Wheat blast disease: danger on the move

Abstract

Wheat blast is caused by the fungus Magnaporthe oryzae Triticum pathotype (MoT). The potential for wheat blast to cause widespread losses demands immediate action to understand and manage this explosive disease. The recent appearance of wheat blast in Bangladesh demonstrates the threat of global spread, which could occur via the movement of infected seed or grain. MoT mainly infects wheat heads, with symptoms closely resembling Fusarium head blight. To date, wheat blast is considered an intractable and dangerous disease and fungicides have shown limited efficacy. Disease management requires identification of new resistance sources and a complete understanding of MoT ecology and wheat blast epidemiology. Understanding the full potential for pathogen variability, including any role for sexual reproduction in the field, is critical. A small number of pathogen avirulence (AVR) genes block other host-adapted M. oryzae pathotypes from infecting wheat; so potential AVR gene mutations leading to new host jumps remain a threat. Indeed, some strains of the closely related Lolium pathotype, causing gray leaf spot of turf grasses, already infect wheat. This review provides the current status of wheat blast research and disease control strategies indicating similarities and differences to rice blast and gray leaf spot. Critical knowledge gaps are discussed.

Gene Flow between Divergent Cereal- and Grass-Specific Lineages of the Rice Blast Fungus Magnaporthe oryzae

“…Additionally, host species specificity is not strictly maintained. Under controlled conditions, most lineages have at least one host in common (40), and strains within one lineage can still cause rare susceptible lesions on naive hosts (21,47). Moreover, a single plant infected by a single genotype can produce large numbers of spores in a single growing season (48), allowing the pathogen to persist on an alternative host even if selection is strong and promoting the rapid and repeated creation of genetic variation (6).…”

“…Therefore, understanding the evolutionary origin and structure of populations causing wheat blast is a top priority for researchers studying disease emergence and for regulatory agencies. Wheat blast was first discovered in southern Brazil in 1985 (16), and the disease subsequently spread to the neighboring countries of Argentina, Bolivia, and Paraguay (17)(18)(19), where it represents a consider-able impediment to wheat production (20,21). Until recently, wheat blast had not been reported outside South America.…”

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

“…Because wheat blast has now escaped from its endemic areas in South America and is causing epidemics in South‐East Asia, the primary global concern is to prevent additional spread of the pathogen to disease‐free countries and to avoid potential outbreaks in new regions. As infected seeds can spread the pathogen over long distances (Cruz and Valent, ; Goulart and Paiva, ; Silva et al ., ; Urashima et al ., ), the strengthening of quarantine and seed trading laws will provide the best course of action to prevent the further spread of wheat blast (Mezzalama, ; Sadat and Choi, ; Singh, ; Valent et al ., ). Trade in wheat seeds from the wheat blast endemic areas in Latin America and South‐East Asia should be strictly regulated, if not totally prohibited.…”

“…Because wheat blast can be seed‐borne and seed‐transmitted (Goulart and Paiva, ; Martins et al ., ), infected seeds are thought to be the primary source of inoculum for long‐distance dispersal (Cruz and Valent, ; Goulart and Paiva, ; Silva et al ., ; Urashima et al ., , ). Spores of Pygt are able to survive and remain infectious for up to 2 years on both the surface and inside seeds.…”

Pyricularia graminis‐tritici is not the correct species name for the wheat blast fungus: response to Ceresini et al. (MPP 20:2)

“…In laboratory studies, adapted fungal strains rapidly kill primary host plants because nearly all penetration sites result in large coalescing lesions. In contrast, the fungus causes fewer, often smaller, lesions on secondary, non‐adapted hosts because only a few penetration sites succeed and colonization is reduced at successful sites (Cruz and Valent, ). Indeed, Castroagudin et al .…”