Manipulating morphological traits of cassava to enhance host plant resistance and biological control of cassava green mite in Zambia.
Understanding direct and indirect defense mechanisms that enhance host plant resistance (HPR) and biological control is critical for successful development of an integrated pest management (IPM) approach. Cassava green mite (CGM) (Mononychellus tanajoa Bondar (Acari: Tetranychidae)) is a major arthropod pest of cassava (Manihot esculenta Crantz) in Africa. Strategies to control CGM include HPR and biological control by use of exotic natural enemies particularly the predatory mite Typhlodromalus aripo DeLeon (Acari: Phytoseiidae). The success of the latter depends on continuous survival of the natural enemy which requires suitable host plants and weather conditions. Various plant morphological traits have been recognized as indirect defense mechanisms that enhance HPR to CGM, and/or attract T. aripo in cassava. It was envisaged that integration of HPR and classical biological control approaches through manipulation of such indirect defense traits would lead to a more sustainable management of CGM in view of anticipated climate change. Lack of information on farmers’ perception of CGM and preferred varietal attributes, and gene action controlling the inheritance of CGM resistance also limits success of resistance breeding and adoption of varieties. This research was undertaken to gather information on farmers’ perceptions of cassava varietal attributes and cultural practices in relation to CGM resistance, identify suitable sources of resistance and environments for future breeding; and to determine the nature of gene action controlling CGM resistance and the inheritance of plant morphological traits that enhance the ability of cassava to host and support continuous survival of natural enemies. High fresh storage root yield (FSRY), high storage root dry mass percentage (SRDM%), earliness combined with extended underground storability, and resistance to foliar pests and diseases are the major factors that influence adoption and retention of genotypes by farmers. Moles, termites and CGM are the most widespread and most damaging pests. However, due to the non-conspicuous nature of CGM, its effects are under-estimated and are given limited attention by farmers. The majority of the farmers are familiar with CGM leaf damage symptoms but they cannot associate them with the actual pest. Participation of farmers in field training and field research activities helps them to know CGM. Crop rotation, intercropping, removal of shoot tips, selective pruning of infested shoots, and burning of cassava fields are some of the ways used by farmers to manage CGM. Farmers associate hairy broad-leaved, tall cassava genotypes and pink leaf pigmentation (anthocyanin) with low CGM damage. There is substantial genetic variability in the Zambian cassava germplasm for CGM resistance and associated plant morphological traits such as leaf pubescence (Pbs), leaf retention (LR), stay green (SG), tip size (TS), tip compactness, and plant height (PH), stem diameter (StD), SRDM% and FSRY. Genotypes with wide or specific adaptability for these traits have been identified, and should be recommended for general or localized production and for use as sources of desired genes in crop improvement. Genotypes L9.304/147, 92/000, TME2, 4(2)1425, I60/42 and L9.304/175 combine wide adaptability with high levels of resistance to CGM. Genotypes Kapeza, L9.304/147 and 4(2)1425 are able to produce 13-15 t ha ¯¹ at 9 months after planting suggesting their potential for early bulking. This study has shown that both additive and non-additive gene effects play a role in the expression of CGM resistance and associated plant morphological traits. The best combinations of parents for resistance against CGM were 4(2)1425 x L9.304/147 and Mweru x L9.304/147, while L9.304/147 x I92/000 displayed combined resistance to CGM and cassava mosaic disease (CMD). The resistance of cassava to CGM is positively correlated with Pbs, LR, and TS, SG, PH, StD. Overall, the study has shown that there is wide diversity in the expression of valuable indirect defense traits among genotypes, indicating that there is scope for integration of biological control and host plant resistance for CGM in Zambia. The release of genotypes that exhibit high level of intra-season and inter-season stability for enhanced expression of LR, SG, and Pbs will minimize the impact of CGM on FSRY and SRDM% that results from seasonal effects. Such genotypes should also provide habitat for and thus help to ensure the survival of T. aripo in cassava fields. The study has contributed to the promotion of food security through identification of early-bulking genotypes which also have good potential for extended underground storability of roots. Early-bulking, high FSRY and SRDM% and SRR resistance are farmer-preferred traits. Therefore, enhancement of such traits through plant breeding is likely to increase the adoption of new genotypes by farmers.
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