Coconut (Cocos nucifera L.) genetic improvement in Vanuatu: Overview of research achievements from 1962 to 2002. Part 2: Improv

ARTICLE

Auteur(s) : Jean-Pierre Labouisse¹, Tiata Sileye², Jean-Paul Morin³, Chantal Hamelin³, Luc Baudouin³, Roland Bourdeix³, André Rouzière³

¹CIRAD, c/o VARTC, BP 231, Santo, Vanuatu
²VARTC, BP 231, Santo, Vanuatu
³CIRAD, BP 5035, 34398 Montpellier, France

Introduction

To overcome the limitations of this method of increasing production potential, hybridization was tested, using the search for tolerance of coconut foliar decay virus as the major constraint.

This article describes the main steps in the hybridization programme, the methods used for assessing the performance of the hybrids, and the main results obtained.

Steps in the hybridization programme

Creation of a collection

Creation of hybrids

Discovery of coconut foliar decay disease

On the MRD, which is one of the most susceptible varieties, the first symptoms are a yellowing of leaflets at the base of middle fronds and lateral necrosis on the petioles of affected fronds. These fronds die prematurely, hanging from the petiole down through the crown (( figure 2 )). Then, the other upper fronds turn yellow then brown, and die. Susceptible cultivars succumb to the disease between one and two years after symptoms appear.

A small, circular single-stranded DNA was shown to be associated with CFD [4]. The nucleotide sequence of the DNA was determined. It was confirmed that the virus represented a new taxonomic group and it has been tentatively assigned to the genus Nanovirus [5, 6]. The plant-hopper Myndus taffini was shown to be the vector of CFD virus [7]. This insect has a breeding host, Hibiscus tiliaceus (or burao), a local shrub, very common in Pacific countries. However, the disease has never been reported outside Vanuatu [8].

Remission of symptoms has sometimes been observed on individual palms of susceptible cultivars 1 or 2 years after symptoms developed. Those palms then appeared to remain disease-free even if exposed to high infection pressure in the field [9]. The mechanisms of remission and of acquired immunity remain unknown.

There are two efficient ways of controlling this disease. The first is to remove of the insect breeding host, H. tiliaceus, for several hundred metres around the cultivation area. This strategy has been successfully applied at Saraoutou for the conservation of susceptible cultivars in collections and trials, and inside a few large commercial plantations on Santo, but cannot be extended to the whole area cultivated by smallholders. The second is to use CFD virus-tolerant cultivars.

Searching for CFD-tolerant cultivars

For the other varieties, and for the hybrids, screening for CFD virus susceptibility was done by planting palms in a field where they were exposed to high natural infection pressure i.e. in an area with a high density of H. tiliaceus around the plot. At least four years’ exposure to CFD virus in the field was necessary to judge whether an unaffected coconut type (after the susceptible MRD control had been affected) was likely to prove highly tolerant. A complementary test was developed by Julia [11] for a more rapid evaluation. Nursery seedlings were raised in cages where they were exposed to 1 500 Myndus taffini insects captured in the field, but sometimes the artificially inoculated palms recovered, making susceptibility to the disease difficult to assess (nursery effect). However, for each variety, a good correlation was shown between the susceptibility observed in the field and the expression of early symptoms on young seedlings.

The results of these experiments were published [11, 12] and can be summarized as follows: for Dwarfs, the MRD and MYD were the most susceptible varieties and the Vanuatu Red Dwarf (VRD) showed a high level of tolerance. The most susceptible Tall was the Markham Valley Tall introduced from Papua New Guinea. The other Tall and Dwarf varieties showed intermediate levels of susceptibility. All the hybrids were more or less susceptible (table 2( Table 2 )), except the Vanuatu Tall × Rennell Island Tall (VTT × RIT) and the Vanuatu Red Dwarf × Vanuatu Tall (VRD × VTT), which displayed a high level of tolerance. For the VTT × RIT hybrid, a very small percentage of palms (3.5%, 13 years after planting in trial GC1) expressed slight CFD symptoms, but all of them recovered. As for VRD × VTT, no diseased palms were recorded in on-station trials. Rare cases of attacks in farmers’ fields have been reported.

Once these results had been obtained, the breeding programme focused on assessing the performance of the VTT × RIT and VRD × VTT hybrids, and on improving their tolerance of CFD virus.
Table 2 Results of screening 16 cultivars for CFD virus tolerance in collection plot P01, 27 years after planting (1975-2002).

