CHAPTER 7: OVERVIEW OF THE RESEARCH FINDINGS AND IMPLICATIONS FOR
• Other constraints pointed out by the farmers included distance to markets and lack of transport to ferry the produce, lack of capital and labour to work their fields and inadequate planting materials.
• Most of the farmers grew local landraces on small fields of less than 1 ha.
• Farmers intercrop cassava with either bean or maize. Cassava is also grown as a sole crop in some districts.
7.3 Cassava geminiviruses and satellites occurring in Luapula province
The comprehensive cassava virus and satellite survey conducted in this study was the first to be carried out in the five districts (Samfya, Mansa, Mwense, Kawambwa and Nchelenge) of Luapula province in Zambia. The outcome of the survey formed the basis of the research study.
The following were the findings of the survey:
• The two viral strains, EACMV, ACMV and associated satellites (II and III) were present in all the five districts surveyed signifying the potential to cause severe yield losses. This is the first report of CMD associated satellites in Zambia.
• Cassava brown streak virus and Ugandan variant (UgV) strain of EACMV was not detected in the surveyed areas.
• East Africa cassava mosaic virus was more predominant than ACMV in the districts surveyed.
• The ACMV and EACMV were found in single and mixed infections. In plants with mixed infection, the plants were characterised by severe symptoms with yellow and green patches.
• Leaf distortion with reduced leaf blade characteristic of satellites was observed on a number of infected plants.
• Variation of CMD on cassava cultivars was observed implying that the genotypes have an influence on disease expression. To select plants with CMD resistance the two virus strains have to be taken into account.
• Generally CMD incidence was high in all the districts surveyed with an average of 59%.
CMD incidence ranged from 13.3 to 93.3% while disease severity ranged from 1.9 to 2.6 on a 1-5 scale
7.4 Evaluation of genotypes to CMD
The reaction of selected cultivars to CMD was evaluated at Mansa Research Station in Luapula province. The findings were:
• Genotypes significantly (P<0.001) varied in reaction to CMD.
• Bangweulu, Mwakamoya, Chila7, Kalaba, Namuyongo and Chila11 were the most susceptible cultivars.
• Mweru, Nalumino, and Kampolombo were some of the local cultivars which exhibited tolerance to CMD
• All the traits evaluated were significantly (P<0.001) different
7.5 Evaluation of F1 progeny
F1 seedling evaluation trial was conducted at Mount Makulu Research Station. The main findings were:
• Seed germination averaged 70.5%.
• Variation in the F1 progeny was observed in all the traits measured
• Significant correlations between branching height, dry mater content and leaf retention were recorded. However, there was significant correlation between fresh root yield and dry matter content
• The overall mean root dry matter content was high at 39.4%.
• The fresh root yield ranged from 0.1 to 5 kg plant-1 with a mean of 1.57 kg plant-1.
7.6 Combining ability analysis
The trial to determine combining ability for CMD analysis of cassava to cassava mosaic disease was conducted at Mansa Research Station. Eight hundred genotypes were evaluated for CMD using spreader rows and wedge grafting. Wedge grafting was performed to ensure that susceptible genotypes did not present as false resistant genotypes. The following were the findings:
• General combining ability (GCA) and specific combining ability (SCA) MS were significant for CMD
• GCA SS for male parents accounted for 12.5% and, GCA SS for female parents accounted for 19.6%, while SCA SS accounted for 67.9% of the crosses for CMD
• The high SCA SS for resistance to CMD signified predominance of non-additive gene action over additive gene action for this trait.
• Female parents, Bangweulu and Kampolombo had desired negative GCA effects for CMD. In terms of male parents, TMS190, Nalumino and TME2 were good combiners with negative GCA effects.
• The GCA SS at 70.2% of the crosses SS predominated over SS for fresh root yield.
• Bangweulu and Chikula had significant GCA effects for fresh root yield and are considered as good general combiners for the trait.
7.7 Progress in breeding for CMD resistance
Major strides were made in terms of developing CMD resistant progeny arising from different cross combinations using local cultivars and introductions as parents. The progeny on average had a lower disease severity than the parents an indication of a major improvement as reflected in the general negative heterosis for CMD. Similarly there was noticeable fresh root yield improvement in some of the families and specific clones. Among the best performing clones were genotype 8444 (3.35 kg plant-1) from TMS3001 x Bangweulu and 3101 (2.85 kg plant-1) from Bangweulu x TMS190. Mid parent heterosis for the above clones was more than 220%.
These clones did not only perform better for fresh root yield but were also resistant to CMD.
Furthermore, the progeny exhibited earliness at 7 MAP.
7.8 Further research
Further research is required to understand the implication of using the grafting technique on movement of the viruses within the plant. In addition, there is need to understand why the buds immediately below the graft showed symptoms (in susceptible genotypes) unlike buds further away from the grafted area of the same branch. There is need to evaluate the promising clones in other ecological regions for all the agronomic traits.
7.9 Implications of the research findings for cassava breeding and the way forward Since there have never been formal efforts to develop cassava genotypes with CMD resistance and associated satellites in Zambia, it was important to develop a local breeding programme with specific emphasis on breeding for resistance to this disease. Breeding for resistance