Genetic Improvement of Groundnut in India: Priorities and
Prospectus
Overcoming the yield barriers
In India, the pronounced unpredictability
of the groundnut crop is attributed to its confinement mainly to dry
areas prone to various biotic and abiotic stresses.
The abiotic stresses are moisture stress, end-season
rains, cold, salinity, acidity, micronutrient deficiency, shade, water
logging, etc.
The biotic stresses include insect pests, foliar
diseases, viral diseases, soil-born diseases, aflatoxin contamination,
weeds, etc.
Leaf spots and rust are the most economically important
of the diseases and may reduce yields upto 70 per cent.
Genetic studies on host resistance to these diseases
have been conducted more systematically than on any other groundnut
diseases.
Rust
Rust resistance in groundnut is governed by two or more
recessive genes interacting in various ways. Since the resistance has
high heritability with considerable additive component, it is possible
to select for rust resistance in early segregating generations.
Highly resistant sources for rust have been identified
among the primary gene pool which are being directly used in breeding
programmes throughout the country. Source of disease resistance in A.hypogaea
Disease
Sources of resistance
Early leaf
Tifton 1108, Ahs 17, 29, 477, 698, 7188, Pis 109839
The moderately rust resistant groundnut varieties
developed so far are Girnar 1, ICG (FDRS) 10, ICGV 86590, ICGV 87199,
ICGV 87187 and ALR 1.
A high level of resistance available from th3
wild relatives of groundnut has also been transferred to A.hypogaea
and several rust resistant interspecific derivatives produced in ICRISAT,
NRCG and TNAU.
Leaf spots
The genetic resistance to clear and late leaf spots
have been reported to be independently controlled by two to several
genes.
A number of resistance sources for both the leaf
spots have been identified so far. Although the leaf spot resistance
is predominantly governed by additive generation, the very low to medium
heritability levels (0 to 0.8) render this character a relatively difficult
one to select in early generations. Moreover, presence of an unspecified
number of modifier genes and linkage of resistance with undesirable
characters makes the task of breeding leaf spot resistant cultivars
difficult.
Breeding partially resistant cultivars with stable
resistance is desirable for control of leaf spots under natural epiplhytotic
conditions with affordable number of fungicidal sprays.
Alternaria disease
This disease is gradually becoming important in
rabi/summer crop.
Although some resistant sources have been identified
at NRCG the genetic studies on this disease and breeding procedures
to develop resistant cultivars have yet to be initiated.
Aflatoxigenic fungi
Aflatoxin contamination is a major problem in hand
picked selected (HPS) genotypes.
At least three different mechanisms of resistance
to aflatoxin producing fungi. A.flavus and A.parasiticus
have been recognised in groundnut.
They are dry seed resistance (DSR), aflatoxin production
resistance (APR) and pre harvest infection resistance (PRI).
The three mechanisms have different genetic controls
and have heritability ranging from 0.2 to 0.7. Seed coat resistance
has been shown and maternal genotypes.
Although the genetic mechanisms governing the
aflatoxin resistance in host seem to be simple, the character is known
to be influenced by several other factors like humidity, microbial activity,
cultural practices, drying methods, etc.
Hence, cultivar improvement is only apart of the
integrated approach needed to control aflatoxin contamination in groundnut.
As the acceptable aflatoxin levels for human consumption
are lowered regularly by the developed countries which happen to be
the main importers of HPS types, the problem has to be tackled immediately
for ensured foreign exchange earnings.
Several genotypes possessing resistance to A.flavus
colonization and aflatoxin contamination have been identified at the
NRCG and the ICRISAT .
At ICRISAT, a mass pedigree method of selection
and its modifications have been practiced to develop eight stable derivatives
(ICGVs 86168 to 86171 and 86173 to 86177) having seed coat resistance
levels equal to those of the donors J 11 and UF 71513.
