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Abstract

Cocoyam, Xanthosoma spp., is cultivated in tropical regions as a source of carbohydrates for human nutrition, for animal feed, and cash income for the farmers. It is the most important export crop of all the root and tuber crops grown in Nicaragua, but the national yield and total planted area have drastically decreased during the last few years. The crop has received little research attention, and previous to the studies presented in this thesis there was hardly any information available regarding the number of genotypes cultivated in the country, the agronomic performance of the different genotypes, which diseases that infect the crop, and to what extent these diseases influence the yield. Three cocoyam genotypes were evaluated regarding agronomic traits in four field locations during two years. Significant interactions between genotype and location as well as between genotype and year were found for many of the traits. A differential response of the genotypes to the varying climatic conditions at the locations is one of the possible causes of these interactions. Different harvest times for the three genotypes are suggested, due to differences in time to reach the maximum growth. Dasheen mosaic virus (DsMV) is the most important virus that infects cocoyam. Virus-free plants were produced through meristem culture, and protocols for the regeneration of plants via callus and protocorm-like structures were established. Although a genotypic response to tissue culture was observed, all the tested genotypes produced shoots from meristem and from meristem-derived calli. Under field conditions the virus-free plants performed better, regarding yield, than infected in vitro plants. The fact that the plants were virus-free could not, by itself, explain the yield increase, suggesting that the process of in vitro re-invigoration also played an important role. Aphis gossypii was the only winged aphid observed during the field trial, suggesting that this insect is the virus-transmitting vector. cDNA fragments containing the coat protein (CP) and 3’-UTR regions of nine DsMV isolates were molecularly characterised. One isolate had a 19 aa long deletion in the N-terminus of the CP region. The Nicaraguan isolates formed two subgroups correlated with geographic origin of the cocoyam genotypes. Author’s address: Guillermo Reyes Castro, Facultad de Agronomía, Universidad Nacional Agraria (UNA), km 12 ½ Carretera Norte, Apdo. 453, Managua, Nicaragua. E-mail: Guillermo.Reyes.Castro@una.edu.ni