High yielding CIMMYT spring wheats with resistance to Ug99 and other rusts developed through targeted breeding
Targeted breeding to develop high yielding wheat germplasm resistant to Ug99 and other rusts initiated at CIMMYT in 2006. Ug99 resistant materials, especially those with adult plant resistance (APR), were used in crossing. F3 and F4 populations from simple, BC1 and top crosses were grown for two generations under high rust pressures at Njoro, Kenya in a Mexico-Kenya shuttle breeding scheme. Parallel populations were also grown in Mexico for comparison. Approximately 5,000 advanced lines were tested for grain yield performance at Ciudad Obregon, Mexico in 2009/10 season, and phenotyped for resistance to Ug99 and other rusts. The 728 retained lines were evaluated for grain yield performance in five environments during the 2010/11 season in Mexico. About 68% of the 728 lines had nearimmune (16.5% entries) to adequate APR to Ug99. An additional 13.6% lines carried one of the six (Sr25, Sr26, SrTmp, SrHuw234, SrSha7, and an unidentified gene) race-specific resistance genes often in combination with APR gene Sr2. About 80% entries were highly resistant to yellow rust in Kenya and Mexico, and 90% entries to leaf rust in Mexico. Yield distribution of lines derived from Mexico-Kenya shuttle breeding was similar to lines selected only in Mexico. Sufficient lines with >5% superior yields than the Mexican checks varieties in 2 years testing were identified. Our results indicate that targeted crossing and shuttle breeding are powerful tools for a simultaneous improvement of grain yield potential and resistance to rusts.