Monitoring physiologic variability in wheat rust pathogens and its role in rust management in India

Wheat rusts (Puccinia species) are known to be historic diseases. These are shifty pathogens, capable of spreading over long distances, changing forms and rendering resistant varieties susceptible. Rust epidemics have been recorded recently even in most advanced and neighbouring countries whereas there had been no epidemic of wheat rusts in India during the last 35 years. Identification of pathotypes in initial stages backed by anticipatory breeding, evaluation of germplasm for rust resistance, deployment of varieties based on pathotype distribution have been the key factors in containing wheat rusts in India. Monitoring of new rust virulences is not only important for resistance breeding but also in predicting epidemics. Wheat rusts are monitored by surveying farmers fields during the growing season, trap plots consisting of the sets of differential genotypes and other cultivars either susceptible or resistant planted at strategic locations. Pathotype analysis data are used in determining the virulences of the rust population over time and in different wheat growing areas. Accordingly, varieties are deployed for different zones, the strategy is known as “gene deployment”. Cultivation of varieties with vertical resistance has led to the evolution of new virulent forms recently on Yr9, Lr9, Lr19, Lr28 and Sr25. To counter the erosion of vertical resistance genes and increasing the self life of wheat varieties, strategy has now shifted to diversity. The features added in the resistance diversification are slow rusting, adult plant resistance, multigenic resistance (combination of specific or minor resistance genes) with a target on durability of resistance. Further wheat rust management system is in place and prepared to counter the threat of virulent forms of stem rust on Sr31 (Ug99 variants) identified in African countries to south Asian nations and emergence of Yr9 virulent pathotypes in north western plains zone.