151 CHAPTER SEVEN
General overview
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• Only a few farmers (18.5%) are familiar with nematodes and the related damage on maize in Uganda.
• Pratylenchus zeae occurred at generally higher frequencies than Meloidogyne spp. in nematode susceptible cultivars.
• The landraces and the cultivar Longe 5 supported high nematode populations compared to cv. Longe 1. Farmers also reported low yields for Longe 5 when compared to the rest of the cultivars they grow.
• Farmers’ most preferred traits were pest and disease resistance, high grain palatability, long storage duration and large kernels.
7.2.2 Monoxenic culture of P. zeae on carrot discs
Twenty live nematodes were transferred to the margins of each of the 40 sterile carrot discs contained in 3.5 cm diameter sterile glass Petri dishes. All cultures were maintained in the dark at 25 ± 1°C.
• The study revealed higher reproduction rates of P. zeae on carrot discs compared to excised maize roots. Each P. zeae inoculated on the carrot discs reproduced 5,090 times after three months of incubation compared to a reproduction rate of 26.4 on excised maize roots.
• The study indicated that sterile carrot discs are a more productive medium for culturing P.
zeae than excised maize roots, and is therefore the recommended practice.
7.2.3 Combining ability and genetic effects for nematode resistance in maize
A 6 x 6 full diallel was conducted to study the genetics of nematode resistance in maize. The 30 F1 hybrids were evaluated in an 8 x 4 alpha-lattice design in split plots at three locations in Uganda.
Combining ability effects
• General combining ability contributed most of the phenotypic variance (72-93%) in reduction of P. zeae and Meloidogyne densities, and the increase in root mass. The SCA contributed most of the phenotypic variance (43-58%) in plant height and grain yield under nematode infestation.
• Parents CML444, CML312 and CML395 enhanced grain yield with positive GCA effects under both nematode infested and nematicide treated conditions. Parents MP709 and CML206 were the best general combiners associated with P. zeae resistance.
• The SCA effects for grain yield were positive and significant for 11 hybrids under nematode infestation. However, hybrids which showed the best SCA effects for grain
153 yield were MP709/CML312, 5057/CML395, 5057/CML312, CML206/CML312, CML444/CML312 and CML395/CML312.
• Hybrids MP709/CML444 and MP709/CML395 had negative significant reciprocal effects for grain yield resulting from the negative maternal effects observed in parent MP709 under nematode infestation. Therefore, grain yield was being compromised in the two hybrids of this parent when used as the female under nematode infestation.
Genetic effects
• Overdominance gene action explained the non-additive portion of the genetic variance observed for plant height, grain yield, number of root lesions, P. zeae and Meloidogyne spp. densities.
• For plant height and grain yield, the dominance was largely unidirectional specifically in the direction of tall plants with high grain yield.
• Parents MP709, 5057, CML206 and CML444 contributed most of the dominant genes towards P. zeae resistance, whereas CML312 and CML395 contributed recessive genes associated with susceptibility to P. zeae. Reciprocal effects were, however, recorded for P. zeae densities.
• Parents CML312 and CML206 contributed most of the dominant genes for susceptibility and resistance to Meloidogyne spp., respectively. Parents MP709 and CML395 had similar frequency of both dominant and recessive genes.
• Parent CML444 contributed dominant genes for high grain yield whereas MP709, CML395 and 5057 had similar frequency of both dominant and recessive genes towards grain yield.
7.2.4 Nematode resistance, grain yield, heterosis and yield losses among the maize hybrids
The 30 F1 hybrids generated from the diallel cross were evaluated in an 8 x 4 alpha-lattice design in three locations in Uganda for resistance to nematodes, but also in the screenhouse for P. zeae resistance. Two checks were included, one susceptible to nematodes and another resistant.
• Based on the reproduction factor, 24 hybrids were P. zeae susceptible whereas six hybrids were P. zeae resistant under screenhouse conditions.
• Grain yield was higher by about 400 kg ha-1 under nematicide treated plots than under nematode infestation. The nematode resistant hybrids exhibited high yields ranging from 5.0 to 8.4 t ha-1 compared to 5.0 t ha-1 obtained from the best check.
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• Highest grain yield loss due to nematodes was 28%, observed in hybrid CML206/CML444, indicating high levels of damage by nematodes.
• Under field conditions, favourable heterosis was recorded in 18 hybrids for P. zeae, and in three hybrids for Meloidogyne spp.
• Under nematode infestation, only 16 hybrids had higher relative yield compared to the mean of both checks, the best check and the trial mean, whereas it was 20 hybrids under nematicide treated plots. Hybrids CML312/CML206, CML444/CML395, CML395/CML444, CML444/CML312, CML312/CML444, CML395/CML312, CML312/CML395, CML312/5057, CML395/5057, 5057/CML444, 5057/CML206, CML395/MP709, CML444/MP709 had higher relative yield compared to the mean of both checks, the best check and the trial mean, both under nematode infestation and nematicide treatment.
• Overall, the most outstanding hybrids under nematode infestation were CML395/MP709, CML312/5057, CML312/CML206, CML312/CML444, CML395/CML312 and CML312/CML395.
7.2.5 S1 progeny recurrent selection
Two cycles of S1 progeny recurrent selection were used to improve the nematode resistance and grain yield of three OPVs (Longe 1, Longe 4 and ZM521).
• Results revealed that grain yield was negatively correlated with the nematode densities.
• The net gains in grain yield following the two cycles of selection were 6.3%, 10% and 22% for Longe 1, ZM521 and Longe 4, respectively. The average net gains in plant height and root mass were 6.3% and 44%, respectively for the three maize populations.
• The P. zeae densities reduced by 57%, 59% and 55%, and the Meloidogyne spp.
densities by 65%, 39% and 59% for Longe 1, Longe 4 and ZM521, respectively, after two cycles of selection.
• Net reductions in the number of root lesions were 16%, 28% and 40% for Longe 1, Longe 4 and ZM521, respectively, after two cycles of selection.
• Realized heritability (h2) for P. zeae and Meloidogyne spp.ranged from 66-96% at cycle 2. For grain yield, h2 ranged from 80-86% at cycle 2. Broad sense heritability (H2) for grain yield at cycle 2 ranged from 74-97% for the three maize populations.