Abstract:

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 F,
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 St progeny recurrent selection in three tropical maize
populations.
In the participatory rural appraisal, data were collected from 120 households in two maizegrowing
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 + W. 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 F, 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 ppreponderance of dominant genes and SCA effects would favour
pedigree and various sib tests to improve grain yield under nematode pressure.
The 30 Ft 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 Si 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 (hz) 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 S! progeny recurrent selection improved grain yield in the three maize
populations through reduction of nematode densities.