ASSOCIATION MAPPING AND STABILITY ANALYSIS OF RESISTANCE TO CASSAVA GREEN MITE IN CASSAVA (Manihot esculenta Crantz)
Cassava is a unique crop, because it is the primary source of carbohydrate for more than 1 billion people. In Nigeria, cassava has become a major sustenance crop and currently and is the world’s largest producer of cassava. However, the yields on smallholder farms are relatively low largely due to pests’ infections and other various cassava production constraints. A participatory rural appraisal study was therefore conducted to gather information on farmers’ preferences, perception and knowledge of cassava green mite (CGM) and other production constraints and to lay the foundation for the development of CGM resistant cultivars in Nigeria. Individual interviews and focus group discussions involved 360 farmers in Abia, Anambra and Benue states. Termites, CGM and whitefly were recognized as the major pests that contribute to low yields and abandonment of some cassava cultivars by farmers. Majority of the farmers in the surveyed areas had little or no knowledge of CGM. However, farmers depend on traditional cultural practices such as weeding, selective pruning, use of barriers and setting traps as effective measures to reduce pest populations in their fields.
These methods interfere with the survival of natural enemies of CGM. There is need to educate the farmers about the importance of CGM and the benefit of natural enemies. Resistance to pests and diseases, high yield and early bulking were highly ranked by farmers as their preferred traits. Cultivars which lack in most of these traits have been abandoned by farmers. Therefore, there is need to look for genetic resources for these farmers desired traits and incorporate them into new cultivars through plant breeding methods. To evaluate the presence of resistance genes in the available germplasm, a diverse panel of 845 advanced breeding lines obtained from the International Institute of Tropical Agriculture (IITA), the International Center for Tropical Agriculture (CIAT) and National Root Crops Research Institute (NRCRI) were evaluated for cassava green mite severity (CGMS), leaf pubescence (LP), leaf retention (LR), stay green (SG), shoot tip compactness (STC) and shoot tip size (STS). A genome-wide association mapping using mixed linear models detected 35 single nucleotide polymorphisms (SNPs) markers significantly associated with CGMS, LP and LR on chromosome 8. Co-localization of the most significant SNP associated with CGMS, LP and LR at chromosome 8 is possibly an indication of the presence of pleiotropic effects or closely linked genes that regulate these traits. Seventeen candidate genes were found to be directly linked to cassava green mite resistance. These candidate genes were subdivided into seven categories according to their protein structure: zinc finger, pentatricopeptide, MYB, MADS, homeodomain, trichome birefringence related protein and ethylene-responsive transcription factor genes. Genome wide association study revealed the presence of CGM resistant genes, which might represent new sources of resistance for the on-going effort to develop improved cassava cultivars.
A combined additive main effect and multiplicative interaction (AMMI) analysis of variance revealed that there were significant genotypic variations for all the traits indicating opportunity for selection and prospects for the improvement of cassava for the traits. The impact of environment was highly significant for cassava green mite severity at 6 months after planting (MAP) (CGMS6), leaf pubescence at 9 MAP (LP9), leaf retention at 9 MAP (LR9), stay green at 9 MAP (SG9), shoot tip compactness at 9 MAP (STC9), shoot tip size at 9 MAP (STS9), fresh root yield (FRY), root dry matter content (RDMC) and biomass justifying the need for multilocational testing to identify good performers for specific locations. There were significant responses of genotype by environment interaction for CGMS6, LP6, LP9, LR6, LR9, SG6, STC6, STC9, STS6, FRY, RDMC and biomass. This implies different adaptation by the different genotypes suggesting the need to identify and select location specific genotypes for different environments. 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. Stable high yielding genotypes such as IBA131866, IBA131746, IBA131872, IBA131767 and IBA131770 which combine high FRY and resistance to CGM were identified, suggesting it is possible to combine these traits in cassava as desired by farmers.