Participating
institutions
Institute for Plant Sciences, ETH Zurich
International Institute of Tropical Agriculture IITA, Ibadan Nigeria
University of Ibadan, Nigeria
Cassava is an important staple for millions of people in Sub-Saharan Africa. Its tuberous roots provide carbohydrate and the leaves serve as source of protein, minerals and vitamins. Constraints to improvement, difficult to address using conventional breeding, have been identified for priority attention through biotechnology. Routine use of genetic transformation in cassava to develop enhanced germplasm for delivery to farmers requires reproducible regeneration and transformation systems to be applicable across a wide range of genotypes. Regeneration methods based on somatic embryogenesis and organogenesis, compatible with Agrobacterium-mediated transformation and biolistics, have been developed using model cassava genotypes but their general applicability to African germplasm was not assessed. The objective of this project is to transfer and improve existing regeneration and transformation protocols to a range of African cassava germplasm at the International Institute of Tropical Agriculture (IITA).
The project succeeded in adapting the organogenesis-based regeneration system to more than 10 African cassava genotypes. Cyclic somatic embryogenesis, organogenesis, and plant regeneration were achieved. Friable embryogenic cultures were successfully established and maintained using the model cassava genotype (TMS 60444) and induction of friable embryogenic callus (FEC) was also demonstrated in two genotypes. Agrobacterium-mediated transformation, compatible with shoot organgenesis and FEC were successfully tested and transient GUS expression has been demonstrated in cotyledon pieces, embryogenic units and clusters.
In 2002 emphasis was placed on genetic transformation and production of transgenic plants. By combining our results on the Agrobacterium–mediated transformation and antibiotic sensitivity tests with various cassava tissues and regeneration systems, transformation experiments using marker genes were carried out. Hygromycin-resistant embryogenic units and clusters, and shoot buds were obtained from transformation studies using FEC and organogenesis regeneration systems respectively. Shoots regenerated from co-cultured cotyledon pieces were all escapes. However many GUS-positive embryogenic units and clusters were obtained from the hygromycin-resistant embryogenic units and clusters resulted from co-cultivation of FEC with three Agrobacterium strains. Plantlets were recovered from hygromycin resistant embryogenic units and clusters, which were transferred, to embryo maturation and germination media, and then to elongation medium. Shoot apex, root and leaf obtained from regenerated shoots as well as whole plantlets were tested GUS positives. These indicated that the regenerated shoots and plantlets were putative transgenic. The transgenic lines were multiplied and molecular characterisation will be carried out on these plants.
The fellow has successfully completed his PhD program. This is the first report of success in obtaining transgenic cassava plants in Africa and it will serve as a model for future testing of the capability of other farmers-preferred cultivars to be transformed and to integrate agronomically useful genes into this germplasm at IITA.
Regeneration and transformation of African cassava germplasm
Integrated Pest Management of Cassava Whiteflies
Contact: Anthony Bellotti
An integrated pest management (IPM) program for cassava pests in Valle del Cauca is being designed and implemented. Because farmers’ surveys show that the predominant pest, for both small and large farmers, is whitefly, emphasis is being given to the complex of whitefly species associated with the crop. At present, research activities for whitefly control, many funded by the Colombian Ministry of Agriculture and Rural Development (MADR), include host-plant resistance, biological control, cultural practices, and chemical control. CIAT and CORPOICA have jointly developed a cassava variety that is resistant to whiteflies. Released in November 2002, this variety is expected to greatly aid small farmers to control this pest.
Various biological control agents, including predators, parasites, and entomopathogens have been identified and are being evaluated. An isolate of the entomopathogenic fungus Verticillium lecanii (CIAT 215) has shown highly positive results in preliminary trials, and has the potential to be commercially formulated. A predator species, Chrysopa sp., has been collected from whitefly infested cassava fields, and is being mass reared in the laboratory. It will then be evaluated in the field for whitefly control. Numerous parasitoid species of whiteflies have been identified in field surveys in three countries (Colombia, Ecuador, and Venezuela) but their efficiency in controlling high whitefly outbreaks is still being assessed.
A 2-month prohibition on growing cassava at CIAT’s Palmira station dramatically reduced whitefly populations in subsequent plantings. The prohibition is now recommended practice, especially for large-scale cassava farmers in Valle del Cauca.
Most farmers, small or large, resort to pesticide applications to control whiteflies. In a series of field trials, Confidor® (imidacloprid) gave the best results for controlling cassava whiteflies, with Actara® (thiamethoxan) also giving favorable results. However, a cost-benefit ratio study shows that, while pesticide use for larger farmers is profitable because of a guaranteed higher price for cassava roots, it may be uneconomical for small farmers. These results reinforce the idea that alternative methods must be sought for whitefly control that are efficient and cost effective for the small farmer, as well as being environmentally sound.
Contact: Anthony Bellotti
Adobe PDF document Further Information
Cassava Entomology, Annual Report 2002 (1135 kb)
The System Wide Tropical Whitefly IPM Project: From Phase 1 to Phase 2, Annual Report 2002 (180 kb)
hyperlink.gif (169 bytes) Related Web Site
Tropical Whitefly IPM Project
A Cassava Variety Resistant to Whitefly
Nataima-31: A cassava variety resistant to whiteflyWhitefly resistance in agricultural crops is generally rare, and control usually requires the continued use of agrochemicals. Pesticide use reduces farmers’ income, and is especially detrimental to small farmers who do not have easy access to credit for purchasing costly inputs. Host-plant resistance (HPR) offers a low-cost, efficient, and easy-to-use technology for controlling major pests in cassava, such as whiteflies.
Several good sources of resistance to whiteflies have been identified in cassava, and high-yielding, whitefly resistant, cassava hybrids are being developed. The hybrid CG 489-31 (CIAT breeding code) is officially being released by CORPOICA in November 2002, under the varietal name ‘Nataima-31’, probably the first of a food crop to be released for whitefly resistance. This variety was developed over 15 years in a collaborative effort between CIAT and CORPOICA. Especially for small cassava farmers, ‘Nataima-31’ is a high quality, high-yielding, cassava variety that will require little or no pesticide use. It is also moderately resistant to thrips and mites.
Frogskin Cassava
Detecting a Phytoplasm Associated with Frogskin Disease in Cassava (Manihot esculenta Crantz) in Colombia
Frogskin (FSD) is an important cassava disease that affects roots. The causal agent remains unknown, even though it has been reported with increasing frequency in Colombia, Brazil, and Venezuela. Yield losses close to 90% have been reported in commercial fields in Colombia.
The specific primers R16mF2/R16mR1 and R16F2n/R16R2 were used in a nested PCR assay to detect and confirm that phytoplasms were associated with FSD. To characterize and subsequently classify the phytoplasms, two pairs of universal primers (P1/P7 and R16F2n/R2) were used to amplify the 16S rDNA gene. Root, stem, leaf, and flower samples were taken from symptomatic plants, and 1.2-kb fragments amplified.
Sequence analysis of the cloned fragments revealed that the phytoplasms found were similar to the Chinaberry yellows phytoplasm (GenBank acc. no. AF495657, 16SrXIII Mexican periwinkle virescence group) and the Cirsium white leaf phytoplasm (GenBank acc. no. AF373106, 16SrIII X-disease group), both with a sequence homology of 100% and 99%, respectively.
The presence of phytoplasms was confirmed by the DAPI and Dienes’ staining methods and by grafting. This is the first report of phytoplasms associated with FSD in cassava.
Contact: Elizabeth Alvarez
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