GENETIC IMPROVEMENT OF LOCALLY ADAPTED GROUNDNUT VARIETIES FOR RESISTANCE TO PRE-HARVEST AFLATOXIN CONTAMINATION IN GHANA
Groundnut is an important food and oilseed crop across the globe. It is cultivated in Ghana under rain-fed mainly in areas bedeviled with periodic drought. This exposes the crop to end-of-season drought, resulting in pre-harvest Aspergillus invasion and subsequent aflatoxin accumulation. As a consequence, the potential for aflatoxin post-harvest management becomes limited, leading to further accumulation in storage. The study was conducted to (i) assess the distribution of Aspergillus species and aflatoxin in stored groundnuts from Ghana, (ii) determine the aflatoxigenicity of isolates of Aspergillus flavus derived from groundnuts in Ghana, (iii) identify groundnut genotypes with resistance to pre-harvest aflatoxin accumulation by local aflatoxigenic Aspergillus flavus, and (iv) determine the mode of gene action governing resistance to pre-harvest aflatoxin contamination. Groundnut samples were collected from farmers’ storage sites in 20 locations spanning four major producing regions in Ghana. Groundnut samples were plated on potato dextrose agar and examined for the presence of Aspergillus species. The samples were analyzed for aflatoxin with high performance liquid chromatography (HPLC). Sixty A. flavus isolates, originating from groundnut samples collected from the major producing regions were assessed for their aflatoxin producing ability using cultural and multiplex polymerase chain reaction (PCR) methods. Cultural method involved the culture of isolates in yeast extract sucrose broth followed by extraction and determination of aflatoxins by HPLC. Multiplex PCR method involved screening of isolates for presence or absence of four key aflatoxin biosynthesis genes. Twenty-seven groundnut genotypes (22 introduced lines and 5 local varieties) were evaluated at Fumesua and Nyankpala for resistance to kernel infection by A. flavus and aflatoxin accumulation under terminal drought. Twelve F1 progenies were generated from four local varieties (females) and three introduced lines (males) in North Carolina II mating design and evaluated at Fumesua and Nyankpala for resistance to kernel infection by A. flavus and aflatoxin accumulation. The results revealed the predominance of Aspergillus flavus and Aspergillus niger in groundnut samples. A. flavus was widely distributed across all four regions while A. niger was predominant in samples from two regions. A. flavus was identified in majority of the samples. All four regions had samples with aflatoxin levels higher than the European Union and Ghana Standards Authority thresholds. All sixty A. flavus isolates produced aflatoxins by the cultural method while only seven isolates showed aflatoxigenicity (amplification of all four aflatoxin biosynthesis genes) by multiplex PCR method. Significant genetic variability was detected among the groundnut genotypes for kernel infection by A. flavus and aflatoxin contamination at both locations. Genotypes ICGV 03401, ICGV 03331, and ICG 4729 had aflatoxin concentrations lower than the 20 ng/g (Ghana Standards Authority standard) at Fumesua and Nyankpala, and across locations. These genotypes also exhibited low to moderate kernel infection by A. flavus at individual and both locations. Significant female x male interaction, male, and female was found for both kernel infection by A. flavus and aflatoxin accumulation, suggesting that both additive and non additive gene effects were implicated in the expression of the traits. The males contributed to the largest proportion of the variation in the hybrids for both traits, suggesting paternal influence on the traits. GCA to SCA variance ratio was more than unity for both traits, implying the preponderance of additive gene effect in the expression of the traits. The study has revealed the abundance of A. flavus and high levels of aflatoxins in groundnut from the major growing regions, requiring urgent need for management. Breeding approach for management could be effective with the identification of genotypes with low A. flavus infection and aflatoxin accumulation and also with additive gene action controlling the traits. However, a holistic approach involving pre- and post-harvest strategies is required to achieve effective management of aflatoxin contamination.