Soil borne diseases
The severe seedling mortality caused by soil-borne
disease like stem rot, collar rot and dry root rot results in "patchy
crop" which results in 20 to 40 per cent yield losses.
Although a few resistance sources have been identified
, relatively little efforts have been put in so far either in understanding
the genetics of resistance or breeding for resistant cultivars, probably
because of the difficulties associated with development of sick plots
and screening large populations.
Reliable sources of resistance to the viruses are
practically not available in the cultivated germplasm and the wild Arachis
species are the only sources available for incorporation.
As the problem of viral diseases is increasing
alarmingly year after year, conscious efforts should be initiated hereafter
aiming at breeding for resistance to viral diseases.
Bud necrosis disease
It is the most economically important viral disease
and known to reduce the yields upto 90 per cent under severe conditions.
The epidemiology and virus vector relationships
in case of BND have been worked out by the ICRISAT. So far, four cultivated
varieties, Kadiri 3, ICGS 11, ICGS 44 (all selections from population
of Robout – 331) and TAG 24, besides several intra and interpseicfic
derivatives developed at ICRISAT have been reported to show low incidence
of BND.
The breeding programmes on BND should aim to resistance
to both virus and vector (thrips) for effective control of the virus.
Peanut mottle and peanut stripe
Besides BND, the peanut mottle virus (PMV) and
of late the peanut stripe virus (PStV) have been recognised as potential
viral diseases of groundnut. Both the viruses are seed-borne; the range
of seed transmission being 0.1 to 3.5 per cent in case of PMV and 0.2
to around 20 per cent for PStV.
While resistant sources are absent in cultivated
germplasm a few sources of resistance have been identified among wild
Arachis species.
A plethora of insect pests attack groundnut crop
from sowing to storage. The changing scenario of cropping patterns and
multiple cropping seasons have aggravated this problem.
Among the insect pests, sucking pests like aphids,
jassid, thrips and leaf miner and leaf cutters such as Heliothis, Spodoptera
and economically important.
Though moderate level of resistance to the sucking
pests is available in A.hypogaea, a directed transfer of these
into high yielding cultivars has not been very successful due to lack
of genetic information either on the host resistance or on the virulence
of the pest.
Three recessive genes with additive effects are
reported to confer resistance to jassid in two crosses.
Non additive gene effects were found to be significant
for host resistance to leaf miner.
Among the released cultivars, Girnar 1 and TAG
24 are known to have resistance to jassid. At NRCG, several jassid resistant
culture have been developed through intra and interspecific breeding.
Resistance to leaf eating insects is mostly confined
to wild relative although genotypic differences have been found for
this character.
Therefore, the cultivar improvement on this aspect
takes some more time to yield results.
Control of polyphgous species such as white grub,
a menace in several groundnut growing areas in India, may not be possible
through genetic improvement alone, improving the productivity and quality.
Yield per se
The foremost objective of a grower is to increase
the groundnut yield per unit land area and sustain it.
Although 114 varieties have so far been developed
and released in India, the increase in productivity over the years is
not promising.
Owing to domestication and narrow selection pressure
using modern cultivation methods, the cultivated groundnut has acquired
several genetic disorders including susceptibility to a large number
of diseases and insect pests.
The farmer spends lot of time, money and efforts
on control of various diseases, insects and nematodes in groundnut.
Even a marginal reduction in the amount of pesticides used for different
purposes would be economically and ecologically advantageous.
The economically important groundnut diseases
in India are: early and late leaf spots (Cercosporaarachidicola
and Phaeoisariopsis personata), rust (Puccinia arachidis),
alternaria leaf diseases, collar rot (Aspergillus niger), stem
rot (Sclerotium rolfsii), dry root rot (Macrophomina phaseolina),
aflaroot (Aspergillus flavus), bud necrosis disease (BND/TSWV),
peanut mottle virus (PMV), peanut stripe virus (PStV), nematode disease
(root knot and kalahasti malady), etc.
