Detection and characterization of fungus (Magnaporthe oryzae pathotype Triticum) causing wheat blast disease on rain-fed grown wheat (Triticum aestivum L.) in Zambia

Authors: TEMBO, BATISEBA - Zambia Agricultural Research Institute; MULENGA, RABSON - Zambia Agricultural Research Institute; SICHILIMA, SUWILANJI - Zambia Agricultural Research Institute; M'SISKA, KENNETH - Zambia Agricultural Research Institute; MWALE, MOSES - Zambia Agricultural Research Institute; CHIKOTI, PATRICK - Zambia Agricultural Research Institute; SINGH, PAWAN - International Maize & Wheat Improvement Center (CIMMYT); HE, XINYAO - International Maize & Wheat Improvement Center (CIMMYT); Pedley, Kerry; Peterson, Gary; SINGH, RAVI - International Maize & Wheat Improvement Center (CIMMYT); BRAUN, HANS - International Maize & Wheat Improvement Center (CIMMYT)

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2020
Publication Date: 9/21/2020
Citation: Tembo, B., Mulenga, R.M., Sichilima, S., M'Siska, K.K., Mwale, M., Chikoti, P., Singh, P.K., He, X., Pedley, K.F., Peterson, G.L., Singh, R.P., Braun, H.J. 2020. Detection and characterization of fungus (Magnaporthe oryzae pathotype Triticum) causing wheat blast disease on rain-fed grown wheat (Triticum aestivum L.) in Zambia. PLoS ONE. 15:e0238724. https://doi.org/10.1371/journal.pone.0238724.
DOI: https://doi.org/10.1371/journal.pone.0238724

Interpretive Summary: Wheat blast is devastating disease that severely affects the production of wheat in areas where it occurs. The disease was first discovered in Brazil in 1985 and remained confined to South America until 2015 when it was discovered in Bangladesh. The discovery of wheat blast outside of South America established that the disease, caused by the Triticum pathotype of the fungus Magnaporthe oryzae, is an emerging threat to wheat production worldwide. During the 2017 – 2018 growing season wheat blast symptoms were observed on wheat in experimental plots and production fields in the Mpika district of Muchinga Province, Zambia. To determine whether the symptoms were caused by M. oryzae, a study was conducted to isolate and characterize the pathogen using a combination of classical and molecular methods. The fungus isolated from infected plants was determined to be M. oryzae based on physical characteristics, and two molecular diagnostic markers confirmed that the fungus belonged to the Triticum pathotype. Pathogenicity tests performed with pure cultures of two isolates produced typical blast symptoms on all the six inoculated wheat genotypes. The results of this study demonstrate that the Triticum pathotype of M. oryzae is present and causing disease in Zambia. This is the first report of wheat blast in Zambia and on the African continent.

Technical Abstract: Wheat blast caused by Magnaporthe oryzae pathotype Triticum (MoT) is a threat to wheat production especially in the warmer-humid environments. In Zambia, wheat blast symptoms were observed for the first time on wheat (Triticum aestivum L.) grown in experimental plots and five farmers’ fields in Mpika district of Muchinga Province during the 2017-18 rainy season. 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. Incidence of blast symptoms on nearly all wheat heads was high and ranged from 50 to 100%. Examination of diseased plant leaves showed the presence of elliptical, grayish to tan necrotic lesions with dark borders on the leaf often mixed with other foliar diseases. A study was conducted to isolate and identify the causal pathogen(s) using classical and molecular methods and determine the pathogenicity of the detected disease causal agent. Morphobiometrical determination of causal pathogen revealed conidia with characteristic pear shaped 2-septate hyaline spores associated with M. oryzae species. Preliminary polymerase chain reaction screening for six isolates obtained from wheat blast infected samples with diagnostic primers (MoT3F/R) was conducted at ZARI, Zambia, and subsequent analysis of two isolates with MoT3F/R and C17F/R was performed at USDA-ARS, USA. Both experiments confirmed that MoT is the causal agent of wheat blast in Zambia. Further, pathogenicity tests performed with pure culture isolates from samples WS4 and WS5 produced typical blast symptoms on all the six inoculated wheat genotypes. Results of this study indicate that MoT is causing wheat blast in rain-fed wheat grown in Zambia, thus making it the first report of MoT in Zambia and Africa. This inter-continental movement of the pathogen (disease) has serious implication for wheat production and trade that needs to be urgently addressed.