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. 2011 Sep-Dec;43(3-4):209–219.

Grain Yield and Heterosis of Maize Hybrids under Nematode Infested and Nematicide Treated Conditions

Frank Kagoda 1,2, John Derera 1, Pangirayi Tongoona 1, Daniel L Coyne 2, Herbert L Talwana 3
PMCID: PMC3547356  PMID: 23429435

Abstract

Plant-parasitic nematodes are present on maize but resistant genotypes have not been identified in Uganda. This study was aimed at determining the level of nematode resistance among F1 hybrids, and to estimate grain yield, heterosis and yield losses associated with maize hybrids under nematode infestation. The 30 F1 hybrids and two local checks were evaluated in a split plot design with nematode treatment (nematode infested versus nematicide treated) as the whole plot factor, and the hybrids as subplot factors arranged in an 8 x 4 alpha-lattice design. The experiment was conducted simultaneously at three sites. The hybrids were also evaluated in a split plot design under greenhouse conditions at IITA-Namulonge. Results revealed 24 P. zeae susceptible hybrids compared to only six P. zeae resistant hybrids. Grain yield across sites was higher by about 400 kg ha-1 under nematicide treatment than under nematode infestation. The nematode tolerant/resistant hybrids exhibited yields ranging from 5.0 to 8.4 t ha-1 compared to 5.0 t ha-1 obtained from the best check. Grain yield loss was up to 28% among susceptible hybrids, indicating substantial economic yield losses due to nematodes. Under field conditions, desired heterosis was recorded on 18 hybrids for P. zeae, and 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. Overall, most outstanding hybrids under nematode infestation were CML395/MP709, CML312/5057, CML312/CML206, CML312/CML444, CML395/CML312 and CML312/CML395. Therefore, grain yield loss due to nematodes is existent but can be significantly reduced by growing nematode resistant hybrids.

Keywords: Grain yield, heterosis, Maize hybrid, Meloidogyne spp., Pratylenchus zeae, Yield loss

Maize is the most important cereal crop and the second most important food crop after cassava in Africa (DeVries and Toenniessen, 2001; FAOSTAT, 2009) but mostly grown by small-scale farmers, who lack inputs such as fertilizer, chemicals, improved seed, irrigation and labor (Infonet-Biovision, 2009). Consequently, yields barely exceed 1.8 t ha-1 (FAOSTAT, 2009). Pests and diseases are indicated as the most important constraint to maize production among small-scale farmers in East and Southern Uganda (Kagoda et al., 2010a). According to Imanywoha et al. (2005), maize yields have remained low in Uganda because some production constraints have not been addressed in the development of improved cultivars except the key biotic stresses such as turcicum leaf blight (TLB), maize streak virus (MSV), stem borers and weevils. Plant-parasitic nematodes are such constraints which have not been addressed for maize in Uganda and many other African countries.

Over 60 nematode species have been associated with maize (Jones and Perry, 2004; McDonald and Nicol, 2005) across the globe. In Uganda, the nematodes Pratylenchus zeae and Meloidogyne spp. are the most serious root pests of maize (Talwana et al., 2008; Kagoda et al., 2010a), and have potential to cause economic yield losses. Though nematode control options such as the use of nematicides, crop rotation, and bare fallow are effective, they are often inappropriate on a low value crop such as maize (Sikora, 1992). The use of host plant resistance as a nematode control option is cost-effective provided that resistance genes are readily available (Trudgill, 1991). Presence of resistance in maize to nematodes has been demonstrated, for example, to P. zeae (Kimenju et al., 1998; Oyekanmi et al., 2007) and Meloidogyne spp. (Windham and Williams, 1987; Windham and Williams, 1988; Windham and Williams, 1994a). However, the challenge is deploying the resistance trait into commonly acceptable and grown cultivars. The objectives of our study therefore, were to: i) determine the level of nematode resistance in maize hybrids ii) estimate heterosis for nematode resistance in maize, and iii) quantify the grain yield loss associated with nematode infestation in maize.

