Okra germplasm collected from different locations around the world were screened at AVRDC(The World Vegetable Center) in 2011 and 2012 to identify germplasm resistant to the melon aphid (Aphis gossypii) for use in sub-Saharan Africa. A total of 260 okra accessions and varieties were screened (150 at AVRDC Taiwan and 110 at AVRDC Cameroon), which included four varieties commercially available in Cameroon. The experiments were conducted under natural infestation in Shanhua, Taiwan, and at Yaoundé, Cameroon. Since the preliminary screening trials were conducted in Taiwan and Cameroon, the aphid populations in these two countries were compared. A total of 60 insects was used for cytochrome c oxidase I (COI) gene sequencing and phylogenetic analysis. The nucleotide sequences of all the populations showed 100 % similarity and the phylogenetic analysis confirmed the genetic similarity of A. gossypii in Taiwan and Cameroon. Results of the screening trials showed that three accessions (VI033805, VI036213 and VI051114) were resistant to A. gossypii. The basis of resistance of the three okra accessions was elucidated by studying their biochemical and biophysical properties. There was no significant difference between the susceptible and resistant okra accessions in terms of leaf tannins, free amino acids, total sugars and total phenols. Only total nitrogen was significantly different between the susceptible and the two resistant okra accessions with the lowest aphid infestation (VI033805 and VI036213). Thus, higher leaf nitrogen content seems to favour the aphid infestation on okra. For physical parameters, there was no significant difference among the accessions in trichome density of bottom and middle leaves, and leaf toughness. However, trichome density in the younger leaves of resistant VI033805 was significantly higher than susceptible VI057245. Studies on settling behaviour showed that aphids did not discriminate between the susceptible and resistant okra accessions for oviposition and feeding 72 h after release.
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Vol. 22 • No. 2