Aphid resistance was determined by observing the increase in number of the aphid population on the test plants. Identification of the genotypes with high resistance to GRD in this study provides an opportunity to breed more GRD resistant materials.
Background
Importance of groundnut in Malawi
Groundnut production trends in Malawi
Challenges of groundnut production in Malawi
Declining soil fertility due to poor crop management practices and low fertilizer use has also become a major challenge for the groundnut industry in Malawi (Minde et al., 2008). Other factors, such as the loss of important markets due to poor nut quality due to aflatoxins, and the lack of an organized system for seed production and supply, have also limited the expansion of groundnut production (Siambi and Kapewa, 2004).
Research justification
Goal of the research
2006) Highlights, International Crops Research Institute for Semi-Arid Tropics, ICRISAT, Eastern and Southern Africa Region, PO Box 39063, Nairobi, Kenya. 2008) Assessment of the Current Situation and Future Prospects of the Groundnut Subsector in Malawi, International Crops Research Institute for Semi Arid Tropics, Patancheru, India.
Introduction
Origin, distribution and taxonomy of groundnut
- Botany
The groundnut leaves are usually four-leaf and arranged alternately on the stems, but the subsp. The groundnut plant produces flowers within four to six weeks of emergence, until late in the growing season, depending on the genotype and environment (Shokes and Melouk, 1995; Stalker, 1997).
Production, uses and economic importance of groundnut
Groundnuts also provide cash to poor farmers in developing countries in Asia and sub-Saharan Africa, thus contributing significantly to food security and poverty reduction (Naidu et al., 1999). It is also relatively drought tolerant (Stalker, 1997) and thrives well despite minimal inputs, making it suitable for low-input agriculture by smallholder farmers in sub-Saharan Africa (Naidu et al., 1999).
Constraints to groundnut production
- Groundnut rosette disease (GRD)
- Disease distribution
- Disease symptoms
- Disease diagnosis
- Disease epidemiology
- Disease transmission
- Management of GRD
- Breeding for resistance to GRD and its vector
- Sources of resistance and breeding methods
- Screening techniques for GRD resistance
In experiments, GRV can also be transmitted by inoculation and mechanical inoculation (Waliyar et al., 2007). This meant that there was still a need to breed short-duration, GRD-resistant varieties (Naidu et al., 1998).
Genotype by Environment interaction
Screening for resistance to the aphid vector promises to be beneficial for GRD resistance breeding programs. Resistance is determined by the effect of the plant on the aphid physiological aspects such as instar development, reduced survival, lower body weight and reduced fertility of adult aphids.
Summary
1990) Different satellite RNA variants of peanut rosette virus are responsible for chlorotic and green forms of peanut rosette disease. Effect of peanut rosette helper virus on agronomic performance of peanut (Arachis hypogaea L.) genotypes.
Introduction
Lack of access to sufficient quantities of improved seed causes farmers to use low-yielding varieties and plant recycled grains as seed, reducing peanut productivity (Simtowe et al., 2009). For example, farmers in Ghana indicated that resistance to GRD was their most preferred trait in improved groundnut varieties (Adu-Daapah et al., 2007).
Materials and Methods
- Study areas
- The survey
- Assessment of GRD and other diseases
- Participatory Rural Appraisal
- Data analysis
In each of the 3 districts, 10 farmers' fields were randomly selected from groundnut producers with the help of extension officers who were familiar with the farmers and fields in question. An Extension Planning Area (EPA) in each district was selected for farmer interviews and focus group discussions. In each EPA, farmers were initially organized for a focus group discussion (FGD), each group consisting of a total of âĨ20 farmers (both men and women).
The data collected from the EPAs in the 3 districts was analyzed using the Statistical Package for Social Scientists (SPSS).
Results
- Household characteristics, landholding size and labour use
- Household characteristics
- Land holding characteristics
- Labour use
- Cropping systems, crop production and seed sources
- Groundnut varieties
- Farmersâ preferences for different groundnut varieties
- Groundnut production and utilization
- Occurrence of diseases in farmers groundnut fields
- Farmers awareness and perception about GRD
- Other production and marketing problems
The majority of farms (94%) owned by households were inherited, while only 6% were rented or borrowed (Figure 2.4). In addition to family and wage labor, 16% of farmers, mainly in Kasungu and Salima, also used village labor. Yield and good taste were the most common positive qualities expressed by farmers for most varieties grown.
