Most of the initial wheat varieties that had triggered the Green Revolution succumbed to rusts and had to be abandoned. So did many of the wheat strains developed to replace them by incorporating resilience against specific rust diseases into them. Some other approaches tried out to enable wheat varieties to defy onslaught of rusts also did not work for long and rusts have continued to remain a formidable enemy of wheat in India and other wheat-growing countries. An alert issued recently by the UN Food and Agriculture Organisation has cited rusts among the diseases that play havoc with this crop across the globe. Even durum wheat (commonly used to make pasta products), which is deemed to be the most sturdy among all the three main species of wheat —Triticum aestivum, T durum and T dicoccum — has turned susceptible to rusts.
Taming rusts has proved particularly problematic because rust spores (reproductive units, which disperse the infection) are carried from one place to another by air. When there is no wheat crop in the main northern wheat belt, the rust pathogens (called puccinia) survive on wheat grown in the Nilgiri and Palni hills in south. They come back to the north again during the next wheat season. To disrupt this so-termed puccinia path, wheat scientists tried to saturate the southern hills with rust-defiant wheat varieties. Even this has not proved a foolproof defence against the rusts menace.
The latest bid to find the sources of multiple resistance against rusts involved screening of a whopping 19,460 wheat accessions (genetically distinct races) collected from within the country and abroad and preserved in the New Delhi-based national gene bank. The screening was done at three locations — Wellington in Tamil Nadu, Gurdaspur in Punjab and Cooch Behar in West Bengal — which represented different agro-ecological zones. The aim of this research initiative of unprecedented scale, involving scientists of the Indian Council of Agricultural Research and agricultural universities, was to locate the genes capable of withstanding the onslaught of all rusts simultaneously. The results have been published in the December 2016 issue of the multidisciplinary research journal PLOS ONE.
This landmark study led to shortlisting of 498 accessions potentially immune to multifarious rusts and 868 resistant to spot blotch malady. Further testing carried out at the Flowerdale (Shimla)-based research institute, dedicated exclusively to wheat rusts, helped prune this list to 137 strains, which showed innate protection against various rusts at the same time. The scientists also managed to identify different combinations of genetic loci (position of specific genes on the chromosome), which impart resistance against various rusts to wheat plants. This information can help place the desired rust-resilient genes at the right spot in targeted wheat varieties through conventional or biotechnological means.
The outcome of research has, therefore, paved the way for evolving new high-yielding varieties of this key staple crop, which may have lasting freedom from dreaded rust diseases. It is now for wheat breeders to gainfully utilise the identified sources of multiple resistance against rusts to evolve high-yielding wheat varieties, which could enable farmers to grow healthy crops at reduced costs to earn more income.
