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

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 of 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.

Fig 1. Wheat blast symptoms and spores isolated from symptomatic plants.

A. Bleached heads of naturally infected plants of wheat variety Coucal. B. Wheat blast symptoms on inoculated plants in the entry restricted screenhouse. C. Magnaporthe oryzae pathotype Triticum (MoT) culture of isolate WS4. Grey mycelia at the bottom and white aerial mycelia can be observed. D. Color of mycelia on the reverse side of the same isolate looks dark. E, F and G. MoT Spores isolated from inoculated plants in the screenhouse. Spores were identified using a Motic BA310 digital microscope (Motic, Kowloon, Hong Kong).

Fig 2

A. Amplified PCR fragments of expected bands using MoT3 oligonucleotide primers. Lane L is a 100 bp ladder. Lanes 1–6 are positive samples from the six symptomatic wheat head samples collected from both farmers’ fields and experimental fields in Mpika district, Zambia. B. Amplification of expected band sizes from two sub-isolates (WS4 and WS5) and two known MoT-negative (Rb3 –MoO and PL3.1-MoL) and positive (T25 and B2-MoT) samples with primers MoT3F/R and C17 F/R. As expected, there was no amplification from samples Rb3 (MoO) and PL3.1 (MoL). The analysis was performed at Ft. Detrick, MD, USA.