Overall, 63.3% of farmers recognized that GRD was an important problem requiring intervention.
Discussion and conclusion
Most of the farmers interviewed in this study did not have formal jobs from which they can obtain cash. According to the farmers, the trend of GRD levels has increased due to the high frequency of drought periods in Malawi. Moreover, the inadequacy of the extension system has failed to provide essential information to farmers.
Most farmers choose to grow smaller quantities of groundnuts due to the inaccessibility of markets and exploitation by vendors.
Introduction
However, very little polymorphism at the molecular level has been detected in cultivated groundnut (Dwivedi et al., 2003). In general, plant breeding practices reduce genetic diversity within improved crop species (Rauf et al., 2010). In general, simple sequence repeats (SSRs) or microsatellites and single nucleotide polymorphism (SNP) markers are preferred for plant genetics and breeding (Pandey et al., 2012b).
However, the number of molecular markers available for cultivated groundnut is still limited (Wang et al., 2012).
Materials and Methods
- Plant Material
- DNA extraction
- SSR markers and Polymerase Chain Reaction
- Data analysis
This study used 21 SSR markers to assess the genetic diversity of a collection of peanut germplasm assembled for use in a breeding program for the development of cultivars resistant to peanut rust disease. Twenty-one SSR markers were used to assess the genetic diversity among the 106 groundnut genotypes (Table 1). These SSR markers were selected from previous studies (Ferguson et al., 2004; Moretzsohn et al., 2005) based on their informativeness and polymorphic information content.
Summary statistics on major allele frequency, allele number, availability, gene diversity, heterozygosity, and PIC values ââ( Botstein et al., 1980 ) were calculated using PowerMarker V3.25 ( Liu and Muse, 2005 ).
Results
- SSR polymorphism, allelic richness and number of alleles
- Gene diversity
- Polymorphic Information content
- AMOVA to partition the genetic variation
- Gene diversity among the groundnut genotypes
- Genetic relationships among groundnut genotypes
The number of alleles analyzed per population showed that of the 19 polymorphic markers, 17 were polymorphic among the improved cultivars and 9 among the germplasm accessions. A PCoA plot of the first and second coordinates explained 10.42% and 7.52% of the total diversity, respectively, clustering the test genotypes into two different groups (Figure 3.1). However, a plot of first and third axes, explaining 14.51% of the total diversity, clustered the genotypes into three different groups (Figure 3.2).
Most of the groundnut varieties popular among farmers in Malawi (Chalimbana, CG 7, RG 1, Manipintar and ICGV-SM 90704) were classified in CL-3 meanwhile.
Discussion
2003) Genetic variation among South African cultivated groundnut (Arachis hypogaea L.) genotypes as revealed by AFLP analysis. A high-throughput DNA extraction protocol for tropical molecular breeding programs. 2006) SSR analysis of germplasm of cultivated peanut (Arachis hypogaea L.) resistant to rust and late blight. 1997) Identification of polymorphic DNA markers in cultivated peanut (Arachis hypogaea L.). 2010) Implications of plant breeding for genetic diversity.
Registration of peanut cultivar ICGV-SM 90704 with resistance to peanut rosette. 2005) SSR markers associated with rust (Puccinia arachidis Speg.) resistance in peanut (Arachis hypogaea L.).
Introduction
The disease, endemic to the African continent, is caused by a complex of groundnut rosette virus (GRV), groundnut rosette helper virus (GRAV) and satellite RNA (satRNA) (Taliansky et al. 2000). GRD usually occurs in farmers' fields in Malawi at low levels each growing season, however, disease severity increases with late planting (Waliyar et al. 2007). Effects of both forms of GRD on young plants include severe stunting due to shortened internodes and reduced leaf size leading to a bushy appearance (Naidu et al. 1999b).
