Taro Breeding Programme of Papua New Guinea Achievements, Challenges and Constraints

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1 Taro Breeding Programme of Papua New Guinea Achievements, Challenges and Constraints D. Singh 1,3, T. Okpul 2 and D. Hunter 3 1 National Agricultural Research Institute, PO Box 1639, Lae, Papua New Guinea; 2 Agriculture Department, University of Vudal, Private Mail Bag, Rabaul, Papua New Guinea; 3 Secretariat of the Pacific Community, Private Mail Bag, Suva, Fiji. ABSTRACT The Papua New Guinea taro breeding programme was established in the early 1990s with limited resources but has accomplished successful outputs with recent support from the SPC/AusAID-funded TaroGen project. The programme released three leaf blight resistant and high yielding taro varieties (NT 01, NT 02 and NT 03) in 2002, and distributed planting material widely to farmers and extension agencies. In addition to benefiting PNG growers, the taro growers in other Pacific countries will also benefit from these lines, once safe international transfer is guaranteed. The PNG breeding programme has now progressed to its fourth cycle and additional superior lines are expected to be released from these advanced cycles. Major challenges for future breeding programme will be in the areas of multiplication and distribution, safe movement of germplasm, widening the genetic base of crop, extending the selection criteria to taro beetle tolerance and superior post harvest traits, simplifying the breeding using molecular markers, and most importantly the sustainability of the programme. The important constraints to prevail over various challenges will be funding, liaison between the national research and extension agencies, and national scientific capacity. Key words: adaptability, eating quality, G x E trials, TLB resistance, variety release, yield.

2 INTRODUCTION In Papua New Guinea (PNG), taro (Colocasia esculenta) is second important root staple with an estimated annual production of about t from an area of 77000ha (Sar et al., 1998). The crop is not only an important source of food and income, but also plays a vital role in the cultural heritage of Papua New Guineans, and is considered an essential component of many traditional ceremonies. Taro production in PNG has been continuously declining over past few years (Singh and Okpul, 2000). The declining trend in production can be mainly attributed to multiplicity of biotic and abiotic stresses. Of prime significance is taro leaf blight (TLB) disease caused by pathogen Phytophthora colocasiae, which first arrived in PNG in the 1940's and is estimated to cause up to 50% loss in corm yield (Cox and Kasimani, 1988). Since then country opted and practiced various pronged cultural, biological and chemical control strategies, but without any noteworthy success. In 1980s, PNG commenced taro improvement programme with emphasis on TLB resistance breeding but this programme was also not successful due to lack of funds and staff changes. As an effect, the production trend continued to decline and important taro germplasm collected by farmers over years kept eroding. In some cases farmers abandoned the crop and replaced with other crops like sweet potato. Realizing the economic importance of crop, and implications of TLB on production and diversity, National Agriculture Research Institute (NARI) of PNG re-established its taro resistance breeding programme (then under the Department of Agriculture and Livestock) in The programme received a further support from the SPC/AusAID regional TaroGen project to develop taro lines with resistance to TLB, high yield and good eating quality. The present paper brings up to date achievements of NARI breeding programme, and also addresses the future challenges and constraints. MATERIALS AND METHODS Breeding strategy The breeding programme is based on the strategy of improving the population by adopting a modified recurrent selection approach, and has focused on incorporating horizontal resistance to TLB using a systematic cyclic strategy. Base population TLB resistance sources in the base population included a wild variety from Thailand (Bangkok), hybrids from a Solomon Islands breeding programme and three semi-wild taro varieties (Ph 15, Ph 17 and Ph 21) from PNG, and more than 50 agronomically important and popular varieties grown by local farmers. Methodical approach The entire methodological approach used by the breeding programme for the release of new varieties is outlined in Figure 1. The methodological details of each step are outlined in Singh et al. (2001) 2