Materials and Methods

Germplasm: A total of 30 F1 hybrids (including reciprocals) developed from a 6 x 6 full diallel mating design at the International Institute of Tropical Agriculture (IITA) – Namulonge, Uganda were used. The hybrids were developed from four CIMMYT inbred lines namely CML206, CML312, CML395 and CML444, known for their adaptability to maize growing conditions in Uganda (CIMMYT, 2001); one inbred line (MP709) from USDA-ARI Corn Host Plant Resistance Research Unit, Mississippi State known for being resistant to Meloidogyne spp. (Williams and Windham, 1998); and an inbred line (5057) from IITA-Nigeria known for its resistance to P. zeae (Oyekanmi et al., 2007). A nematode susceptible (H614D) and resistant check (DK8031) (Kimenju et al., 1998; Kagoda et al., 2010a), which are known for their adaptability to maize growing conditions in Uganda were included in the evaluation trials.

Evaluation of the maize hybrids under field conditions: Field evaluations were conducted at three sites in Uganda, namely Namulonge (Central Uganda; 1200 masl; 0°32’N, 32°34’E), Bufulubi (Eastern Uganda; 1130 masl; 00° 49’ N, 033° 42’ E) and Kabanyolo (Central Uganda; 1,150 masl; 0° 28’N, 32° 37’E), which are characterized by greater than 40% sandy soils (Kagoda, 2010) and represent major maize growing areas in Uganda (MAAIF, 2011). Fields under natural infestation with nematodes were used for the evaluation trials. Where nematode initial populations (Pi) were low (< 500 P. zeae and < 100 Meloidogyne spp. per 100 g of soil), in the nematode unprotected plots, chopped nematode infested maize roots were applied per plant in the affected block but this only occurred in the Kabanyolo trial 2009B season. The genotypes for evaluation in the different sites constituted the 30 F1 hybrids and the two local checks, DK8031 and H614D. The hybrids were evaluated in a split plot design with nematode treatments (nematode infested versus nematicide treated) as whole plots and the hybrids as subplots with two replications at Namulonge and three replications at Kabanyolo and Bufulubi. The hybrids were arranged in an 8 x 4 spatially adjusted alpha (0,1) lattice design for each of the nematode treatments. Field inter- and intra-row spacing was maintained at 75 cm x 30 cm. Two-row plots were planted per genotype consisting of 16 plants each. Two seeds were planted per hill and later thinned to one plant. Other standard cultural practices such as hand weeding were implemented at all the sites. Fertilizer to boost growth was applied as Di-ammonium phosphate (DAP) at planting at a rate of 7.5 kg N ha-1 and 19.2 kg P2O5 ha-1. Fenamiphos (nemacur™), a non-volatile nematicide, was applied at a rate of 2.5 kg ha-1 (≈ 2.3 g per plant) and incorporated 5 – 8 cm soil depth with a hand hoe prior to planting in the nematode protected plots (Rhoades, 1979; Taylor et al., 1999).

Evaluation of the maize hybrids under greenhouse conditions: Additionally, the 30 F1 maize hybrids and the two local checks were planted in plastic pots of 15 cm diameter containing 2500 ml of a potting mixture of heat-sterilized sandy loam soil and river sand (2:1) in a greenhouse. Two maize seeds were planted in each plastic pot, and for each maize hybrid, 12 pots were planted. The pots were arranged in a split plot design with two replications; the main plot factors were nematode treatments (nematode inoculated and non-inoculated) and the sub-plot factors were the 32 maize hybrids. The pots were placed on metallic mesh tables about 1 m from the ground to avoid contamination. Pots were watered twice a week with 0.5 litres each time. After 10 days, seedlings were thinned to one per pot and inoculated with 5000 P. zeae mixed stages or a mixture of 5000 Meloidogyne spp. juveniles and eggs (Pi).

Pratylenchus zeae and Meloidogyne spp. inoculum preparation: Pratylenchus zeae used for inoculation were initially extracted using the modified Baermann sieve method (Coyne et al., 2007) from infected maize roots, obtained from farmers’ fields in Iganga District, Uganda. The P. zeae were multiplied on carrots (Daucus carota L.), cv. Nantes in the laboratory (Kagoda et al., 2010b) and maintained on susceptible maize hybrid H614D in pots under a shade house at IITA, Namulonge. Meloidogyne spp. juveniles and eggs were collected from galled tomato roots using a method described by Hussey and Barker (1973). The tomato plants were also maintained in pots under a shade house.

