TBRI has been supplying virus-indexed tissue-culture plantlets to growers since 1983. It has also been doing research on improving the efficiency of the in vitro multiplication technology while maintaining the incidence of off-types below 5%.
TBRI’s breeding programme is based on the production of natural mutants, called somaclonal variants, that arise when banana plantlets are multiplied in vitro. Most off-types are usually discarded as undesirable, but some possess agronomically desirable traits, such as shorter stature and higher yields, and, sometimes, resistance to pathogens.
GCTCV-105 (left) growing alongside Pei Chiao (right) in a race 4 infested field.
In Taiwan, the search for disease-resistant somaclonal variants was motivated by the appearance in 1967 of race 4 of Fusarium oxysporum f. sp. cubense, which three years later was affecting 5,536 plants 1. The objective of TBRI’s varietal improvement programme is to make the local Giant Cavendish clone Pei Chiaoresistant to race 4. Screening for resistant somaclonal variants started in 1984, when some 20,000 plantlets were transplanted to a 1-ha field in which the pathogen population was between 300 to 1,000 propagules per gram of soil. The suckers of 6 symptomless clones were selected and further tested: GCTCV-40, GCTCV-44, GCTCV-46, GCTCV-53, GCTCV-62 and GCTCV-119 (GCTCV stands for Giant Cavendish tissue-culture variant)2. As the search continued, 6 more resistant clones (GCTCV-104, GCTCV-105, GCTCV-201, GCTCV-215, GCTCV-216 and GCTCV-217) were obtained by testing tissue-culture plantlets produced from rhizomes collected in different locations 2.
Most resistant clones have agronomic traits that made them unacceptable for the export trade, but when they are planted in large numbers, plants with improved agronomic characters can be found. For example, a further selection, the variant GCTCV-215-1, was registered for commercial production as Tai-Chiao No. 1 in 1992. Meanwhile, a farmer had noticed healthy plants growing among diseased plants in his field. He was able to trace them back to one of the GCTCV suckers he had obtained from his neighbour. He informed TBRI of his discovery, which was confirmed experimentally. The clone was designated GCTCV-218 and registered asFormosana in 2002.
More variants have since been selected, of which two have been registered as Tai-Chiao No. 3 and Tai-Chiao No. 5. The efficiency of selection has been about 2-3 resistant clones from every 10,000 plantlets screened3. In addition to screening in infested fields, selections are also conducted in participating farmers’ fields.
TBRI is also conducting studies to control the Cucumber mosaic virus using silver mulching. It is also experimenting, in collaboration with the National Chung Hsin University, the use of an endophyte as biological control agent against Fusarium wilt.
TBRI has a germplasm collection of 229 accessions, which include diploids, triploids, tetraploids and abaca.