Chapter 1: Literature review
1.7 Breeding pigeonpea
Breeding pigeonpea is a challenge because the objectives and methods chosen in the breeding programme depend on the nature and magnitude of genetic variation, the
involving the crop. High stable yield, with acceptable grain quality, is the major breeding objective. Stable yield is sought by incorporating resistance to biotic stresses such as diseases (wilt, sterility mosaic, phytophthora blight), pests, and abiotic stresses (waterlogging, drought, acidity, and salinity) (Singh et al., 1990). It is essential to breed for a range of resistances to pathogenic organisms in order to reduce the need for chemical controls to a minimum, and thus lower production costs, increase the nutritional value of agricultural products, and improve the environment (Borojevic, 1990). Other objectives have focused on breeding pigeonpea for specific production systems; special traits such as suitability for vegetable products and fodder: high protein content for the animal feed industry, suitability for processing for canning; the milling quality for split peas (dhal); and market preferences (Singh et al., 1990). Breeding programmes in Malawi have focused on high stable yields, fitting into the intercropping system, ratoonability (Sakala, 1992), appropriate maturity period, and market preferences (seed size and colour) (ICRISAT, 2006). Short and extra short-duration, short-statured pigeonpeas, with comparatively low sensitivity to photoperiod and temperature interactions, have been bred by ICRISAT. Medium and long-duration pigeonpeas are principally grown as intercrops with tall cereals (maize, sorghum, and millets), and a variety of other crops. However, selection for competitiveness and high productivity from early generations in mixed cropping systems is not practical (Singh et al., 1990), because selections at early generations are done in pure stands instead of interrows. A participatory rural appraisal was conducted in the current study (see Chapter 2) to identify and set the breeding priorities for Malawi.
1.7.1 Breeding techniques in pigeonpea
Since the beginning of pigeonpea cultivation, farmers have exercised selection for specific traits suitable for their cropping systems and this led to the development of landraces which are still popular today (Singh et al., 1990). Early breeding efforts were aimed at improving yield and acceptability for specific uses and production systems, and selections were made from landraces. Most of the early improvement work in pigeonpea was confined to selection and pedigree evaluation from landraces adapted to the region in which such selection was exercised (Singh et al., 1990).
Diallel and line x tester mating schemes using three or more well-adapted cultivars as testers have been used (Singh et al., 1990). The commonly used breeding methods for a
self-pollinated crop are applicable to pigeonpea, even though a considerable amount of outcrossing occurs in the species. Bulk hybrid advance by single-pod descent, and single-seed descent, have proven successful in breeding for high-yielding lines.
Stratified mass selection and mass selection with progeny testing have been tried in Kenya for yield gains in pigeonpea (Singh et al., 1990), and in India to estimate heritability and genetic advance (Singh et al., 2003). Mass selection, selfed progeny selection, and half-sib progeny selection, have been used to estimate genotypic and phenotypic variance, heritability, and genetic advance for some yield traits in pigeonpea (Singh et al., 2003). Singh et al. (1990) suggested recurrent selection and population breeding methods as ways to accumulate desirable genes and facilitate the breaking of linkages in pigeonpea and other self-pollinated species. Pigeonpea has a substantial amount of non-additive genetic variance and hybrid vigour for yield (Singh et al., 1990).
The discovery of stable genetic male sterility, coupled with its outcrossing nature, has opened the possibility of commercial use of the heterosis in pigeonpea, enabling the economic production of hybrid seed. Successful hybrids are produced from those combinations where specific combining ability effects result in considerable heterosis in the F1 generation. A number of mutants, both induced and spontaneous, for various qualitative characters, have been reported in pigeonpea and a few cultivars have been developed through the use of induced quantitative variability (Singh et al., 1990).
In pigeonpea, and in several self-pollinated species, varietal improvement methods based on pedigree, bulk pedigree, backcross- and multiple-crossing techniques have been useful in recombining simply inherited characters such as disease resistance, seed size and colour, and maturity (Singh et al., 1990). Pedigree selection has been useful in breeding for highly heritable traits such as disease resistance, seed size, seed colour, growth habit, and seed number per pod (Green et al., 1981). There are wide ranges of options to choose from when embarking on breeding in pigeonpea. However, the choice of the best technique depends on the objectives, time involved, and the technical know- how of the breeder.
1.7.2 Breeding for disease resistance in pigeonpea
Fusarium wilt (F. udum) is a major disease of pigeonpea. Other important pigeonpea
(Oidiopsis taurica) (Reddy et al., 1990). In India, sterility mosaic and Fusarium wilt were found to be major diseases of pigeonpea. With the availability of effective screening methods, breeding for resistant genotypes started with the identification of resistant germplasm accessions (Gupta et al., 1988; Okiror, 1998; Reddy et al., 1998). Emphasis was put on understanding the inheritance of resistance for each disease: nature and number of genes involved in governing the resistance (Singh et al., 1990). At ICRISAT, the disease resistance breeding programmes have aimed at breeding varieties with multiple disease resistance to Fusarium wilt, sterility mosaic, and phytophthora blight, using a multiple disease screening nursery. Some sources with combined resistance have been identified (Singh et al., 1990).
1.7.3 Breeding for Fusarium wilt resistance in pigeonpea
Breeding for wilt-resistant genotypes started as early as 1906 in India when the Fusarium wilt pathogen was identified (Reddy et al., 1990). However, the results of studies in the mechanism of inheritance of wilt resistance are still contradictory and not fully understood.
Breeding for Fusarium wilt resistance is usually done by pedigree or mass-pedigree selection, although in some cases, backcrossing has also been successful (Singh et al., 1990). Some resistant varieties, which were bred using various methods, include ICP 9145 (Reddy et al., 1995) and ICEAP 00040 (Silim et al., 2005). In India, the resistance in the cultivars is site-specific, depending on the F. udum races prevalent in the area.