Development of cassava (Manihot esculenta Crantz) cultivars for resistance to cassava mosaic disease in Zambia.
Despite the increasing number of farmers growing cassava in Zambia, yield per hectare has remained low at 5.8 t ha-1. The major constraints contributing to low yields are pests and diseases of which cassava mosaic disease (CMD) caused by East Africa cassava mosaic virus (EACMV), Africa cassava mosaic virus (ACMV) and South Africa mosaic virus (SACMV) is the most important. Breeding of cassava is restricted by limited information on viruses and associated satellites, and farmer preferences. Most of the farmers cannot manage to institute control strategies that require buying of chemicals. The most feasible option remains improving existing cultivars through resistance breeding. The study therefore was conducted to: i) establish farmers’ perception and knowledge of CMD; ii) to identify viruses of cassava occurring in Luapula province; iii) evaluate the performance of local and improved cultivars for agronomic traits; iv) evaluate the performance of F1 progenies for CMD resistance; and v) determine general combining ability and specific combining ability for CMD resistance. The studies were carried out between 2008 and 2011 at different locations in Zambia. The information generated was important in formulating a local breeding strategy for CMD resistance. A participatory rural appraisal and a structured survey was conducted in Mansa, Samfya and Mwense districts in Luapula province involving farmers to ascertain farmers’ perceptions of CMD. The results of the study showed that the majority of the respondents (97.6%) were not aware of CMD. Most of the farmers grew landraces on small pieces of land. Although, the cultivars (local and improved) were widely grown, they were susceptible to CMD. The farmers preferred cultivars with high yielding and early bulking characteristics among others. A CMD survey conducted between April and May 2009 in Samfya, Mansa, Mwense, Kawambwa and Nchelenge districts in Luapula province established East Africa cassava mosaic virus (EACMV), and Africa cassava mosaic virus (ACMV) as the most prominent viruses in the area. Symptoms of satellites were also observed in the farmers’ fields in most of the areas visited. Satellite II and III were detected in leaf samples. The CMD incidence (59.1%) and severity (2.4) was moderate across the districts surveyed. The CMD symptoms on the cassava plants were variable with plants showing mild and severe symptoms characterised with narrowing and reduced leaf blades. The transmission of CMD infections was mainly through cuttings rather than via whitefly infection which means that most of the planting materials used by the farmers were infected. Evaluation of cassava cultivars for CMD resistance was conducted in 2009/2010 and 2010/11 seasons at Mansa Research Station in Luapula province using a 4 x 4 α lattice design. Both introduced and locally grown cultivars had significant (P<0.001) differences in their reaction to CMD. Bangweulu, Namuyongo, Kalaba, Chikula, Mwakamoya, Chila7 and Chila11 were the most susceptible genotypes. Mweru, Tanganyika, and Nalumino were moderately tolerant to CMD. Eight hundred F1 genotypes developed using a North Carolina II mating design were evaluated in a 4 x 5 α lattice design in 2011 at Mansa Research Station, Luapula province to determine combining ability for reaction to CMD, yield and yield components. The plants were harvested 7 months after planting (MAP). Significant (P<0.001) general combining ability and specific general combining ability were recorded for CMD. The SCA effects were more important for CMD than GCA effects suggesting that non-additive gene action was more prominent than the additive gene action in determining CMD reaction. Parent lines with desired significant, negative GCA effects for reaction to CMD were Bangweulu, Kampolombo, Nalumino and TME2. In general, the survey and participatory rural appraisal established CMD as one of the constraints to cassava production and created a basis for the research study. The findings indicate opportunities that exist in creating genotypes with tolerance to CMD. The study identified cassava lines with resistance to CMD. The lines that expressed the above trait should be selected and tested further for release to the farmers in Zambia. Since the clonal evaluation trial was harvested at 7 MAP, there is need to investigate further for earliness trait in best performing lines in different locations.
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