Peanut plays an important role in the livelihoods of poor farmers and in the rural economy of many developing countries. Aflatoxin contamination in peanut seeds, caused by
The problem of aflatoxin (produced by the
The Groundnut Improvement Program was established at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) in 1976 and assigned a high priority to contain the aflatoxin problem in peanut. Taking the lead from Mixon and his group's work, PI 337394 F and PI 337409 were obtained from the USA and extensively used in hybridization programs. However, it was realized that significant invasion of apparently undamaged peanut pods by
The pathogen has to cross two barriers before it can reach cotyledons to derive its sustenance. The first interaction between the pathogen and the host is at the pod wall which is a physical barrier and resistance is attributed to pod shell structure (
The second barrier to the pathogen is the seed coat. This resistance is attributed to thickness and density of palisade layers, presence of wax layers, and absence of fissures and cavities (
The ultimate feeding site for fungi is the cotyledons in the seed and this is where aflatoxins are produced. If cotyledons do not provide sustenance to the pathogen, there would be no or little production of aflatoxin.
The relationships between
Very little is known about the inheritance of resistance to preharvest seed infection, IVSC, or aflatoxin production. A few published reports give information on broad sense heritability (low to moderate) and combining ability of resistance sources (
Sources of all the three types of resistance (preharvest seed infection, IVSC, and aflatoxin production by
In earlier screenings of germplasm accessions for their reaction to IVSC at ICRISAT, Patancheru, India, resistance (15% or fewer seeds colonized) of three genotypes, PI 337394 F, PI 337409 and UF 71513 was confirmed and six new sources of resistance (Ah 78223, J 11, U 4-47-7, Var. 27, Faizpur, and Monir 240-30) were identified (
Over the years, about 2000 peanut accessions have been screened for their resistance to
In spite of significant G × E interaction reported for resistance to seed infection, some accessions such as ICG 1326, ICG 3263, ICG 3336, ICG 3700, ICG 4749 and ICG 7633 showed consistent resistance to seed infection in India and Senegal (
In a recent evaluation of 49
During the 2005 rainy season, 24 aflatoxin resistant germplasm lines were reevaluated in the field and under
The value of resistance sources depends upon levels and stability of their resistances. Many genotypes have shown high levels of resistance against preharvest seed infection, IVSC, and aflatoxin production, significant G × E interaction remains a major issue in screening for aflatoxin resistance. Nonetheless, there are several genotypes including ICG 7633, ICG 4749, ICG 1326, ICG 3263, ICG 9407, ICG 10094, ICG 1859, and ICG 9610 which have shown consistently high levels of resistance across tests and locations, which make them good candidates for use in a resistance breeding program. It would be worthwhile to make an international screening nursery of such genotypes for extensive evaluation across diverse growing environments to identify stable sources with high levels of combined resistance to seed infection, IVSC, and aflatoxin production.
In the past, resistance to IVSC received major attention because of ease of screening and also because aflatoxin contamination was considered a postharvest problem. The available genetic resistance was transferred to superior agronomic backgrounds and several advanced breeding lines were released in the USA and India. However, none showed marked superiority in resistance over the resistant parent. When it was realized that
In a recent 2-year evaluation of introduced genotypes at the Crops Research Institute, Guangzhou, China, ICGV 95440, ICGV 95422, ICGV 94435, ICGV 94434, ICGV 94433, ICGV 95435, and UF 71315 showed high levels of resistance to seed invasion by
Performance of some of the recently developed breeding lines is given in
Performance of some of the newly developed aflatoxin tolerant Spanish breeding lines at ICRISAT Center, Patancheru, India.
Performance of some of the newly developed aflatoxin tolerant Spanish breeding lines at ICRISAT Center, Patancheru, India.
Cultivation of resistant varieties would be a simple and effective option for farmers, but cultivars with high levels of resistance to aflatoxin contamination and good agronomic characteristics are not available. Fourteen advanced breeding lines with resistance to IVSC including, ICGV 91278, ICGV 91279, ICGV 91283, ICGV 91284, ICGV 91315, ICGV 91317, ICGV 91324, ICGV 91328, ICGV 91341, ICGV 92302, ICGV 93305, ICGV 93328, ICGV 93379, and ICGV 94434, were evaluated from 2003 to 2006 period for seed infection and aflatoxin contamination as well as agronomic performance in farmer participatory, multi-locational on-farm trials in Anantapur and Chittoor districts in Andhra Pradesh, India.
All 14 advanced breeding lines along with local variety, TMV 2, were evaluated in nine farmers' fields in three villages in each district following farmers' practices of cultivation. All the test genotypes produced 12–45% higher pod and haulm yields than TMV 2. Aflatoxin contamination ranged between 0–7 µg kg−1 in all the test lines at all locations compared with 0 to >150 µg kg−1 in TMV 2. Based on their performance, farmers in Chittoor selected ICGV 91341, ICGV 93305, ICGV 94379, and ICGV 94434, and farmers in Anantapur chose ICGV 91278, ICGV 91328, ICGV 94379, and ICGV 94434 for further evaluation and adoption. These materials have good tolerance to drought, high pod and haulm yields, good fodder quality, good shelling outturn, and low aflatoxin risk.
