Pyricularia blast – a threat to wheat cultivation

Abstract

Wheat blast disease caused by Pyricularia grisea (telemorph Magnaporthe grisea) has become a serious restriction on increasing the area and production of the crop, especially in the tropical parts of the Southern Cone Region of South America. First identified in 1985 in the State of Paraná in Brazil, it has become an endemic disease in the low lying Santa Cruz region of Bolivia, south and south-eastern Paraguay, and central and southern Brazil in recent years. Severe infections have also been observed in the summer planted wheat crop in north-eastern Argentina. So far, only sporadic infections have been seen in Uruguay, especially during the wet and warm years. Spike infection (often confused with Fusarium head blight infection) is the most notable symptom of the disease and capable of causing over 40% production losses. However, under severe infection, the loss of production can be almost complete in susceptible varieties. Wheat blast is mainly a spike disease but can also produce lesions on all the above ground parts of the plant under certain conditions. Depending upon the point of the infection on the rachis, the disease can kill the spike partially or fully. The infected portion of the spike dries out without producing any grain which can be visibly distinguished from the healthy portion. While virulence diversity in the fungus has been reported in the literature and is under further exploration, genetic resistance in the host species has been more difficult to identify. Earlier, Brazilian cultivars such as BH 1146, CNT 8, several IAC and OCEPAR selections were credited as demonstrating different levels of field resistance, but this was not confirmed under artificial inoculation studies. However, other cultivars such as BR18, IPR 85, CD 113, have shown moderate levels of resistance over the years in many locations. Recently, several cultivars and advanced lines derived from the CIMMYT line, Milan, have been observed to carry a high level of resistance to blast disease throughout the endemic region. However, to date, the genetic basis of this resistance is not very clear due to extreme variation in the pathogen. Cultivars showing complete resistance against a few isolates under controlled conditions in the glasshouse, may or may not show field resistance in commercial cultivation. Due to an increase of the area under Milan based resistant wheat cultivars in Bolivia, Brazil and Paraguay, it needs to be combined with other sources of resistance urgently to prevent the selection of a virulent pathotype in the fungus. Besides genetic resistance, avoidance of early dates of seeding and chemical control can reduce the disease severity. Fungicides combining triazols with strobilurins can, under some situations, be effective in disease control at the heading stage. Even when all components of integrated disease management of wheat blast are not in place yet, it is seen as an essential strategy to reduce production losses in this region. Given the threat that the blast disease may pose to world wheat growing areas in the future, more research efforts are deemed urgent and necessary.