Location: Foreign Disease-weed Science ResearchTitle: First report of wheat blast caused by magnaporthe oryzae pathotype triticum in Bangladesh Author
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2016
Publication Date: 6/6/2016
Citation: Malaker, P.K., Barma, N.C., Tiwari, T.P., Collis, W.J., Duveiller, E., Singh, P.K., Joshi, A.K., Singh, R.P., Braun, H.J., Peterson, G.L., Pedley, K.F., Farman, M.L., Valent, B. 2016. First report of wheat blast caused by magnaporthe oryzae pathotype triticum in Bangladesh. Plant Disease. doi: 10.1094/PDIS-05-16-0666-PDN. Interpretive Summary: Magnaporthe oryzae is a fungal pathogen that is responsible for a number of serious leaf and spike diseases in turf grass and field crops, such as rice. In 1985 a new strain of the pathogen, referred to as the ‘Triticum pathotype’, was reported on wheat in Brazil. The most prominent symptom of the disease is the bleaching of the wheat spike. Infected spikes either do not produce seed or the seed produced is of low quality. Significant yield losses have been reported in the areas where the disease is present, making the disease a major limitation to wheat production. Since first reported in Brazil the pathogen has spread to wheat producing areas in Argentina, Bolivia, and Paraguay, but has not been detected elsewhere. Reported here is the first occurrence of the disease outside of South America in Bangladesh affecting approximately 15% of the total wheat production area during the 2015-2016 crop season. Disease losses were estimated at 10 to 100% in individual fields. Disease symptoms were similar to those observed in South America and the fungal structures were typical of Magnaporthe oryzae. Infected wheat heads were collected throughout Bangladesh and samples were sent to the USDA Agricultural Research Service, Foreign Disease-Weed Science Research Unit at Ft. Detrick, Maryland where the presence of M. oryzae in the infected samples was confirmed based on morphological comparisons. Isolates of the pathogen collected in Bangladesh tested positive using a diagnostic assay specifically developed to identify the Triticum pathotype. Additional characterization of the isolates based on DNA sequence comparisons to South American isolates confirmed that the Triticum pathotype of M. oryzae is present in Bangladesh.
Technical Abstract: Wheat blast or ‘brusone’, caused by the ascomycetous fungus Magnaporthe oryzae B.C. Couch (synonym 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), perennial and annual ryegrass (Lolium pathotype), finger millet (Eleusine 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 for the first time observed 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 symptom 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 and confusing with lesions due to spot blotch. 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 growths of the fungus taken from the infection point on the rachis of several independent spikes were observed. Incubation of several infected spike/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 from these lesions typical of Pyricularia-like fungus. 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 USA for characterization of the pathogen. In the USA, the presence of M. oryzae in the infected samples was confirmed based on morphobiometrical analysis, and strains were preserved in the permanent wheat blast strain collection at Fort Detrick. Molecular analysis with M. oryzae Triticum pathotype-specific markers and comparative genome sequence analysis confirmed that the wheat blast observed in Bangladesh is caused due to M. oryzae pathotype Triticum. This first incidence of wheat blast was significantly widespread accounting for approximately 15% of Bangladesh’s total wheat area. Initial estimates of losses range from 10% (potentially tolerant cultivars) to 90% (late planted and susceptible cultivars) thereby significantly affecting the country’s aggregate wheat production. 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.