Host-plant interactions and resistance mechanisms to banana weevil Cosmopolites sordidus (Germar) in Ugandan Musa germplasm

English: Banana is an important subsistence crop in many tropical regions of the world. In the East African Great Lakes region it constitutes a staple food for more than 17 million people. Among many production constraints, banana weevil (Cosmopolites so rdidusi. is the most serious pest to the crop. The tunnels caused by the boring larvae interfere with water and mineral uptake, and provide entry for fungi and bacteria. Most importantly this weakens the corm, leading to toppling of plants even in slight winds. Host plant resistance is considered the basis of any successful integrated pest management plan, if the banana weevil problem is to be solved by the resource poor farmers. The screening trial of 45 Ugandan Musa germplasm accessions revealed that the plantain subgroup (AAB) was most susceptible to banana weevil followed by East African highland banana clones. The exotic bananas, especially Kayinja, Bluggoe (ABB), Kisubi and Ndiizi (AB) were resistant to banana weevil. Plantain derived tetraploid hybrids of the wild banana Calcutta-4 were also susceptible, indicating dosage effect of the susceptible gene. Mbwazirume, Tereza and Nakyetengu have been found to be relatively resistant local land races and they are recommended as possible resistant selections. Total inner damage was found to be the best criteria for screening, and selecting for weevil resistance, since it scored the highest heritability and was highly correlated with all other weevil damage indices. Significant phenotypic and genotypic correlations were found between corm hardness, dry matter content, sap/resin production, suckering ability and corm size and banana weevil damage. These were therefore considered important mechanisms of resistance in Musa and this indicated that banana weevil resistance is a complex polygenic trait. In agreement with the literature studied. antixenosis was not found to be important as a resistance mechanism in Musa. However. results from various no-choice experiments on hatchabil ity and development revealed significant differences. The previously observed resistance cultivars Kayinja. Yangambi-km5 and FHIA03, unlike the more susceptible plantains and EAHBs, significantly increased developmental time and in some cases caused mortality of immature weevil stages. Preliminary studies on the influence of secondary metabolites were undertaken. The results showed the presence of two or three compounds indicated by peaks on HPLC chromatograms of methanol extracts of corms from resistant cultivars (e.g. Kayinja and FHIA03) that were not present in susceptible cultivars (e.g. Atwalira and Gonja). These substances were also not present in some resistant cultivars like Yangambi-km5 and Cavendish, both of the AAA genome group. This was yet another indication that resistance is complex and these different factors are important in different groups of cultivars.