Several cultivars resistant to GRD and its aphid vector have been developed and released to farmers (Subrahanyam et al., 1998; Ntare et al., 2001).
Materials and methods
- Plant material
- Experimental site
- Experimental Design
- High disease pressure
- Low disease pressure environment
- Evaluation for aphid resistance
- Data collection
- Data analysis
Before planting the JL trials 24 plants were grown in the greenhouse and infected with GRD. At weekly intervals until 80 DAS, viruliferous aphids that had been reared in a greenhouse on rosette plants were placed on the infestant lines and test genotypes using a camel hair brush. The field experiment was laid out in a Randomized Complete Block Design (RCBD) with 10 replications.
The plants were covered with perforated plastic bags to prevent the aphids from escaping after they were placed on each plant.
Results
- Germplasm reaction to GRD under high disease pressure
- Germplasm reaction to GRD under low disease pressure
- Yield under conditions of high and low disease pressure environments
- Resistance of 11 selected groundnut genotypes to aphid infestation
The average values ââof PDI, yield and other traits combined during the two seasons are shown in Table 4.3 and Appendix 4.1. Correlations between traits recorded for genotypes grown under high disease pressure are shown in Table 4.4. Correlations between different traits for genotypes grown under low disease pressure are shown in Table 4.7.
For all genotypes, lower yields were observed under high disease pressure than under low disease pressure.
Discussion
Aphid counts on two count dates were used as an indicator of resistance and susceptibility of the genotypes. This shows that these genotypes are tolerant to GRD, which may be one of the reasons why farmers choose to grow them. 1990) Different variants of satellite RNA of peanut rosette virus are responsible for the chlorotic and green forms of peanut rosette diseases.
The effect of peanut rosette assistant virus on the agronomic performance of four peanut (Arachis hypogaea L.) genotypes.
Introduction
The disease mainly affects groundnut in Africa and has the potential to cause total yield loss in severe cases (Naidu et al., 1999). Additive gene action controls the majority of yield quality traits while seed size is controlled by non-additive gene action (Hariprasanna et al., 2008). Resistance to GRD has been reported to be controlled by two independent recessive genes (Nigam and Bock, 1990; Olorunju et al., 1992).
Evaluation of resistance to GRD has been assessed on a 1â9 scale or as percent disease incidence or both (Waliyar et al., 2007).
Materials and methods
- Genotypes used for hybridization
- Hybridization using a 10 x 10 full diallel mating scheme
- Data analysis
Aphid colonies were reared on the susceptible genotype JL 24 in the glasshouse before planting the experiments. Each of the test plants was regularly checked and evaluated for GRD symptoms at 7, 14, 21 and 28 days after aphid infestation. The percent disease incidence data from the parental and F2 populations were arcsine transformed before analysis to stabilize the error variance (Gomez and Gomez, 1984).
The estimates of genetic components were obtained based on the expectations of the mean squares as under;.
Results
- Reaction of parent lines to GRD infection
- Reaction of F 2 progenies to GRD infection
- Gene action governing GRD resistance
The significance of variance estimates due to GCA, SCA and reciprocals was tested using F-values ââat the PË0.01 and PË0.05 levels, while the significance of GCA, SCA and reciprocal estimates was tested using their standard errors . Estimates for SCA, interaction effects and least squares means are presented in Table 5.6 (see also Appendix 5.1). Overall, 17 out of 90 F2 progeny and reciprocals showed significant negative effects of SCA for GRD resistance.
The remaining 13 crosses and reciprocals that showed significant SCA effects were between resistant and susceptible parents (Table 5.6).
Discussion
The significance of the GCA and SCA effects indicates that both additive and non-additive gene action are important in the inheritance of GRD resistance. These results contradicted a study by Misari et al., (1988) where cytoplasmic and/or maternal effects were not observed in the inheritance of GRD resistance. Mean squares due to general combining ability (GCA), specific combining ability (SCA), and maternal and non-maternal effects were all significant, indicating that both additive and non-additive gene effects are important in the inheritance of GRD resistance.
Genetic studies have shown that resistance to GRD is largely dependent on additive genetic effects.
