COMBINING ABILITY AMONG SOURCE POPULATIONS FOR TROPICAL MIDALTITUDE MAIZE INBREDS

Information about the combining ability of maize (Zen mays L.) germpla sm potentially useful for extraction of inbred lines for the mid-altit ude tropics has been very limited, and is essential for the structurin g of efficient breeding programs. The objective of this study was to d etermine optimal combining patterns among promising source populations for inbred line development in tropical mid-altitude zones. Parents i ncluded the lowland CIMMYT Populations 21, 43, 49 (all 'Tuxpeno') and 32 ('ETO'), subtropical Population 34, the IITA mid-altitude populatio n TZMSR, and the Kenyan based higher altitude heterotic group populati ons Kitale Synthetic II and 'Ecuador 573'. The four test sites ranged in altitude from 1000 to 1800 m above sea level (masl) in Cameroon. Lo wland by highland population crosses were optimal in the three mid-alt itude Tuxpeno sites (1000-1500 masl), with Tuxpeno x Kitale Synthetic II crosses producing the highest yields. Higher yields were associated with increases in maturity and plant height. Kitale Synthetic II and Ecuador 573, while conferring resistance to Puccinia sorghi Schw. and Exserohilum turcicum (Pass.) Leonard & Suggs, contributed to undesirab ly tall plants in the mid-altitudes. Lowland populations, while more s usceptible to mid- and high-altitude diseases, contributed to reduced plant height and loa er ear position. In order to combine the desirabl e shorter stature of the lowland populations with the yield, adaptatio n, and disease resistance contributions of Ecuador 573 and Kitale Synt hetic II, it is suggested that two heterotic populations be formed, co mbining Ecuador 573 with ETO (both flints), and Tuxpeno with Kitale Sy nthetic II (both dents), applying selection pressure for disease resis tance and reduced plant height in the two new populations. In addition , it appears that Ecuador 573 confers a dominant resistance to P. sorg hi, which could be useful in improving other germplasm.