First Report of Wheat Blast Caused by Magnaporthe oryzae Pathotype triticum in Bangladesh

Abstract

Wheat blast or ‘brusone,’ caused by the ascomycetous fungus Magnaporthe oryzae B.C. Couch (syn. Pyricularia oryzae Cavara), was first identified in 1985 in Brazil. M. oryzae is composed of a range of morphologically identical but genetically different host-specific pathotypes that are specialized for infecting rice (Oryza pathotype), wheat (Triticum pathotype - MoT), perennial and annual ryegrass (Lolium pathotype), foxtail millet (Setaria pathotype), and many other graminaceous hosts. Isolates from different hosts are genetically distinct, although cross infection occurs to some extent. Wheat blast has become a serious biotic constraint to wheat (Triticum aestivum L.) production in parts of the warmer wheat growing areas of the Southern Cone region of South America, causing yield losses of 10 to 100% in recent years (Duveiller et al. 2016). Wheat blast was observed for the first time outside of South America during the 2015-16 cropping season in the districts of Kushtia, Meherpur, Chuadanga, Jhenaidah, Jessore, Barisal, Bhola, and several other districts in the south of Bangladesh. Infected plants showed the typical wheat blast symptoms with the spike becoming partially or completely bleached with the blackening of the rachis in a short span of time. Examination of diseased plants showed the presence of elliptical, grayish to tan necrotic lesions with dark borders on the leaf often mixed with spot blotch disease lesions. Additionally, in some fields, blackening of lower nodes was observed. Grains from blast-infected heads were small, shriveled, deformed, and had low test-weight leading to serious yield losses. Diseased plants were collected and brought to the Wheat Research Centre, Bangladesh Agricultural Research Institute, Dinajpur, for further investigation. Grayish mycelium of the fungus taken from the infection point on the rachis of several independent spikes was observed. Incubation of several infected spikes/leaves in a 3-layered moist blotter at room temperature less than 12 h light/darkness cycle for 5 days led to the production of pyriform conidia. Morphobiometrical characteristics of the typical pyriform (pear-shaped) and 2-septate hyaline conidia were in agreement with the identification of the fungus as M. oryzae (Subramanian 1968). The air-dried spike samples have been kept in cold storage and several infected spikes were sent to the USDA-ARS, FDWSRU laboratory in the United States for characterization of the pathogen. There, the presence of M. oryzae in the infected samples was confirmed based on morphobiometrical analysis, and strains were preserved in the FDWSRU permanent wheat blast strain collection. Molecular analysis with MoT-specific markers and comparative genome analysis of isolates (BdBar16-1, GenBank accession no. LXON01000000; BdJes16-1, LXOO01000000; BdMeh16-1, LXOP01000000) confirmed that the wheat blast observed in Bangladesh is caused by MoT pathotype and has strong genetic identity to a strain from South America (B71, LXOQ01000000). This first incidence of wheat blast was significantly widespread accounting for approximately 15% of Bangladesh’s total wheat area. This large scale incidence of wheat blast has underscored a concern about the potential spread of wheat blast to other wheat producing areas in Bangladesh, South Asia, and beyond.

Wheat blast or ‘brusone,’ caused by the ascomycetous fungus Magnaporthe oryzae B.C. Couch (syn. Pyricularia oryzae Cavara), was first identified in 1985 in Brazil. M. oryzae is composed of a range of morphologically identical but genetically different host-specific pathotypes that are specialized for infecting rice (Oryza pathotype), wheat (Triticum pathotype - MoT), perennial and annual ryegrass (Lolium pathotype), foxtail millet (Setaria pathotype), and many other graminaceous hosts. Isolates from different hosts are genetically distinct, although cross infection occurs to some extent. Wheat blast has become a serious biotic constraint to wheat (Triticum aestivum L.) production in parts of the warmer wheat growing areas of the Southern Cone region of South America, causing yield losses of 10 to 100% in recent years (Duveiller et al. 2016). Wheat blast was observed for the first time outside of South America during the 2015-16 cropping season in the districts of Kushtia, Meherpur, Chuadanga, Jhenaidah, Jessore, Barisal, Bhola, and several other districts in the south of Bangladesh. Infected plants showed the typical wheat blast symptoms with the spike becoming partially or completely bleached with the blackening of the rachis in a short span of time. Examination of diseased plants showed the presence of elliptical, grayish to tan necrotic lesions with dark borders on the leaf often mixed with spot blotch disease lesions. Additionally, in some fields, blackening of lower nodes was observed. Grains from blast-infected heads were small, shriveled, deformed, and had low test-weight leading to serious yield losses. Diseased plants were collected and brought to the Wheat Research Centre, Bangladesh Agricultural Research Institute, Dinajpur, for further investigation. Grayish mycelium of the fungus taken from the infection point on the rachis of several independent spikes was observed. Incubation of several infected spikes/leaves in a 3-layered moist blotter at room temperature less than 12 h light/darkness cycle for 5 days led to the production of pyriform conidia. Morphobiometrical characteristics of the typical pyriform (pear-shaped) and 2-septate hyaline conidia were in agreement with the identification of the fungus as M. oryzae (Subramanian 1968). The air-dried spike samples have been kept in cold storage and several infected spikes were sent to the USDA-ARS, FDWSRU laboratory in the United States for characterization of the pathogen. There, the presence of M. oryzae in the infected samples was confirmed based on morphobiometrical analysis, and strains were preserved in the FDWSRU permanent wheat blast strain collection. Molecular analysis with MoT-specific markers and comparative genome analysis of isolates (BdBar16-1, GenBank accession no. LXON01000000; BdJes16-1, LXOO01000000; BdMeh16-1, LXOP01000000) confirmed that the wheat blast observed in Bangladesh is caused by MoT pathotype and has strong genetic identity to a strain from South America (B71, LXOQ01000000). This first incidence of wheat blast was significantly widespread accounting for approximately 15% of Bangladesh’s total wheat area. This large scale incidence of wheat blast has underscored a concern about the potential spread of wheat blast to other wheat producing areas in Bangladesh, South Asia, and beyond.