All the selected breeding lines (four each in both districts) produced 16–61% higher pod and haulm yields and had 36–73% reduction in aflatoxin contamination over TMV 2.
Due to severe drought conditions during the 2006 rainy season in Andhra Pradesh, a trial was conducted with only the two breeding lines where enough seed was available (ICGV 94379 and ICGV 94434) and restricted to six farmers' fields in two villages in Anantapur. Overall, both lines produced about 11% higher mean pod yield at both locations against the control yield 1010 kg ha−1. Both lines also showed 57–71% reduction in aflatoxin contamination as compared to 177 µg kg−1 in control TMV 2.
ICGV 91278, ICGV 94379, and ICGV 94434 are being tested in 42 farmers' fields in six villages in Anantapur, and ICGV 91341, ICGV 93305, ICGV 94379, and ICGV 94434 were evaluated in 15 farmers' fields in three villages in Chittoor. The results from these evaluations are currently being analyzed. Simultaneously, farmers were trained in postharvest aflatoxin management methods and utilization of mechanical threshers for rapid separation of pods from plants instead of beating them against hard objects.
This study has clearly demonstrated that available genetic resistance combined with simple postharvest management practices can be an effective option for mitigating
A resistance breeding program requires that gene(s) for resistance and a reliable, efficient and simple screening technique (including sampling procedures) are available. The presence of the latter is over riding as it helps to detect the presence of resistance genes, their level of resistance and nature of inheritance, prevents escapes in a large-scale breeding program, and gives results that are repeatable across locations and years.
Conventional breeding can reshuffle genes and bring out desirable combinations among and between the available genes to harness their cumulative or complementary benefits, but it cannot create new genes. The level(s) of resistance with available gene(s) in the germplasm pool places a ceiling on the progress that can be made by conventional breeding. Lack of high levels of resistance to preharvest seed infection and aflatoxin production in the field and IVSC and aflatoxin production (although some genotypes have stable resistance), high G × E interaction for these traits, and limitations of screening techniques in giving reliable and repeatable results place severe limitations on the progress in resistance breeding to eliminate aflatoxin. Even if highly resistant gene(s) are utilized from other sources through non-conventional means, the need for an efficient and reliable screening technique still remains for field verification of the resistant products.
Under the present circumstances, genetic resistance alone cannot eliminate the problem of aflatoxin contamination in peanut unless it is accompanied with other cultural management practices such as soil amendments, bio-control, soil water management, soil pest control, and proper drying and curing and storage. Other plant traits such as short-duration (with high partitioning to reproductive tissues) to match the period of soil moisture availability to avert terminal moisture stress, uniform pod maturity, and longer root systems to extract moisture from the deeper soil layers to maintain plant-water status may also help to some extent to mitigate the problem of aflatoxin contamination.
Aflatoxin contamination in the field is the result of host plant × pathogen × environment interactions. Because peanut being a subterranean crop and the pathogen also inhabits the soil, the plant-pathogen interaction is very complex. Aflatoxin contamination is a rare event and occurs only when all the conditions in and around a geocarposphere are favorable. Not all seeds in a plant are infected. Intra- and inter-plant variation in aflatoxin contamination of the seeds can be many fold. Unless screening techniques are able to minimize these variations under field conditions, the success in aflatoxin resistance breeding will remain a chance event. Therefore, the first priority should be on development of reliable screening techniques to make them more reproducible under field conditions. Once a screening technique is perfected, the search for resistance gene(s) can be vigorously pursued. Stability of resistance gene(s) in multiple environments can be evaluated against variable
Wild
Although fatty acid concentrations accounted for significant portions of genetic variation in their laboratory screening for postharvest contamination,
The lack of high levels of resistance to aflatoxin contamination in the cultivated peanut germplasm places a ceiling on the progress that can be made following conventional approach in resistance breeding. The search for better sources of resistance in the cultivated and wild
Principal Scientist, Scientist, Scientific Officer, and Principal Scientist, respectively, ICRISAT, Patancheru 502 324, Andhra Pradesh, India.
Principal Scientist and Director, West and Central Africa, ICRISAT, BP 12404, Niamey, Niger
Virologist, IITA, Oyo Road, PMB 5320, Ibadan, Nigeria.
Principal Research Fellow, Plant Environment Laboratory, Department of Agriculture, University of Reading, Cutbush Lane, Shinfield, Reading, RG2 9AF, UK.
Senior Scientific Officer and Principal Scientist respectively, ICRISAT, BP 320, Bamako, Mali.