Evolution of wheat rust pathogens in the indian subcontinent
534Wheat rusts are dynamic pathogens. There is a tug of war between the rusts and wheat scientists. Rust resistant wheat varieties become susceptible due to shift in virulence which is influenced by the wheat cultivars in the fields. New virulences emerge which render rust resistant wheat varieties susceptible to a within 4–5 years of their cultivation. Over the years more virulent forms go on appearing, however, only fittest one rules the flora for a long period. New virulences arise due to sexual recombinants on alternate hosts. However, in countries where alternate hosts are not functional, new pathotypes arise mostly through mutation and somatic hybridization in few cases. These mutations occur as a natural phenomenon; however, resistant varieties in the field facilitate their selection. Few rust resistance genes are more prone to the mutations than the others. Mutations are generally forward (gain in virulence), however, reverse (loss of virulence) mutations have been observed in few cases. Loss of virulence is specific to few rust resistance genes only. Our experience shows that wheat rusts have even overcome the transgenes and other alien introgressions undertaken for rust resistance in wheat. It means we cannot eradicate wheat rusts but better we learn to live with these pathogens. Intelligent management is to be practiced. A vigil has to be kept for the occurrence of new virulences. More resistance sources, preferably diverse and varieties with more than two effective rust resistance genes will have to be given preference. Taking into consideration the racial patterns, skilful deployment of varieties can help in keeping wheat rusts below the threshold levels.