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Yam : cultivar selection for disease resistance and commercial potential in Pacific Islands. SPYN

Lebot V. (ed.), Jackson G.. 2003. s.l. : s.n., 174 p..

Yam (Dioscorea alata) is a crop with potential for increased commercial exploitation. However, several problems are limiting its development: tuber shape is often irregular making harvest time-consuming and anthracnose disease, caused by the fungus Colletotrichum gloeosporioides, is always a threat. In addition, the lack of information on starches hinders the prospective utilisation of yam as a high quality vegetable. The SPYN project had 5 objectives: 1- Characterisation and evaluation of germplasm, to rationalise collections and to select cultivars of interest in Fiji, Vanuatu, the Solomons and Papua New Guinea. 2- Development of in vitro conservation strategies. 3- International exchange of virus-tested genotypes. 4. Identification of sources of anthracnose inoculum; and 5. Agronomic evaluation of selected cultivars. Overall, 1,040 accessions were collected (Fiji 108, PNG = 209, Solomons = 392, Vanuatu = 331). Based on tuber shape, tolerance to anthracnose and palatability properties, the best varieties were selected and propagated. Samples were taken on 719 plants representing nine Dioscorea species from seven South Pacific countries. ELISA was used to examine the presence of YMV, DAV, DDV, DABV, DBBV, DLV, and CMV. Some samples were later tested for two more viruses of Asian origin: JYMV and CYNMV. DAV (Potyvirus) was the virus most commonly detected (69% of samples) by serology (ELISA) in D. alata, D. esculenta, and D. bulbifera, from across the South Pacific region. YMV (Potyvirus) was commonly found in D. rotundata plants from Africa, but has not been demonstrated in the South Pacific. Eighty-one isolates of C. gloeosporioides were characterised according to radial growth, the morphology of reproductive structures in culture, and 49 isolates according to their pathogenicity on D. alata leaves. Molecular fingerprints of 39 isolates from yam leaves and tubers, and from other plants were obtained using ISSR primers by PCR amplification following extraction of DNA. There were significant differences in pathogenicity among C. gloeosporioides isolates. However, there was no clear association between morphological data and pathogenicity among isolates studied. The detection of C. gloeosporioides in yam tubers (4.8%) confirms that the fungus is able to infect and survive in tuber tissue from season to season. It is likely that the infected tubers could act as a primary source of inoculum playing an important role in the epidemiology of the pathogen in the field. C. gloeosporioides is a polyphagous pathogen with a wide host range and, under optimum conditions, is able to cross-infect different host species. AFLP study on the Vanuatu collection indicated that D. alata was closer to D. nummularia than D. persimilis, one of the putative parents of D. alata. D. nummularia accessions fall into two groups, with three cultivars of intermediate type closely associated. The data suggest that they are a distinct species from D. alata and D. nummularia, but share a common ancestor. Comparisons of these varieties and D. transversa from New Caledonia suggest they belong to that species. D. alata accessions from different parts of the world clustered together indicating that there has been wide distribution of clones. Tetraploids, hexaploid and octoploid varieties of D. alata are present in the collection, although hexaploids are rare. Hexaploids and tetraploids are probably auto-polyploids. Analyses were made for percentage starch, amylose, lipids, minerals, proteins, sugars and gelatinisation temperature range on 110 accessions. Significant variation exists within each country for each of these major characteristics. Good varieties are characterised by high dry matter, starch and amylose contents. Chemotypes are genetically controlled. SPYN has produced scientific data that will be used in the long term: one Ph.D. thesis and one MSc thesis have been completed and another Ph.D. thesis will be defended in 2004. The project has emphasised ...

Mots-clés : dioscorea alata; maladie fongique; glomerella cingulata; infection; germplasm; variété; sélection; pouvoir pathogène; résistance aux maladies; Îles du pacifique

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