3 Selection criteria and trait assessments The programme is based on selection criteria, which emphasizes on identifying genotypes with moderate levels of TLB resistance, high yield and acceptable eating quality. A popular local variety (Numkowec) was included for comparison in all trials. In preliminary trials, yield was measured as corm weight (grams), but in advanced trials it was estimated as tones/hectares (t/ha). Yield stability was estimated on the basis of the Eberhart and Russell (1966) model. Severity of TLB was recorded using standard area diagram developed by Gollifer and Brown (1974). The percentage diseased leaf area (DLA) was estimated for each leaf and the mean disease rating for each plant was calculated by dividing the total of the assessments for each leaf by the total number of leaves examined as outlined by Hunter and Pouono (1998). Only genotypes with moderate resistance were further selected and genotypes with hypersensitive reactions or very high DLA were discarded. Eating quality was assessed in terms of eating quality score (EQS). For eating quality, five parameters (texture, acridity, fibre, aroma and colour) were evaluated. The total score (TS) was calculated for each parameter. After calculating the TS, a final score (FS) was calculated for each parameter by multiplying the TS of that parameter with its constant parameter weighting (CPW: 0.50 for texture, 0.30 for acridity, 0.10 for fibre and 0.05 each for aroma and colour). A final EQS was then calculated by summing FS of all five parameters. The higher the value of the EQS, the better the eating quality was assumed to be. RESULTS NARI taro breeding programme has successively advanced to its fourth cycle. The progressive achievements of each of the four cycles is presented below: Cycle 1 Cycle 1 was developed in 1994 by crossing the resistant base population with superior local taro varieties. The detailed evaluation results of Cycle 1 superior lines are presented in Okpul et al. (1996). No recommendations were made from Cycle 1 since most lines retained undesirable wild characteristics, although some recombinants were partially superior. Cycle 2 Cycle 2 was created in 1996 by inter-crossing partially superior genotypes from Cycle 1. In 1998, 32 lines were selected from Cycle 2 on the basis of preliminary assessments on yield, TLB resistance and eating quality. Further testing enabled selection of 12 TLB resistant lines for inclusion in replicated advanced yield trials. The replicated trials identified seven lines (C2-E1, C2-E3, C2-E4, C2-E7, C2-E8, C2-E10 and C2-E11). These seven lines were advanced to genotype x environment (G x E) trials for assessing their adaptability under major agro-ecological sites of PNG. The details of these results are presented in Okpul et al. (2002). The G x E trial results were reviewed by a panel of scientists and were presented to the national Taro Improvement Coordinating Committee (TICC) with a recommendation to release three lines, C2-E3 and C2-E4 and C2-E8. These lines were selected on the basis of high yield, yield stability over a range of environments, resistance to TLB and good eating quality. Full description of the three lines is presented in Table 1. 3

4 Table 1. Numkowec Descriptions of PNG released taro varieties and a popular standard cultivar Trait Variety NT 01 (C2-E3) NT 02 (C2-E4) NT 03 (C2-E8) Numkowec (control) Yield (t/ha) Average corm weight (g) 525 g 380 g 380 g 300 g Yield stability Stable Stable Unstable Stable Taro leaf blight (TLB) Resistant Resistant Resistant Susceptible TLB diseased leaf area (%) Taro beetle Susceptible Susceptible Susceptible Susceptible Taro beetle damage (%) Eating Quality Good Good Good Good Eating Quality Score Time to maturity (c. months) Sucker production Growth habit Erect Erect Erect Erect Plant height Tall Medium Tall Medium Leaf lamina Light green Dark green Dark green Dark green Petiole colour Light green Purple green Purple Light green Petiole junction Purple Purple Purple Purple Flowering Rare Rare Frequent Frequent Corm shape Cylindrical Elliptical Conical Conical Corm skin Smooth Smooth Smooth Smooth Flesh colour Pink Pink Pink Pink Corm dry matter content (%) A final release document paper was prepared and presented to NARI management. The release of lines was endorsed by NARI, and the lines were officially named (NT 01, NT 02 and NT 03) and released on 13 December This is a first report of any taro varieties released in PNG. Cycle 3 Cycle 3 was created by inter-crossing 21 selected Cycle 2 lines in a half-diallel design. More than 300 crosses were attempted and a population of over seedlings was created and evaluated. Forty-nine superior lines in terms of TLB resistance, high yield and good eating quality were recovered in preliminary trials. Twenty-six lines were selected from intermediate trials and finally six lines were selected from advance trials. The selected six lines are being evaluated in G x E trials for adaptability. Any superior lines identified from these trials will be recommended for new release(s). 4

5 Cycle 4 In advancing to fourth cycle of recurrent breeding, more than 200 selective partial diallel crosses were performed among 49 genotypes selected from Cycle 3 in Approximately seedlings were generated and 237 superior genotypes were selected on basis of TLB resistance, high yield and eating quality. The selection criteria in this cycle were extended to wider adaptability during preliminary screening to widen the genetic base of population for better adaptation of the identified genotypes. Therefore, the selected 237 lines were tested under three varied agro-ecological sites to identify wider adaptable lines in addition to high yield, TLB resistance and good eating quality. On this basis, 22 elite lines were identified from Cycle 4 for undertaking G x E trials and simultaneously for creating Cycle 5. Compared to previous cycles, the genetic gains were considerably higher for Cycle 4 in terms of recovering superior progeny and traits of interest. DISCUSSION PNG is one of the few countries in the Pacific region, where taro varieties have been successfully bred for superior yield, TLB resistance and good eating quality. The released varieties will help growers wishing to improve taro production for subsistence purposes in order to maintain a traditional food staple with cultural significance. There will also be benefits to farmers who previously grew taro for domestic markets, but in recent years have had to abandon their production because of TLB. The superior traits associated with these lines should enable successful acceptance and adoption by smallholders, subsistence and semi-commercial growers. The yield of these lines should be sustainable over time, since these varieties are derived from genetic improvement and are widely adaptable. The TLB resistance will be durable since it is based on horizontal resistance relying on additive effects of multiple genes against the pathogen. The released lines are being multiplied at four different sites covering different regions of PNG. The material is being distributed to farmers nationally in collaboration with TICC, extension workers, non-government organizations and church groups. Till now, more than 5000 planting suckers of each line have been distributed to farmers and communities through out PNG. In future, more lines are expected to be released from post Cycle 2 recurrent cycles. It is likely that those lines released from advanced cycles will be more superior in their attributes, especially eating quality because of the polygenic breeding approach (which relies on accumulation of superior genes from cycle to cycle) adopted by PNG programme. PNG breeding programmes could be used as a vehicle for breeding network to control TLB disease in the Pacific region. The material has been transferred to the Regional Germplasm Centre (RGC) Fiji for distribution to other Pacific island countries in the region, once safe quarantine movement is guaranteed. 5