Quantification of nematode densities and assessment of root damage: For the field experiments, soil samples per plot were collected for nematode (vermiform) population (Pi) counting by species shortly before planting. The soil samples were collected in each plot using a trowel to a depth of 15 cm, discarding the upper 5 cm (Todd and Oakley, 1996; Coyne et al., 2006). Ten soil sub-samples per plot were combined to form one sample. From 50% flowering, root samples were taken from the root system of 10 randomly selected plants in each plot for final nematode (Pf) assessment in the field. In the greenhouse, all plants were uprooted at flowering stage (≈ 60 days after planting) for nematode assessment. At this stage, the nematodes were expected to have completed two generations (Dropkin, 1989).

In the laboratory, nematodes were extracted from a 100 ml soil sub-sample (Pi), and from a macerated 5 g fresh root mass (frm) sub-sample (Pf), using the modified Baermann sieve method. The samples were examined after a 48-hour extraction period, and nematodes counted using a stereomicroscope. Both Pi and Pf were estimated from three x 2-ml aliquots (taken from a 25-ml suspension). Therefore, Pf – Pi refers to the nematode populations present in the roots after subtracting the initial populations in the soil in that plot at the time of planting. In the greenhouse, Oostenbrink’s (1966) reproduction factor (RF), calculated as Pf/Pi, was used to assess resistance to nematodes with RF ≤ 1.5 indicating resistance to nematodes, 1.5 < RF ≤ 2.0 indicating moderately resistant, 10 ≥ RF > 2.0 indicating susceptibility and RF > 10 indicating very susceptible to nematodes (Ferris et al., 1993).

Assessment of yield and other agronomic traits: In the field, plant height was recorded at 100% flowering as described by Magorokosho et al. (2007). Grain yield was recorded per plot at harvest and adjusted to 12.5% moisture (CIMMYT, 1985) using the formula:

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A tolerance index (TI) for each genotype was calculated by comparing yield from nematode infested plots with yield from nematicide treated plots using the formula: (Yield of nematode infested plot/yield of nematicide treated plot) x 100. Specifically, TI < 100 represents least tolerant hybrids whereas TI ≥ 100 represents most nematode tolerant hybrids. Percentage yield loss was calculated as:

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Root damage was assessed from fresh root mass and the number of root lesions on root pieces in a 5 g root sample. Plant growth parameter assessment in the greenhouse was similar to that described for the field experiments.

Statistical analysis: Data from the field and greenhouse trials were tested for normality using the Proc Univariate normal plot procedure in SAS statistical package. Log and square root transformations were used where appropriate to transform the data prior to analysis. The nematode densities were log(x+103) transformed whereas grain yield was sqrt(x) transformed. The data were then subjected to analysis of variance as split plot experiments using the General Linear Model (Proc GLM) in SAS statistical package to enable separation of the variance components (Steel and Torrie, 1980). Differences between means were compared using Tukey’s studentized range test at P = 0.05. Models used for analysing data followed procedures laid down for split plot designs (Steel and Torrie, 1980).

Pearson correlation and regression analyses were run using Proc corr and Proc reg procedures in SAS, respectively, to determine the type of relationships among traits. Heterosis (hybrid vigor) was computed for P. zeae under greenhouse and field conditions, and for Meloidogyne spp. under field conditions. Mid-parent heterosis (MPH) for nematode resistance were calculated as the performance of the F1 hybrid compared with the average performance of its parents (Falconer, 1981; Srivastava, 1991). The formula used was: [(F1 – MP)/MP] x 100, where F1 = mean of the F1 hybrid performance, MP = mean of the two parents of the cross, i.e., (P1 + P2)/2, where P1 and P2 are the means of the inbred parents. To ascertain any differences in vigour between pairs of reciprocal hybrids, a t-test of significance was carried out on mid-parent heterosis values obtained per replicate for P. zeae and Meloidogyne spp. The hypothesized mean difference between reciprocals was zero.

