Cassava Germplasm Conservation and Improvement in India

Genetic diversity of cassava (Manihot esculenta Crantz), obtained by open or controlled pollination and selection, as well as from germplasm introductions from abroad, is being conserved, both in the field and in an in vitro gene bank. Nine hundred and eighty five germplasm accessions are being conserved under slow-growth conditions using an osmotic retardant medium. The major thrust given in the varietal improvement program are on Indian cassava mosaic disease (ICMD) resistance, improvement of dry matter and starch content, early bulking and culinary quality. Collaborative activities with international organisations like CIAT, and EMBRAPA (Brazil) and with the national All-India Coordinated Research Programme (AICRP) on Tuber Crops have been beneficial. Field evaluation of breeding lines received as in vitro cultures from CIAT has resulted in the identification of one line, MNga 1, showing resistance to ICMD. For the past eight years this line has shown only very mild symptoms of ICMD in the field. The resistant character was confirmed by a grafting test. This variety had a root yield of 34 t/ha, had 43% dry matter and 33% starch content, while showing normal flowering and seed set. This variety has now been crossed with both released and popular local varieties in order to incorporate the ICMD-resistant gene. Interspecific hybridization of cassava with other Manihot species, such as M. caerulescence, M.tristis, M. glaziovii, M. epruinosa, M. esculenta var flabellifolia, M. esculenta var peruviana have been carried out at CTCRI for the past ten years with the objective of transferring ICMD resistance. The interspecific hybrid of cassava with M. caerulescence has shown stable resistance, which was confirmed through wedge grafting. The ICMD-free backcross breeding lines produced root yields ranging from 0.5 to 8.5 kg/plant, and had starch contents ranging from 19.2 to 36.6%. Efforts to improve the culinary quality and root shape continue. A new cycle of interspecific crossing has now started using M. tristis, M. caerulescence (both cultivated and wild), M. catingae, M. dichotoma, M. pseudoglaziovii and M. epruinosa as parents. Starch and dry matter content have also been improved through triploidy breeding, which resulted in the release of the first triploid cassava variety, Sree Harsha, which is presently undergoing testing in industrial areas of Tamil Nadu. Triploids with early bulking and good cooking quality (at 7-month harvest) have been selected from an intervarietal cross of the local variety Ambakadan with H-2304 (4x); hyperploids of varieties with good culinary quality (M4, Sree Jaya, Sree Vijaya) have also been developed. Inbreeding was found to be very effective in breaking up of populations into widely divergent but reasonably uniform groups. Crossing of such early generation inbred lines with high-yielding hybrids like Sree Visakham has resulted in the production of superior top-cross hybrids. Sree Rekha and Sree Prabha with yields of 48.4 and 42.3 t/ha, respectively, with good culinary quality, and high starch content (28 and 27%) were released in Kerala State. They are now being tested and popularized in the other states. True cassava seed (TCS) technology was developed to enhance the rapid spread of crops to far-away and non-traditional tribal areas, overcoming the problem of shortage of planting material. Crosses made between the male-sterile line Ambakadan and selected male parents resulted in progeny with nearly uniform root yields and starch contents comparable with those of H-165 and H226. First clonals (C-1 clones) of Ambakadan hybrid generations were found to be suitable for the starch industry. The All-India Coordinated Research Programme (AICRP) on Tuber Crops has been instrumental in the collection and conservation of cassava germplasm at its various Centers in India, as well as the widespread testing of the new varieties in diverse locations. 1 Central Tuber Crops Research Institute (CTCRI), Thiruvananthapuram, 695 017, Kerala, India.