Key messages

Wheat blast (WB) caused by the fungus Magnaporthe oryzae Triticum (MoT) lineage (synonym Pyricularia oryzae Triticum) is one of the potential future disease threats to the wheat industry in Asia and Australia. This paper is designed to create awareness among growers, professionals, and researchers about WB disease and its potential risk to wheat production systems in Australia.

Aims

• Present a brief overview of WB disease globally, to give a clear understanding of its potential impact on wheat production systems in Australia
• Provide information on the identification and management of the disease

Introduction

Wheat blast is a dreadful disease with limited control options that presents a global risk to wheat production. Fortunately it has not yet been reported in Australia. First detected in Brazil in 1985, it subsequently spread across south American countries including Bolivia, Paraguay, and Argentina (Igarashi et al 1986; Kohli et al 2011). In 2016, wheat blast was introduced to Bangladesh and in 2018, Zambia (Islam et al 2016; Malaker et al 2016). The genetic identity and origin of the wheat blast pathogen were determined as a South American lineage of MoT (Callaway 2016; Islam et al 2016) and it is assumed that it spread from South America to Bangladesh and Zambia through international wheat trade. There is high risk of spread of MoT across the Asian and African continents with the possibility of further spread to adjacent continents, such as Australia (Ceresini et al 2018, Islam et al 2019).

Wheat blast is a fast-acting, severe disease which causes wheat heads to become fully or partially bleached, resulting in inferior quality, small, shrivelled grains with a reduced test weight. Yield losses are often significant; up to 100% crop loss when environmental conditions are conducive for disease epidemics. Wheat blast reduced national wheat production in Bangladesh and in Brazil by 30% in 2016 and 2009 respectively. Rice blast (Magnaporthe oryzae Oryzae (MoO)) is genetically distinct from the wheat-infecting pathotype and generally does not infect wheat (Chiapello et al 2015; Yoshida et al 2016). Wheat is the main host of MoT, it can also infect triticale, barley, durum wheat, black oats, and some grass weed species (Urashima et al 1993 and 2004). International seed and grain trade, similar environments, and global warming all increase potential risk to spread WB disease from epidemic regions to disease-free regions of south Asia and the east coast of Australia.

This paper was written by reviewing journal articles, information available on websites, consultation with researchers and knowledge gathered from previous experiences.

Results

What is the WB disease distribution pattern globally?

Wheat blast is a disease that poses a global risk to wheat production. After first being detected in Brazil in 1985, it progressively spread to adjoining countries. In 2016, it was detected in Bangladesh, and by 2018 it was confirmed in crops in Africa, in Zambia (Figure 1). In both cases its spread has been attributed to the international wheat trade. It now threatens wheat production in south and southeast Asia, Australia, and southern Africa. Cao et al (2011) predicted vulnerable regions in mid-east South America, southeast and Midwest Africa, southeast of south Asia, east coast of Australia and south China. Contaminated seed and grain trade are the potential source of WB spread between continents. There are also concerns that wind-blown spores might bring the disease from south-east Asia into Australia’s north. Once established it can be spread naturally from plant to plant and from paddock to paddock through the forces of wind and rain. Movement of contaminated farm machinery, vehicles, and people will speed up the spread. Global warming, cultivation of susceptible cultivars, increasing virulence of the pathogen, fungicide resistance, potential sexual recombination, and possible cross-host infections could lead to more frequent outbreaks and spread of the disease to other major wheat-producing countries.

Figure 1. Intercontinental spread of wheat blast attributed to the grain trade (Source: Singh et al. 2021)

What weather conditions favour WB disease?

The disease is of greatest threat to grain growing regions in warm, humid and wet environments. Rainy and humid weather conditions during heading stage of wheat crops have been found to enhance the occurrence and development of WB. The most vulnerable growth stage is between anthesis and early grain development. The most severe field infections occur in seasons when there is continuous rainfall during the period of anthesis, with an average temperature of 18–25 °C, followed by a period of sunny, hot, and humid weather (Kohli et al 2011). Another study reported that an optimum temperature ranging between 25 and 30°C and heads under wet conditions for 25–40 hours can lead to a severe outbreak of the disease (Cardoso et al 2008).

What are the disease symptoms?

The MoT fungus can infect all above-ground parts of wheat, but the most noticeable symptom is detected on the spikes. The first identifiable symptom of the disease is usually observed at the reproductive stage of the crop in a scattered patch in wheat field. With time, the patches coalesce and the whole field is severely damaged. Spikes in the infected field become silvery colour while the leaves may remain green. Partially or completely bleached spikes are the most notable symptoms of WB. Symptoms on the leaf include the presence of elliptical, elongated or eye-shaped, greyish to tan necrotic lesions with dark borders. Stem lesions include lesions that are elongated or elliptical in shape with a white centre surrounded by a dark-brown or blackish margin. Sometimes WB of heads in the field can be incorrectly diagnosed because it looks somewhat like head blight and spot blotch, caused by Fusarium graminearum and Bipolaris sorokiniana, respectively.

Development of WB disease from seed

Figure 4. Wheat blast disease cycle (Source: Recovery Plan for Wheat Blast, National Plant Disease Recovery System, USA.)

