PRACTICAL APPLICATION OF VIRUS DIAGNOSTICS IN 'CLEANING-UP' YAM AND CASSAVA GERMPLASM FOR DISTRIBUTION

Abstract

The genebank at the International Institute of Tropical Agriculture (UTA) has 3,428 accessions of Dioscorea spp. and 2,385 of Manihot esculenta. These are maintained wherever possible as virus-tested in vitro plantlets. However, many of the accessions are virus-infected or are in the process of field rejuvenation, and therefore their health must be re-assured as they are taken into tissue culture for long-term conservation and subsequent multiplication for international germplasm distribution. The most important viruses infecting cassava in Africa, and prevalent in some of the genebank accessions, are the cassava mosaic geminiviruses, while Dioscorea spp. worldwide are infected by over 12 viruses, of which at least six are found in Africa, with Yam mosaic virus genus Potyvirus being the most prevalent. Diagnostic tests for the viruses infecting cassava and yams used in the clean-up of genebank accessions through meristem-tip culture include both protein-based and nucleic acid-based tests. The protein-based tests are serological tests, which are usually direct or indirect enzyme-linked immunosorbent assays, using monoclonal or polyclonal antisera. The nucleic acid-based tests are a range of polymerase chain reaction (PCR) tests to detect both ELVA and DNA viruses. These tests, with meristem tip culture and in vitro conservation, were carried out on cassava and yam plantlets for certification by the Nigerian Plant Quarantine Service. Out of the 45 Dioscorea spp. plantlets tested, a total of 27 were found to be infected with viruses infecting yams, while 124 cassava plantlets were infected out of the 422 tested. These results showed that both ELISA and PCR were effective in the production of virus free yam and cassava plantlets for healthy germplasm distribution.