Evidence for Enhanced Resistance to Diverse Isolates of Pearl Millet Downy Mildew through Gene Pyramiding

Abstract

Breeding for downy mildew resistance in pearl millet
\n[Pennisetum glaucum (L.) R. Br.] at ICRISAT-Patancheru
\nin India, is currently focused on developing hybrid
\nparental lines with resistance to one or more pathogenic
\nvariants of Sclerospora graminicola (Sacc.) Schroet. that
\nexist in India. Resistance incorporation in inbred lines is
\ndone through conventional pedigree and bulk-pedigree
\nselection, pure-line selection within elite inbreds originally developed by bulk-pedigree methods, backcrossing with conventional selection, and backcrossing with markerassisted selection (Hash et al. 1999). Stability of
\nresistance to downy mildew in pearl millet inbred lines
\nand hybrid cultivars has become elusive in India due to
\nhost-directed evolution of pathogenic variation in S.
\ngraminicola populations (Thakur et al. 1992). Average
\ncommercial life spans of popular hybrids have been
\nreduced to only 3–5 years before they must be withdrawn
\ndue to pathogen virulence changes (Thakur et al. 2003).
\nThere are currently several pathogenic variants of S.
\ngraminicola prevalent in different parts of pearl millet
\ngrowing regions of India, and new variants with higher
\nvirulence levels keep on appearing with deployment of
\nnew cultivars (Thakur et al. 2004; Pushpavathi et al. 2006).
\nA number of quantitative trait loci (QTLs) for host plant
\nresistance effective against one or more pathogen isolates
\nof S. graminicola have been identified (Jones et al. 1995,
\nJones et al. 2002; Hash and Witcombe 2001), and resistance
\nalleles for some of these QTLs have been transferred by
\nmarker-assisted backcrossing to elite parental lines of
\npopular hybrids.