Relative yield (standard heterosis) was calculated using the formula:

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Ranking of hybrids based on grain yield was performed in Microsoft excel using the sort & filter procedure. Spearman rank correlation was then run using Proc corr procedure in SAS software to determine the differences between ranks of hybrids under nematode infestation and nematicide treatment.

Results

Analysis of variance of the various maize traits: Site effects had variations (P ≤ 0.05) for plant height, root mass and grain yield (Table 1). Nematode treatments (Nematode infested versus Nematicide treated conditions) were different (P ≤ 0.05) for only P. zeae and Meloidogyne spp. densities. The hybrids (including reciprocals) were different (P ≤ 0.05) for all traits measured except number of root lesions. Site x Hybrid interactions were different (P ≤ 0.05) for number of root lesions and grain yield.

Table 1.

Mean squares for the various maize traits

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Performance of maize hybrids across sites and treatments: Plant height among the hybrids was higher (P < 0.05) in five hybrids but lowest in four hybrids including the resistant check (Table 2). Four hybrids had the highest root mass, whereas five hybrids including both checks displayed the lowest root mass. The P. zeae densities were lower (P < 0.05) in five hybrids, including the resistant check, but highest in eight hybrids. Grain yield was higher (7.0 – 8.4 t ha-1) in hybrids CML444/MP709, CML395/5057, CML312/CML206, CML444/CML395, CML444/CML312, CML312/CML444, CML395/CML312 and CML312/CML395 compared to only 4.7 to 5.0 t ha-1 obtained in MP709/5057, 5057/MP709, MP709/CML206, CML206/5057 and DK8031.

Table 2.

Performance of individual F1 hybrids across sites and treatments

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Yield losses and tolerance indices: At Bufulubi, significant differences in grain yield were recorded among hybrids under nematode infestation (Table 3). Yield loss ranged from 1 to 37% with highest loss observed in hybrids MP709/5057 and CML206/CML444. However, a total of 11 hybrids were tolerant (TI ≥100) to nematodes. At Kabanyolo, grain yield was higher (P < 0.05) than the mean of the trial in 14 hybrids, but lowest in only two hybrids (MP709/5057 and MP709/CML206) under nematode infestation. The yield losses at Kabanyolo ranged from 1 to 33% with the highest loss recorded in the hybrid MP709/5057. A total of six hybrids, including the resistant check, exhibited tolerance (TI ≥ 100) to nematodes. At Namulonge, no significant variation in grain yield was recorded under nematode infestation. Under nematicide treatment, the highest grain yield was recorded in CML312/CML444, CML206/CML444, CML395/5057, whereas the lowest grain yield was recorded in the hybrid check H614D and MP709/5057. Grain yield loss ranged from 1 to 66% with hybrid CML206/5057 having the highest loss. Tolerance was recorded in 14 hybrids.

Table 3.

Grain yield, tolerance index (TI) and yield losses of individual hybrids and their reciprocals

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Across sites, mean grain yield was higher (P < 0.05) under nematicide treated plots than nematode infested plots. Among hybrids, grain yield was higher (P < 0.05) in seven hybrids and lowest in only three hybrids under nematode infestation. The same hybrids with higher grain yield under nematode infestation maintained high yields under nematicide treatment. Hybrids with the highest TI recorded no grain yield loss under nematode infestation. The most nematode tolerant (TI > 110) hybrids were CML206/CML395, CML206/MP709 and CML395/CML312 compared to a TI of 85 obtained for the susceptible check. Overall, yield loss ranged from 1 to 28% with hybrids CML206/CML444 and MP709/CML395 exhibiting the highest yield loss.

