| dc.description.abstract | 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. | en |