Assessing the agronomic performance of the Vanuatu Tall × Rennell Island Tall hybrid

Characteristics of the Rennell Island Tall parent

The RIT is susceptible to CFD virus. Nine years after planting in trial GC1, 27.1% of the palms had been affected by the disease [17]. In collection plot P01, 28 years after planting, 56% of RIT palms had died from this disease (table 2). To improve the level of RIT and VTT × RIT tolerance of CFD virus, unaffected RIT palms in the collection were self-pollinated and the progenies planted in a field exposed to high infection pressure. After 10 years, all the progenies were affected but the percentage of diseased palms within each of them varied from 11% to 96%. Many palms recovered, but only three progenies expressed good tolerance of CFD virus, with no or very few palms succumbing to the disease (0% for 2 progenies, 4% for the third). We used pollen from those three progenies to create a second generation of VTT × RIT hybrids that we presumed to be more tolerant of CFD virus than those created with an unselected RIT parent.

On-station trials

The origins of the hybrid parents are given in table 4( Table 4 ). For trial GC29, we used pollen from a Rennell Island Tall palm selected for better tolerance of CFD virus.

Germination rate

As observed for most hybrids, the nature of the female parent (here the VTT) seemed decisive and determined germination characteristics.
Table 3 Simplified description of trials with the VTT × RIT hybrid.

Flowering precocity

Description of the palm

Yield characteristics

In trials GC1 and GC14, the first harvest took place four and a half years after planting, and the number of nuts was not significantly different for the VTT and the VTT × RIT hybrid. In trial GC1, the copra content of the hybrid nut was significantly higher (+ 21%) than the VTT copra content. Production calculated from year 5 to year 12 was 22% higher for the hybrid. This trial suffered from drought in years 9 and 10, and from cyclone Gordon in year 10 (January 1979). Similarly, in trial GC14, due to a higher copra content, hybrid production was significantly higher (+ 31%) than VTT production. In trial GC 29, the first harvest took place 4 years after planting. In this trial, the hybrid was very promising with a yield of 3.2 tons per hectare as opposed to 2.3 tons for the Elite VTT (+ 39%) in the fifth year after planting.

For the VTT, copra content was significantly higher in trial GC29 than in trials GC14 and GC1. This was the result of the mass selection process applied to the Vanuatu Tall, as described in Part I. For the VTT × RIT hybrid, copra content was lower in trial GC29 than in trial GC14. The reason could have been that the RIT parents in GC29 were chosen for their tolerance of CFD virus, whereas the RIT parents in GC14 were only chosen for their high copra content.
Table 5 Comparison of production data (annual means) for the VTT and the VTT × RIT hybrid.

^bmeans calculated from year 5 to year 12 after planting.

^cmeans calculated for year 5 only.

Assessment of the agronomic performance of the Vanuatu Red Dwarf × Vanuatu Tall hybrid

Characteristics of the Vanuatu Red Dwarf parent

Whatever its origin is, the VRD is well adapted to the ecology of Vanuatu and is highly tolerant of CFD virus, even though some slight symptoms have occasionally been observed. It is tolerant of strong winds, when compared to other Dwarfs. In the VARTC field genebank, in 1999, cyclone DANI toppled 4% of the VRD palms, as opposed to 100% of the Malayan Yellow Dwarfs (the palms were broken at the bottom of the stem).

The Vanuatu Red Dwarf has a rather thin stem and no bole. The leaves are yellowish. This variety bears numerous small bright orange fruits. In the VARTC collection, the average weight of the whole fruit, calculated over a 4-year period (1997-2000), was 0.692 ± 0.195 kg, and the fresh meat weight was 261.1 ± 60.5 g. The copra content was 117.7 ± 33.1 g.

In 1985, a study by Chant showed that the VRD has a very slow germination speed and a low, irregular germination rate [18]. Germination started between 70 and 100 days after sowing, as opposed to 30 days for the Malayan Yellow Dwarf (MYD). The average germination rate was 43%, but high variability was recorded (22 to 68%) depending on the origin of the nuts, the nutrition of the palm and the stage of maturity.

The VRD bore its first flower 26 months after planting (21 months for the MYD) and 50% of the palms had flowers after 32 months (25 months for the MYD). In the VARTC genebank, the annual number of nuts was over 100, and yield was about 12 kg of copra/palm/year.

However, due to the low copra content of its nuts, the VRD is not used for copra production. It is mainly planted as an ornamental in parks and gardens.

On-station trials

Germination rate

Flowering precocity

These results suggest that VRD × VTT flowering precocity depends on the environment and the stage of seedling development when planted in the field. In most of the cases, the first significant harvest took place 4 years after planting.

