Groundnut (Arachis hypogaea L.) is one of the most important legume crops in Malawi.
However, production among smallholder farmers has declined in recent years. One of the
constraints affecting groundnut production is groundnut rosette disease (GRD). Therefore,
the main objective of this study was to develop appropriate groundnut cultivars that are
resistant to GRD, combined with other traits preferred by farmers, in order to improve
income and food security of smallholder farmers in Malawi and beyond. The specific aims
were; (i) to assess groundnut cropping systems used by smallholder farmers in Malawi, their
varietal preferences, and production challenges (ii) to assess the genetic diversity among
groundnut germplasm collected from ICRISAT, the Chitedze gene bank and farmers (iii) to
identify sources of resistance to GRD and to its aphid vector (iv) and to understand the type
of gene action governing GRD resistance, and to identify groundnut genotypes suitable for
use as parents in breeding for GRD resistance.
Assessment of groundnut cropping systems used by smallholder farmers, their varietal
preferences, and production challenges was done by using a field survey and participatory
rural appraisal (PRA) tools. The field survey was done in Lilongwe, Mchinji and Salima while
the PRA was done in Kasungu, Lilongwe, and Salima. The assessment of genetic diversity
among 106 groundnut genotypes collected from ICRISAT, Chitedze gene bank and farmers
was done using 19 SSR markers. High throughput DNA extraction was done followed by
polymerase chain reactions (PCR) after which the amplified products were analyzed.
Evaluation of genotypes to identify new sources of resistance to GRD and its aphid vector
was conducted under two test situations, one with high inoculum levels and one with low
inoculum levels. Under high inoculum level, the infector row technique developed by Bock
and Nigam (1990) which employs a susceptible variety as a disease spreader was used.
While under low inoculum level, an aphid resistant variety instead of the infector row was
used to control the aphids. Aphid resistance was studied under field and glasshouse
conditions. Plants were planted in rows and at 14 DAS, 2 aphids were place on each plant.
Aphid resistance was determined by observing the increase in number of the aphid
population on the test plants. Gene action governing inheritance of resistance to GRD was
studied under high disease pressure created by using viruliferous aphids. Parents and F2
generations and their reciprocals were used in the study. The trials were laid out in a
glasshouse and aphids were infested a week after germination and were killed after 7 days
using Dimethoate. Disease data was collected at 7, 14, 21 and 28 days after aphid
infestation.
The study on groundnut cropping systems, varietal preferences and production challenges
revealed that most farmers grew groundnut alongside maize (Zea mayis L.) and beans
(Phaseolus vulgaris L.) as food crops and tobacco (Nicotiana tabacum L.) and cotton
(Gossypium hirsutum L.) as cash crops. The most preferred groundnut varieties grown by
farmers were Chalimbana and CG 7. GRD was observed in half of the fields visited.
However, 98% of the farmers interviwed had experienced it in their fields at some point, and
63.3% of the farmers believed that GRD was a major problem. Other challenges noted by
farmers included lack of quality seed, poor extension support, lack of inputs, manipulation of
the markets by buyers, and the failure of groundnut crops to meet the high standards
required by the market. The examination of genetic diversity among 106 groundnut
genotypes revealed a total number of 316 alleles with a mean of 17 alleles per locus.
Polymorphic information content (PIC) and gene diversity values were high, which indicated
that genetic diversity among the groundnut genotypes was high. The analysis of molecular
variance indicated that 72.9% of the genetic variation observed in the genotypes was due to
the variation between individuals within rather than between specific population groups. The
evaluation of genotypes for resistance to GRD revealed five highly resistant genotypes
namely ICG 9449, ICG 14705, ICGV-SM 05701, MW 2672 and MW 2694. Farmer preferred
genotypes were rated as either moderately resistant or susceptible to GRD. Aphid resistance
was only recorded in ICG 12991. Yield and GRD incidence were negatively and moderately
correlated, which confirmed that GRD has the potential to reduce yield in groundnuts. The
highly resistant genotypes were also high yielding except for genotype ICG 9449. Farmer
preferred genotypes CG 7, Chalimbana and Tchayilosi, also gave above average yields,
despite high disease incidence levels, which showed that these genotypes have tolerance to
GRD. The study on gene action governing GRD resistance revealed information on
combining ability effects of GRD resistance. The diallel analysis showed that GCA, SCA,
reciprocal, maternal and non-maternal effects were all significant, which indicated that both
additive and non-additive gene effects played a role in governing GRD resistance. The
significance of SCA and reciprocal effects indicated that maternal parents played an
important role in the expression of GRD resistance. However, the additive effects were
predominant over non-additive gene effects. Four of the resistant genotypes, ICG 14705,
MW 2694, ICGV-SM 05701, and MW 2672, were the best combiners for GRD resistance.
Generally, the study indicates that there is still a need to develop new varieties with
resistance to GRD having traits preferred by farmers to enhance adoption. There is also a
need for breeders to work with extension staff in promoting new varieties and also there is
need for extension staff to actively provide information to farmers on production and
marketing of groundnut. Groundnut is widely known to have a narrow genetic base which
has been a bottleneck to its improvement. However, the high genetic diversity observed in
this study provides a basis for selection of appropriate parental genotypes for breeding
programmes which can enhance further the broadening of the groundnut genetic base.
Identification of the genotypes with high resistance to GRD in this study provides an
opportunity to breed more GRD resistant materials. The observation that additive gene
effects are predominant in governing GRD resistance means that GRD resistant materials
can be improved by introgressing additive genes using recurrent selection breeding
procedures. There is also a need to employ molecular techniques which can help in
shortening the entire breeding process. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10055 |
| Date | 21 November 2013 |
| Creators | Chintu, Justus M.M. |
| Contributors | Githri, Mwangi., Laing, Mark D., Sibiya, Julia., Monyo, Emmanuel. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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