Pearson correlations between grain yield and other traits: Under nematode infestation, grain yield was positive and correlated (P < 0.001) with plant height and root mass (Table 4). However, grain yield was negatively correlated (P < 0.001) with number of root lesions, and negative but non-significantly correlated with P. zeae and Meloidogyne spp. densities. Under nematicide treatment, grain yield displayed a positive correlation (P < 0.001) with plant height, root mass and number of root lesions, but maintained negative and non-significant correlations with P. zeae and Meloidogyne spp. densities. Pratylenchus zeae densities were negative and correlated (P < 0.05) with plant height and root mass under nematode infestation but had a positive correlation (P < 0.05) with Meloidogyne spp. densities. Under nematicide treatment, P. zeae had a positive correlation (P < 0.001) with root mass, number of root lesions and Meloidogyne spp. densities but had a negative correlation with plant height. Meloidogyne spp. densities displayed a negative correlation (P < 0.05) with plant height and number of root lesions under nematode infestation.

Table 4.

Pearson correlation coefficients based on pooled data across the three sites

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Linear regression analysis: Plant height and root mass had a positive regression coefficient (P < 0.001) with grain yield under both nematode infested and nematicide treated plots (data not shown). The number of root lesions, P. zeae densities and Meloidogyne spp. densities had a negative and non-significant regression coefficient with grain yield.

Response of the hybrids to P. zeae infection in the greenhouse: Root mass was significantly higher (23 g) in the uninoculated pots than in the P. zeae inoculated pots (22 g) (data not shown). Mean P. zeae density was 44007 per 100 g frm in the inoculated pots whereas the uninoculated pots had no P. zeae (Table 5). Similarly, mean RF in the inoculated pots was 8.8. Based on RF, only five hybrids (including the resistant check) displayed resistance to P. zeae, whereas two hybrids were moderately resistant. The most resistant hybrids were 5057/MP709, 5057/CML444, CML206/CML312, CML395/CML312.

Table 5.

Hybrid performance in pots inoculated with P. zeae in the greenhouse

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Hybrids 5057/MP709 and 5057/CML444 were among the most P. zeae resistant in the greenhouse. They also performed well in the field with grain yields of 5.0 and 6.6 t ha-1, respectively, despite being mostly from exotic parents. Similarly, the hybrid CML395/CML312 exhibited P. zeae resistance in the greenhouse and yielded well in the field with 8.4 t ha-1. Hybrids MP709/CML312 and CML312/CML206 were relatively resistant to P. zeae in the greenhouse and in the field resulting in higher grain yields (6.3 and 7.2 t ha-1, respectively) compared to the resistant check (5.0 t ha-1).

Relative yield and heterosis of the maize hybrids: Negative heterosis for P. zeae and Meloidogyne spp. is an indication of an F1 hybrid which is superior in resistance to nematodes compared to the mid-parent, susceptible parent or resistant parent. A total of 14 hybrids displayed negative heterosis for P. zeae resistance, whereas 16 hybrids had positive heterosis under greenhouse conditions (Table 6). Differences between reciprocals were significant (P < 0.05) for MP709/5057 and CML444/CML312. Under field conditions, negative heterosis for P. zeae was recorded on 18 hybrids, whereas 12 hybrids had positive heterosis (Table 7). For Meloidogyne spp., negative heterosis was recorded on three hybrids, whereas 27 hybrids displayed positive heterosis. No significant reciprocal differences were observed for both P. zeae and Meloidogyne spp. densities in all the hybrids.

Table 6.

Heterosis for resistance to Pratylenchus zeae under greenhouse conditions

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Table 7.

Heterosis for resistance to P. zeae and Meloidogyne spp. under field conditions across the three sites

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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 (standard heterosis) 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 under stressed and non-stressed environments (Table 8). Spearman rank correlation (Table 8) showed a change in rank order in grain yield in most of the hybrids under nematode infestation when compared to nematicide treated plots (r = 0.636; P = 0.0002).

Table 8.