(Ae), Wheat blast infection on ears will result in seed infection (B, C), providing inoculum for either local or long-distance dispersal of the pathogen. Crop residues serves as a niche for sexual reproduction (D, 1-4), mature fruiting bodies release ascospores (D1), giving rise to new fungi by germination (D2), conidia form and are released (D3-4), airborne inoculum for leaf infection on other grass hosts, either invasive or contiguous to wheat fields (E, F). Sexual fruiting bodies can also form in other infected grasses and ascospores released out onto a nearby wheat crop (E). Seedborne inoculum (B, C) results in primary infections in a newly established wheat crop. (F4) conidia released from leaf blast lesions on other grass hosts nearby wheat crops also contributes inoculum for wheat blast on ears. Conidia on leaves (Af) in the lower canopy of certain wheat cultivars coinciding with spike emergence under field conditions and could be an important trigger for wheat blast epidemics on ears (Ae).

How does the WB pathogen survive and spread?

As a seed-borne pathogen, WB can survive and spread from seed for up to 22 months. It survives in stubbles as well as alternate grass weed hosts. Airborne conidia can travel 1km from their point of infection. Sexual spores (ascospores) can travel longer distances as they are lighter than conidia.

Why does the MoT pathotype pose more risk than other pathotypes of Magnaporthe?

Research shows that the Triticum pathotype (MoT) population evolves quickly, resulting in a high level of genetic diversity that is higher than that of other pathotypes. This might be related to its ability to undergo sexual reproduction The sexual cycle of M. oryzae is not observed in nature on any host because of self-incompatibility, however Triticum pathotype (MoT) shows high levels of sexual fertility, functioning as a hermaphrodite and crossing to produce abundant viable ascospores. This result raises the possibility that the WB pathogen could undergo sexual recombination in the field and might be able to produce more virulant strains that cause higher risk than other pathotypes.

Can we restrict movement of the WB pathogen?

Implementing quarantine and biosecurity regulations and conducting routine surveillance activities are needed to restrict the WB pathogen moving via infected seeds into disease-free sites.

Current WB disease management strategies adopted by growers in epidemic regions

Plant disease develops when the host, pathogen and favourable environment coincide, and management strategies must consider all three of these factors. In epidemic regions, growers generally apply fungicides and use resistant varieties (where available). Other cultural management practices include adjusting planting time, crop rotation with non-host crops, alternate and collateral host eradication, stubble management and suspending the cultivation of wheat in disease-prone areas (wheat holiday policy). Managing WB using fungicides is possible but has had varying effectiveness. Certain seed treatments when used with foliar sprays have had a degree of success overseas. Use of resistant varieties is the safest option to manage pathogens. However, sources of resistant varieties are limited and the ability of WB pathotype to undergo sexual reproduction might lead to development of new virulent races or strains, and this presents the greatest challenge to the use of resistant cultivars. The most effective measure used in regions where WB occurs, is an integrated management approach including varietal resistance, cultural management, use of biopesticides and biocontrol agents and judicial use of chemical fungicides application (seed treating and foliar fungicides).

Current research to combat WB pathotypes

Developing resistant varieties is the most important research underway using conventional and modern breeding approaches supported by various molecular techniques. However, resistance genes and sources of screened germplasm are so far limited. To date, a total of 11 R genes have been identified as sources of resistance against the WB fungus.

GRDC funded the Australian National University (ANU) to assess the WB performance of some Australian wheat varieties about six years ago under the supervision of Professor Peter Solomon. Four of the twenty lines screened showed some resistance to the WB pathogen. However, while Australian conditions are less conducive to wheat blast, an outbreak would significantly impact wheat yields in affected areas (GRDC Ground Cover Issue 132, Jan–Feb 2018) and although resistant cultivars exist most varieties remain susceptible. Subsequent research conducted by the ANU is being performed at the Australian Centre for International Agricultural Research (ACIAR) by Dr Eric Huttner. The project aims to identify and describe new sources of WB resistance to foster rapid delivery of resistant wheat varieties for Australia and the international community, including Bangladesh.

Other research areas include epidemiology research into the impact of changing agroecological conditions and farming systems and development of safe and biodegradable agrochemicals, bioagents and organic chemicals.

Conclusion

Wheat blast is recognised as a significantly damaging disease of wheat worldwide. From its origin in Brazil in 1985, it has spread to many South American countries and made intercontinental jumps to Bangladesh in South Asia and Zambia in Africa, infecting around three million hectares of wheat within a decade. Although most wheat-growing countries of the world are still free from the disease, it has potential to spread on seed into other countries especially Asia and Australia where it could be very damaging. This is an alarming situation for future world food security.

The WB pathogen is fast-evolving, aggressive, and potentially devastating in various agroecological zones; therefore, a globally intensive effort is required to prevent its damage and limit its introduction and spread. Strengthening biosecurity and quarantine, adequate routine surveillance, information sharing, awareness creation, pre-preparedness and proactive research planning are the best ways to tackle the invasion and manage WB disease.

Acknowledgments

This work was supported by Grain Producers Australia (GPA).

References

Islam MT et al. 2019. Wheat Blast in Bangladesh: The Current Situation and Future Impacts. Plant Pathol. J. 35(1): 1-10 https://doi.org/10.5423/PPJ.RW.08.2018.0168

Singh PK, et al. 2021.  Wheat Blast: A disease spreading by intercontinental jumps and its management strategies. Frontiers in Plant Science | www.frontiersin.org; July 2021, Volume 12, Article 710707.

Contact details

Dr Zia Hoque
Department of Primary Industries and Regional Development
75 York Road, Northam, WA 6401
Ph: (08) 9690 2141
Email: zia.hoque@dpird.wa.gov.au