All the groundnut growing regions in India are
drought-prone. Enough efforts have not been put in to understand the
physiological basis of drought and identification of reliable parameters
of moisture stress.
The researches to identify sources of drought
resistance and the mechanisms of resistance are gradually beginning
to gain momentum with the active involvement of ICAR, ICRISAT and ACIAR.
Sources for drought resistance are available within
the cultivated and in the alien germplasm of groundnuts.
However, understanding the genetic nature of drought
resistance and the interplay of different combination of traits conferring
drought resistance are the prerequisites for the successful execution
of the drought resistance breeding programme.
Among the released cultivars GG2, Girnar 1 and
ICGVs 87187, 86876 and 87054 show drought tolerance.
Sprouting of seeds when caught in end season rains
is a problem in rabi/summer groundnut in India which accounts
for over 14 per cent of the total groundnuts produced. In situ
sprouting is a problem in pish bunch varieties which lack fresh seed
dormancy as a rule.
At present none of the released pish varieties,
except TG17, is endowed with fresh seed dormancy. a pish culture,
CGC 7 (CGS 1-19) with fresh seed dormancy upto 30 days was developed
from cross, J 11 x Robout 33-1 at the NRCG. However, the factors responsible
for seed dormancy appear to rest with testa, cotyledons and embryo,
making it a difficult character to bred for. Also, a high level of interplant
variation has confounding effect on dormancy and comes in the way of
understanding genetic mechanism of dormancy.
Cold
Cold temperatures in northern Indian affect seed
germination and seedling growth of rabi sown crop.
The work done so far on different aspects of tolerance
to cold temperatures has been meager.
The NRCG has identified germplasm lines viz., NRCGs
1339, 6408 and 7782, wild species. A.monticola and a cross derivative
of GNLM x A.monticola which can germinate and show seedling growth
under 12/18oC temperature cycles.
Salinity
Soil salinity is a problem in the coastal areas.
Groundnut can not germinate and grow at salinity
levels exceeding 8 EC.
A large number of genotypes were screened in
vitro at NRCG and genotypes which can germinate at high salinity
levels were identified.
Iron deficiency
Availability of iron is a problem in calcareous soils of India, especially
under irrigated conditions.
This results in chlorosis which accounts for upto 20 per cent yield
losses.
The iron absorption efficiency of groundnut seems to be governed by
relatively few number of genes and hence cultivar improvement should
be possible on this aspect.
Several "iron efficient" lines were identified at NRCG and
efforts are on to incorporate the resistance to the iron inefficient
cultivars.
Loss of viability
The produce of rabi summer rapidly loses viability because
the high drying temperature and high humidity lead to loss of integrity
of seed testa and causes efflux of curicalenzymes and other solutes
on soaking.
Hence, use of the produce rabi summer produce has been detected
and genotypes retaining over 70 per cent seed viability after 8 months
of storage under ambient conditions were identified. However, genetic
and breeding studies on this important character have yet to be initiated.
Late season drought spell, particularly in the semi-arid region is
a major factor associated with aflatoxin contamination.
Reduced metabolic activity associated with decreased pod moisture
content under drought stress seems to increase susceptibility of groundnuts
to A. flavus infection.
However, another possible role of drought stress in pre-harvest fungal
infection could involve suppression of microbial competitors of aflatoxin
producing fungus by elevated temperatures in the pod zone.
Pod splitting is another factor contributing to aflatoxin contamination.
Pod maturing under fluctuating soil moisture conditions during seasons
of inadequate or irregular rainfall, are prone to pod splitting.
Seed in split pods are frequently invades by A. falvus and subsequently
become contaminated with aflatoxins.
It is well established that A. flavus invasion can occur in soil during
pod development and maturation; the fungus directly penetrates the pod
wall or enters passages created by pod pests and diseases/lesions.
However, the exact mode of infection of groundnut pod has not been
fully elucidated.
Genetic Improvement