Relative yield of hybrids and their rank under nematode infestation and nematicide treated plots across the three sites

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Discussion

The study revealed variations in plant height, root mass and grain yield of hybrids between sites. The site x hybrid interaction observed for grain yield is an indicator of the differences in adaptability of the hybrids regardless of nematode infestation levels. The site x hybrid interaction effects recorded for number of root lesions could be explained by the different levels of P. zeae damage at the different sites. Traits such as plant height and root mass were generally higher under nematicide treatment than under nematode infested plots at all sites. These traits are known to improve once nematode populations are very low in most crops. Hybrids which were taller did not necessarily have significantly lower P. zeae populations except for MP709/CML206. Therefore, taller plants are not necessarily nematode free, which justifies the need to assess nematode densities. However, the high P. zeae densities recorded in stunted plants confirms reports that nematodes restrain plant growth. These results are consistent with previous observations (Kimenju et al., 1998; Patel et al., 2002; Luc et al., 2005). Meloidogyne spp. densities were quite low in most of these hybrids compared to P. zeae densities. This confirms earlier findings that P. zeae is more aggressive on maize than Meloidogyne spp. in Uganda (Talwana et al., 2008; Kagoda et al., 2010a). According to Olowe and Corbett (1976), P. zeae has a higher reproductive rate and tolerance to environmentally related stress compared to other nematode species, thus the high densities recorded in the current study. Hybrids with the highest root mass also had relatively lower P. zeae densities (< 6000 P. zeae per 100 g frm) and their yields exceeded 6.0 t ha-1, indicating that these hybrids were resistant to nematodes. Patel et al. (2002) recorded considerable reduction in root mass and an almost ten-fold increase in P. zeae densities in maize inoculated with P. zeae indicating high damage potential especially on susceptible varieties. Kimenju et al. (1998) similarly observed nematodes to cause significant reductions in root mass of maize open pollinated varieties and hybrids. Similarly, P. zeae has been reported to limit root growth and eventual yield in rice (Oryza sativa) (Prot and Savary, 1993). Hybrids which had a relatively lower number of root lesions also exhibited lower P. zeae densities, which confirms the positive correlation obtained between root lesions and P. zeae densities. Presence of root lesions is characteristic of damage by root lesion nematodes. These results are consistent with previous observations (Olowe, 1977; Norton and Nyvall, 1999).

More nematodes were recorded at Bufulubi than at other sites. This is probably because sandy soils were more predominant in the experimental site at Bufulubi (61.1%) than the rest (41 - 49%) of the experimental sites (Kagoda, 2010). Both P. zeae and Meloidogyne spp. proliferate more in sandy soils than other soil types (Norton, 1978; Dropkin, 1989). However, yield losses due to nematodes manifested more at Namulonge (9.6%) than at Bufulubi (4.8%) probably because of maize being more adapted in Eastern Uganda (Bufulubi inclusive) than in the central region (Namulonge) (NARO, 2002).

Hybrids such as CML206/CML395 and its reciprocal; MP709/CML206 and CML395/MP709 had lower P. zeae populations compared to the resistant check (DK8031). Such hybrids are characterized by penetration of fewer P. zeae, delayed egg laying and nematode reproduction, less root necrosis and cell wall thickening around the parasitic zone (Kathiresan and Mehta, 2002). Relatively lower Meloidogyne spp. densities were observed in some hybrids with CML206 or MP709 constituting the parental combination, when compared to the resistant check. This is because genotypes CML206 and MP709 possess genes for resistance to nematodes (Williams and Windham, 1998; Kagoda, 2010). Root-knot resistance is characterized by slow nematode development or no development when compared with susceptible hosts (Lawrence and Clark, 1986; Windham and Williams, 1994b).

Grain yield across sites was higher by about 400 kg ha-1 under nematicide treated plots when compared to the nematode infested plots, a clear indication that nematodes are associated with yield loss in maize. Similarly, yield losses due to nematode damage among hybrids rose to 28% compared to a yield loss of 15% in the susceptible cultivar (H614D) across sites. The tolerance index was below 100% in hybrids which registered yield losses, which indicates that nematodes played a significant role in reducing grain yield in such hybrids. The nematode resistant/tolerant CIMMYT lines such as CML444, CML395 and CML206 are adapted to the sub-tropical conditions in Uganda, and greatly influenced high grain yields compared to inbreds MP709 and 5057 which are exotic despite being nematode resistant. However, the hybrid CML395/MP709 had low nematode densities and a grain yield of 7.0 t ha-1 under nematode infestation signifying its adaptability to the environment.