6 Taro varieties identified in PNG although successful locally, may not suit individual countries with a wide diversity of environments and cultures, and therefore regional G x E evaluations will be a challenge. Additional challenges are developing innovative ways to increase the rate of multiplication of planting material, safe germplasm exchange, capacity to meet the needs of larger number and isolated farmers for enhanced distribution of new and improved germplasm. The sustainability of breeding programme is a major challenge and participatory plant breeding can offer a scope to address this challenge as pointed earlier by Hunter et al. (2001) Future genetic gains of taro breeding will challengingly rely on widening the genetic base of the crop. Recent biochemical and molecular studies on taro germplasm from Asia and the Pacific has identified the existence of two distinct regional taro genepools representing these two regions. The level of genetic diversity of PNG germplasm compared to Asian germplasm is very low. Future taro improvement programmes will require introgression of selected exotic and local germplasm into PNG breeding programme to create genetically diverse progeny and further broaden the genetic base of the crop. Once the genetic base is broadened, focus should also be given to selection of post harvest traits like shelf life, and marketable traits demanded by the export markets. Although difficult, breeding for tolerance to taro beetle or selecting for factors reducing taro beetle attack will be a challenge. In addition, there is a considerable potential of molecular markers to simplify selections and breeding programme. The foremost constraints to prevail over various challenges are funding, liaison between the national research and extension agencies, and scientific capacity once donor project technical advice is terminated. It is however stressed that whatever the constraints are, the country has to take initiative to sustain its breeding programme, if it wants to overcome the declining production trend and explore development potential of this major crop. ACKNOWLEDGEMENT The support of funding agencies AusAID, EU and ACIAR, and implementing agencies NARI and SPC is gratefully acknowledged for backing PNG breeding programme over a stretch of time. REFERENCES Cox, P.G. and C. Kasimani. (1988) Control of taro leaf blight using metalaxyl. Tropical Pest Management 34, Eberhart, S.A. and W.A. Russell. (1966) Stability parameters for comparing varieties. Crop Science 6, Gollifer, D.E. and J.E. Brown. (1974) Phytophthora leaf blight of Colocasia esculenta. Papua New Guinea Agricultural Journal 25, Hunter D.G., T. Iosefa, C.J. Delp and P. Fonoti. (2001) Beyond taro leaf blight: a participatory approach for plant breeding and selection for taro improvement in Samoa. Pp in Proceedings of the International Symposium on Participatory Plant Breeding and Participatory Plant Genetic Resource Enhancement (1 st to 5 th May 2000), Pokhara, Nepal. 6

7 Hunter, D.G. and K. Pouono. (1998) Evaluation of exotic taro cultivars for resistance to taro leaf blight, yield and quality in Samoa. Journal of South Pacific Agriculture 5, Okpul, T., A. Ivancic and A. Simin. (1996) Evaluation of leaf blight resistant taro (Colocasia esculenta) varieties at Bubia, Morobe Province, Papua New Guinea. Papua New Guinea Journal of Agriculture, Forestry and Fisheries 40, Okpul, T., D. Singh, M. Wagih, G. Wiles and D. Hunter. (2002) Improved taro varieties with resistance to taro leaf blight for Papua New Guinea farmers. NARI Technical Bulletin Series No. 3. National Agricultural Research Institute, Lae, Papua New Guinea. Sar, S. A., B. M. Wayi and R. D. Ghodake. (1998) Review of research in Papua New Guinea for sustainable production of taro (Colocasia esculenta). Tropical Agriculture (Trinidad) 75, Singh, D. and Okpul, T. (2000) Evaluation of 12 Taro (Colocasia esculenta (L.) Schott) Leaf Blight Resistant Clones for Yield and Eating Quality in Papua New Guinea. SABRAO Journal of Breeding and Genetics 32, Singh, D., D. Hunter, T. Iosefa and T. Okpul. (2001) Guidelines for undertaking on-farm taro breeding trials in the south Pacific. Secretariat of the Pacific Community, Suva, Fiji. 7

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