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 growin...

Approximately 9 million ha of wheat (Triticum aestivum and T. durum) is sown in the Southern Cone of America (Argentina, Brazil, Chile, Paraguay, and Uruguay). Two rust epidemiological zones separated by the Andean mountain range have been described in the region. Presently, leaf rust (caused by Puccinia triticina) is the most important rust disease of wheat. The utilisation of susceptible or moderately susceptible cultivars in a high proportion of the wheat area allows the pathogen to oversummer across large areas, resulting in early onset of the epidemics. Severe epidemics cause important economic losses if chemical control is not used. The pathogen population is extremely dynamic, leading to transitory resistance in commercial cultivars. Lr34 is commonly present in the regional germplasm, but there is limited knowledge about the presence of other genes conferring resistance in cultivars. Genes Lr28, Lr36, Lr38, Lr41, and Lr43 provide effective resistance in the region. The best strategy for the stabilisation of the pathogen population and resistance is considered to be the use of adult plant resistance conferred by minor additive genes including Lr34 and Lr46. Sources of this type of resistance from CIMMYT and the region have been made available to breeding programs in the Southern Cone. Stripe rust (P. striiformis f. sp. tritici) is endemic in Chile where chemical control is required to prevent severe losses in stripe rust susceptible cultivars. Although new virulent races emerge frequently, resistance genes Yr5, Yr8, Yr10, Yr15, and YrSp are currently effective in Chile. Some important stripe rust epidemics have occurred in Argentina, Brazil, and Uruguay. Avoiding the use of highly susceptible cultivars appears to be an effective strategy to prevent stripe rust epidemic development in this area. There have been no serious stem rust (P. graminis f. sp. tritici) epidemics for over 2 decades; the disease was controlled by resistant cultivars. The most important genes conferring resistance in Southern Cone germplasm at the present time are probably Sr24 and Sr31. Other effective genes are Sr22, Sr25, Sr26, Sr32, Sr33, Sr35, Sr39, and Sr40. Several stem rust susceptible wheat cultivars have recently been released. The increased cultivation of susceptible cultivars may lead to higher stem rust incidence, increasing the probability of appearance of new virulent races. Since the 1BL.1RS translocation possessing Sr31 is present in a high proportion of the regional germplasm, the possible introduction of stem rust with Sr31 virulence from Africa is of great concern.

No abstract

No abstract

Wheat blast disease, caused by Magnaporthe oryzae (anamorph Pyricularia oryzae), produces severe damage to wheat production in South America. It was observed that many resistant cultivars contain the 2NS/2AS translocation from Triticum ventricosum. In this study, we evaluate the presence of the 2NS/2AS translocation in 57 advanced breeding lines and one variety ‘Caninde 1’ from Paraguayan wheat germplasm, using VENTRIUP‐LN2 primers. The germplasm ‘Caninde 1 and 22’ of the breeding lines, found positive for the presence of 2NS/2AS translocation, were inoculated with a single aggressive Magnaporthe pathotype P14‐039, to assess their response to wheat blast infection under controlled conditions. Based on the disease infection score, ten of the breeding lines, ‘Caninde 1’ and ‘Milan’ (positive control), were classified as resistant. Three of the remaining breeding lines were classified as moderately resistant, five as moderately susceptible and other four as susceptible. Our results show that the expression of 2NS/2AS‐based blast resistance is more dependent on genetic background of the inserted germplasm than previously envisioned.