Inheritance of resistance to kernel infection by aspergillus flavus and to accumulation of aflatoxins in selected groundnut genotypes in Uganda.
Ozimati, Alfred Adebo
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Groundnut is prone to infestation by two closely related species of fungi, Aspergillus flavus and Aspergillus parasiticus. A. flavus is the most common specie in Africa and Asia, while A. parasiticus is predominant in America. Both species produce highly toxic mycotoxins known as aflatoxins that suppress body immunity, leading to diseases such as Liver cancer, hepatic failure and hepatitis B virus, and is one of the most important constraints to international groundnut trade. In Uganda there is little available information on breeding for resistance to A.flavus and to accumulation of aflatoxins. The aim of this study was to determine the level of resistance to groundnut kernel infection by Aspergillus flavus and accumulation of aflatoxins in eighteen selected groundnut genotypes, establish gene action and estimate heritability. These genotypes included two aflatoxins resistant lines obtained from the groundnut breeding program of ICRISAT, Mali and 16 locally adapted genotypes from NARO groundnut breeding program, based in National Semi-Arid Research Resource Institute Serere (NaSRRI). Two parameters were used for screening A.flavus that is percent-severity kernel infection (PSKI), scored on each kernel on a scale of 1-5, while percentage kernel incidence (PKI) covered with green-yellowish spores was determined by counting the number of kernels with visible spores and expressed as the percentage total of the kernels in an experimental unit. Scores were done on the third and seventh day from the initiation of the experiment. There was significant variation for both PSKI on the third and seventh day of the experiment (p≤0.001), while PKI was significant (p≤0.001) on the third day and significant (P≤0.05) on the seventh day of the experiment. There was significant variation (P≤0.001) among the 18 genotypes for aflatoxins accumulation, measured in parts per billion (ppb) and their log transformed values to equalize variance. There was no significant correlation between PSKI and aflatoxins accumulation measured in ppb, while a significant correlation (P<0.05) existed between PKI and aflatoxins accumulation. The study identified three genotype resistant to both A.flavus and aflatoxins accumulation (Serenut 4, AGRA 99044 and Erudu red). Four identified resistant lines (male parents) to either A.flavus or aflatoxins accumulation, or resistant to both were crossed with five susceptible genotypes (female parents) in North Carolina v II design to obtain F1‟s and the F1‟s were advanced to F2. The F2‟s were evaluated for resistance to A.flavus and aflatoxins accumulation in the laboratory. North carolina II analysis of variance of F2 population for PSKI3 and PSKI7, indicated that the, general combining ability (GCAf) for female parent, general combining ability (GCAm) for the male parents were highly significant (P≤0.001) ( Table 10). Specific combining ability (SCA) was significant for both PSKI3 and PSKI7; however the SCA for PSKI3 was more significant (P≤0.001) than PSKI7 (P≤0.01) (Table 10). The GCA‟s were much higher than SCA for both PSKI3 and PSKI7 as shown by the high baker‟s ratio of 0.73 and 0.67 respectively. Combining ability analysis for aflatoxins (ppb) and log transformed value revealed the GCAf, GCAm, and SCA were all highly significantly high (P≤0.001) (Table 11). Generally the GCA was approximating to SCA for log transformed aflatoxins value, as shown by the low baker‟s ratio of 0.49 (Table 11). Two genotype of AGRA 99044, ICGV 93474, had a negative GCA effects and therefore were considered the best combiners for both resistance to A.flavus and aflatoxins accumulation, implying they can be used in hybridization effectively in multiple crosses. SCA effect was negative and significant (P≤0.05) for a cross between Serenut 1x Serenut 4 for A.flavus resistance, while for aflatoxins accumulation the SCA effects were negative and significant for crosses of Serenut 1x ICGV 94374, Red beauty x Erudu Red, AGRA 99044 x AGRA 99031, and AGRA 99031 x Serenut 4 (P≤0.001), suggesting that cross combinations would be desirable in hybridization, since the produce high frequency of resistant progeny. Estimates of NS- CGD from variance components was 66%on genotype mean basis and from mip-parent offspring regression was 74% for PSKI7 with A.flavus. indicating that the performance of the off springs could be predicted from the performance of the parents used .There were both low estimate of NS-CGD (46%) on genotype mean basis, and narrow sense heritability h2 from mid-parent offspring regression (35%) for aflatoxins accumulation, suggested that, prediction of offspring performance from the parents is limited by non-additive and environmental variances. High BS-CGD obtained for resistance to A. flavus (85%) and aflatoxins accumulation (96%), suggested that variance due to environment in the overall phenotypic expression of the resistance traits was minimum.