The negative correlations and regression coefficients observed between grain yield and number of root lesions, grain yield and P. zeae densities, and grain yield and Meloidogyne spp. densities are evidence that nematodes are associated with reduced grain yield in susceptible maize cultivars. Similarly, Tarte (1971) found a highly significant negative correlation between P. zeae densities and yield of maize. However, even under nematicide treatment, negative correlations were observed between grain yield and the low nematode densities. This calls for use of management practices which completely give the maize plant a comparative advantage over the nematodes such as breeding for resistant varieties.

Hybrids MP709/CML312 and CML395/CML312 maintained the highest plant heights in the greenhouse and in the field. These hybrids were, therefore, tolerant to nematodes. According to Begna et al. (2000), taller hybrids produce a higher dry matter yield but the translocation rate of assimilates to the kernels of taller hybrids is lower than for shorter hybrids. This implies that breeding for maize varieties with short stature but with resistance to nematodes and other stresses offers higher grain yields than the tall varieties. The difference in root mass in the uninoculated pots compared to the P. zeae inoculated pots can be attributed to feeding of P. zeae on the root system of the P. zeae inoculated plants. However, the difference in root mass between the P. zeae inoculated and uninoculated pots was by a small margin. This is because roots sometimes proliferate at a higher rate to absorb nitrogen in the subsoil depleted by nematodes in the top soils leading to high root mass in nematode infested plots than the non-infested (Evans, 1982; Haverkort et al., 1994). Mean P. zeae density was 44007 per 100 g frm in the inoculated pots, which indicates that the nematodes increased by 8.8-fold in the two months the experiment was conducted. However, the five most resistant hybrids had P. zeae densities far below the mean (< 6000 P. zeae per 100 g frm) and RF < 1.5. This demonstrates that a nematode resistant hybrid should have the capacity to reduce entry and rapid multiplication of nematodes in its root system. According to Kathiresan and Mehta (2002), nematode penetration in resistant crops is reduced by mechanical and biochemical barriers present in the plant. A number of P. zeae resistant hybrids in the greenhouse trial recorded high grain yields compared to the resistant check when planted in the field. This indicates that greenhouse data was quite reliable in explaining performance in the field. Similar observations were reported by Speijer and De Waele (1997).

Inbred lines MP709, 5057, CML206, and CML444 evidently had genes for P. zeae resistance and tolerance (Kagoda, 2010), which explains the negative heterosis observed in their hybrid combinations. Likewise, field and greenhouse evaluation provided evidence that inbred lines MP709, 5057 and CML444 have genes for resistance to Meloidogyne spp. (Kagoda, 2010). Notably, these are dominant or epistatic genes for P. zeae and Meloidogyne spp. resistance since dominance and epistasis are the underlying genetic basis for heterosis (Falconer, 1981). According to Cromley et al. (2002), single cross hybrids would have adequate level of resistance if at least one parent has resistance.

The wide range in relative yield among hybrids under nematode infestation than under nematicide treatment suggests that the yield benefit of hybrid vigour declines sharply among nematode susceptible maize hybrids compared to nematode resistant hybrids. The change in rank order for grain yield observed in most of the hybrids, based on spearman correlation, under nematode infestation when compared to nematicide treated plots confirms that hybrid performance can be affected by the presence of nematodes.

To sum up, 1) there is a considerable improvement in grain yield (≈ 400 kg ha-1) when nematicides are used against nematodes in maize; (2) a grain yield loss of up to 28% can be obtained due to nematode infestation in maize, which seriously compromises yield when susceptible varieties are grown; 3) desired heterosis for P. zeae was recorded on 60% of the maize hybrids, which suggests that such hybrids are good sources of resistance to P. zeae; 4) hybrids such as CML395/MP709, CML312/5057, CML312/CML206, CML312/CML444, CML395/CML312 and CML312/CML395 would be recommended for advancement in breeding programs since they exhibited high levels of either nematode tolerance or resistance by displaying high grain yields, high mid-parent heterosis to P. zeae and high relative yields compared to the checks and the trial mean under nematode pressure. These hybrids would be advanced by evaluating them for adaptability across more environments before release.

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