in a crop rotation, wheat decreases pathogen inoculum for other crops in the rotation, including the soybean crop that is vital for Brazil’s export economy. The Brazilian Cerrado is closer to the major cities located in central and northern Brazil, resulting in significantly lower transportation costs and making wheat produced in the Cerrado more economically competitive than imported wheat, especially from Argentina (Ceresini et al ., ; Maciel, ). However, the fear of yield losses caused by wheat blast leads many growers to avoid cropping tropical wheat (Ceresini et al ., ; Maciel, ).…”