Genetic studies and recurrent selection for nematode resistance in maize.
Plant-parasitic nematodes cause grain yield loss in maize. The most important genera of plant-parasitic nematodes demonstrated to be of economic importance to maize are Pratylenchus spp., Meloidogyne spp. and Heterodera spp. In Uganda, the most prevalent species are Pratylenchus zeae and Meloidogyne spp. The current study was initiated with the following objectives: (i) assessing farmers’ awareness of maize nematodes, other maize production constraints, and desirable agronomic traits; (ii) assessing the efficiency of sterile carrot discs for mass culturing of Pratylenchus zeae; (iii) Characterising the inheritance of nematode resistance in maize, through, estimation of the general combining ability (GCA) of various parents, and the specific combining ability (SCA) of a parent in a cross with another parent; and through determining the contribution of cytoplasmic effects to inheritance of resistance to nematodes; and (iv) determine the level of nematode resistance among F1 hybrids and estimate grain yield, heterosis and yield losses associated with maize hybrids under nematode infestation; (v) comparing the gains in nematode resistance and grain yield obtained following two cycles of S1 progeny recurrent selection in three tropical maize populations. In the participatory rural appraisal, data were collected from 120 households in two maize growing districts. Maize roots and soil samples were also collected from farmers’ fields, and nematode incidence determined. A small percentage (18.5%) of farmers was familiar with nematodes and the damage they cause in maize. Pratylenchus zeae occurred at generally higher frequencies than Meloidogyne spp. in susceptible cultivars. The landraces and the cultivar Longe 5 supported high nematode populations. Farmers also reported that Longe 5 had low yields when compared to the rest of the cultivars. Farmers’ most preferred traits were pest and disease resistance, high grain palatability, long storage duration and large kernels. These findings justify the need for a programme to raise farmers’ awareness on nematodes, their effects on crops as well as control strategies, and also a breeding programme that incorporates nematode resistance with farmer-preferred characteristics in maize. 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 had reproduced 5,090 times after three months of incubation compared to a reproduction rate of 26.4 on excised maize roots. Carrot discs are therefore particularly useful for culturing P. zeae. Thirty F1 hybrids generated from a 6 x 6 diallel and two local checks were evaluated in three sites in an 8 x 4 alpha-lattice design in order to estimate GCA, SCA and genetic effects associated with nematode resistance in maize. The evaluations were done under nematode infestation and nematicide treated conditions. The nematode infested plots comprised an average Pi of 500 P. zeae and 100 Meloidogyne spp. per 100 g of soil per plot and lesser populations of other nematode species in the field trials. The GCA was more important for the reduction of P. zeae and Meloidogyne spp. densities and an increase in root mass, with a contribution of 72 to 93% of the phenotypic variance to these traits. Inbreds MP709 and CML206 had the highest GCA for P. zeae resistance, whereas for grain yield, CML444, CML312 and CML395 had the highest GCA. The SCA was important for heterosis in plant height and grain yield, contributing 43% and 58% of the phenotypic variance, respectively, under nematode infestation. Hybrids MP709/CML444 and MP709/CML395 had significant negative reciprocal effects for grain yield resulting from the negative maternal effects observed in parent MP709 when used as the female parent under nematode infestation. Using the graphical approach of the Hayman and Jinks analysis of genetic effects, overdominance gene action explained the non-additive variance observed for plant height, grain yield, number of root lesions, P. zeae and Meloidogyne spp. densities recorded under nematode infestation. Parents MP709, CML206, 5057 and CML444 contributed most of the dominant genes for P. zeae resistance. Parent CML444 contributed most of the dominant genes towards improved grain yield. The high GCA effects among some parents in the different sites suggest that breeding of widely adapted nematode resistant cultivars is possible. Whereas a preponderance of dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure. The 30 F1 hybrids generated from the diallel cross were further assessed for nematode resistance, grain yield, heterosis and yield losses under nematode infestation and nematicide treated conditions. Results revealed more (24) P. zeae susceptible hybrids and a few (six) resistant hybrids. Grain yield across locations was higher by about 400 kg ha-1 under nematicide treated plots than under nematode infestation. Under both nematode infested and nematicide treated plots, the nematode resistant hybrids exhibited high yields ranging from 5.0 to 8.4 t ha-1 compared to 5.0 t ha-1 of the best check. Grain yield loss ranged between 1 and 28% among susceptible hybrids, and up to 12% among resistant hybrids, indicating that nematodes can cause economic yield losses especially when susceptible cultivars are grown. Under field conditions, favourable heterosis was recorded on 18 hybrids for P. zeae, and only on 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, indicating stability of performance between stressed and non-stressed environment. In general, hybrids with the most outstanding performance under nematode infestation were CML395/MP709, CML312/5057, CML312/CML206, CML312/CML444, CML395/CML312 and CML312/CML395. Therefore, grain yield loss due to nematodes can be reduced by growing nematode resistant hybrids. Two cycles of S1 progeny recurrent selection were used to improve nematode resistance and grain yield of three tropical open pollinated varieties (Longe 1, Longe 4 and ZM521). The net gains in grain yield after the two cycles of selection were 6.3%, 10% and 22% for Longe 1, ZM521 and Longe 4, respectively. Each cycle of selection for nematode resistance improved grain yield by 200 to 600 kg ha-1 in the three maize populations. The damage caused by P. zeae reduced by 57%, 59% and 55%, and the Meloidogyne spp. by 65%, 39% and 59% for Longe 1, Longe 4 and ZM521, respectively, following the 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. Therefore, the two cycles of S1 progeny recurrent selection improved grain yield in the three maize populations through reduction of nematode densities.
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