Combining ability for grain yield and other agronomic traits and F1 maize streak virus disease expression in diverse genotypes of maize (zea mays L.)
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Maize streak virus (MSV) disease drastically reduces the yield levels of the maize crop in sub- Saharan Africa. Resistance to MSV has been identified in several maize genotypes. The objectives of this study were to investigate combining ability among six diverse maize genotypes for grain yield and to examine the interaction in Fl hybrids for MSV resistance. The germ plasm used were lines previously identified for MSV resistance: CML 202 and CML 197 from CIMMYT, C92 from CIRAD, Tzi 3 from IITA, VHCY from South Africa and EMll-133, a susceptible line from Kenya. The six parents, 15 Fl hybrids derived from half-diallel among the parents and three susceptible checks (H614, H513 and EM12-210) were evaluated in three locations, Kiboko, Embu and Muguga. An additional trial was conducted under artificial MSV disease inoculation in Muguga. MSV disease severity scores, grain yield (GY) and its associated traits: plant height, ear height, days to silking, days to pollen shed, ear length, ear aspect and lodging were collected. The measured traits were subjected to analyses of variance and correlations. General (GCA) and specific combining ability (SCA) effects were estimated and mid-parent (MPH) and high-parent heterosis (HPH) calculated for all traits measured. The results showed that besides grain yield, the materials tested showed diversity for all other traits measured and that environments affected the expression of these traits. High yields averaging 8.2 t ha' across locations were observed among the test hybrids. The high yields recorded were complemented by selectable traits for high yields such as long ears and good ear aspect. The highest correlations were obtained between grain yield and plant height although short varieties with high yields such as Tzi 3 X CML 202 were also identified. The correlations between grain yield and days to pollen shed (+0.25) and silking (+0.27) were low and noni significant contrary to findings by other workers. The highest yielding single cross hybrid across locations was CML 197 X CML 202 with 11.1 t ha'. Inbred line CML 197 was a unique parent in the top yielding crosses indicating a high degree of GCA, which was in agreement with the data obtained. CML 197 also had the highest positive GCA for grain yield (0.88) among all the lines examined. CML 202, the male line in the highest yielding SC, had the second highest GCA of 0.77. The GCA: SCA ratios for most of the traits were closer to unity (1) implying that progeny performance could be based on GCA alone during selection to improve grain yield. Preponderance of additive effects was thus indicated for grain yield, MSV and most of the other agronomic traits. The high significance of GCA X location indicated that the best GCA effects changed over environments. This suggested the need to select different parental lines for hybrid development at different locations. Dominance effects were also important for grain yield meaning that the germ plasm used should be useful in recurrent selection procedures. The majority of the Fl hybrids deviated little or not at all from the mid -parent for MSV disease score showing that either co-dominance or partial dominance were of major effects. Preponderance of additivity was moreover noted for MSV disease expression. There also seemed to be differences in expressivity of disease resistance where individual plants in particular crosses showed varied disease scores. This suggested that the parents were possibly not completely inbred for MSV resistance. The high yield levels under infected conditions were not correlated to low disease levels suggesting some hybrids had high tolerance to MSV. C92 and CML 202 had the highest negative and significant GCA effects for MSV symptoms thus these could have contributed highest with favorable alleles for MSV resistance. The materials used in the study presented differences in the degree of inbreeding which could have affected their response to stresses and subsequently the amount of heterosis. The highest mid-parent and high parent heterosis values were obtained in CML 197 X CML 202 with values of 234% (MPH) and 210% (HPH) across locations. High heterosis observed in CML 197 X CML 202 might have been due to restoration of heterozygosity from inbreeding rather due to than divergence of the lines. In conclusion, sufficient genetic diversity existed in the test materials used in these studies that could be used to improve both grain yield and MSV resistance and that progeny performance could be inferred from an assessment of GCA alone. In this regard, CML 197 X CML 202 and C92 X EMl1-133 should be evaluated further for possible release to farmers. Also, it was possible to identify relatively early maturing varieties that were particularly good grain yielders such as VHCY X CML 202 and T~i 3 X CML 202 and which yielded 16 % and 14 % more grain yield than the best check (H614) despite these varieties being earlier than the check by 9 days.