Description of the palm

Production

In trial GC12, the first hybrid yields were disrupted by cyclone Nigel which passed over the station in January 1985. This trial was prematurely halted.

In trial GC16, the hybrid yields were greatly improved. But this was probably the result of a high level of selection applied to the VTT parents, which had the following characteristics: very large number of fruits (167 fruits per palm on average) and high copra content (203 g per nut on average). The average theoretical yield of these outstanding VTT palms was 4.7 tons per hectare!

The performance of the hybrid obtained through the PDICC trials was more difficult to analyse. These trials were severely affected during the first years of production by cyclones Dani and Ella in 1999, and cyclones Paula and Sosé in 2001. Moreover, from the beginning of 2000, all the palms in these trials were infected by a fungus, Corticium penicillatum. The older leaves dried and fell prematurely, and production was seriously affected.
Table 7 Production data for the VRD × VTT hybrid in different trials.

^bcalculated from year 6 to year 8.

^ccalculated from year 5 to year 11.

Seednut supplies to growers, and recorded performances

Performance for copra processing

After extraction, the kernel of the cultivars was loaded into a hot-air dryer for 48 hours. At the same time, samples were taken and dried in a laboratory oven in order to determine the percentage of dry matter. The results are presented in table 10( Table 10 ). VRD × VTT kernel had the highest initial water content (55%) but it dried more rapidly (because of the lower kernel thickness). The quantity of copra obtained at the end of the drying process was higher for the VTT compared to the VTT × RIT (+ 4.5%) and the VRD × VTT (+ 12.5%). The difference in copra weight between the laboratory and the hot-air drier resulted from losses during dryer loading/unloading and during the drying process.

We also determined the oil content from kernel samples using the Soxhlet method with hexane as the solvent. The oil content was not significantly different for the three cultivars, varying from 65.3% to 66.2% of dry matter.
Table 9 Evaluation of the labour required for fruit splitting and kernel extraction. Number of fruits and time required to prepare one ton of copra.

Discussion

The VTT × RIT hybrid stands out through its precocity, its vigour and its good adaptation to cyclones. When compared to the VTT, the copra content of its nuts is better and the number of nuts produced per year is slightly larger. The difference in production with the VTT is significant but remains modest for a Tall × Tall hybrid. For instance, in Ivory Coast, yields of the WAT × RIT and WAT × VTT hybrids are more than double those of the local West African Tall (WAT) [22]. This moderate heterosis observed in the hybrid might be explained by the limited genetic distance between the VTT and RIT parents. Indeed, despite dissimilar phenotypic traits, we showed with molecular markers that these varieties are genetically close and, along with the New Caledonia Tall and Solomon Island Tall, form the same sub-group among the Melanesian Tall coconut palms [23]. However, this hybrid still represents true genetic progress compared to the VTT, through its higher yield and a reduction in the work time required to prepare copra.

Unlike the VTT, it has the drawback of not being reproducible by farmers from their own plantation. It can only be reproduced in centralized seed gardens, as RIT parents cannot be maintained in a smallholder environment because they succumb to CFD virus. In addition, the mother palms (VTT) rapidly reach a considerable height, thereby reducing the working life of the seed garden and making work less easy than with Dwarf × Tall hybrids. All this makes it more expensive to produce and disseminate this hybrid than the improved VTT.

For its part, the VRD × VTT hybrid displays good hybrid vigour, though it remains average in relation to that found in Dwarf × Tall hybrids. However, it does have some defects. The first is slow germination and a highly irregular and often low final germinated nut rate. This holds back its distribution to farmers in nut form. In addition, as the female parent is a Vanuatu Red Dwarf with very small nuts, farmers who are used to selecting the largest nuts from their plantation, are reticent to set up and maintain seed beds for such small nuts over a long period. The hybrid can therefore only be distributed in seedling form, usually raised in polybags, from the central VARTC nursery or from regional nurseries supervised by agriculture technicians. This considerably increases planting material and transport costs. The second defect is susceptibility to cyclones, which results in the uprooting or breakage of a not insubstantial number of young palms, but also, and especially, in substantial immature nut fall. Premature nut fall is sometimes seen in young palms without it being attributable to a cyclone. The factor(s) causing it has (have) not been clearly established. Such nut fall may be due to a transient nutrition imbalance, an excessive fruit-set rate or low intrinsic peduncle resistance. This is reflected in highly variable yields depending on environmental conditions. The third defect is the mediocre quantity of copra obtained from one nut, which increases work time when preparing copra. Lastly, other elements of assessment were recorded during field surveys [20]. Whilst farmers say they are satisfied with precocity and with young palm yields, they often express the fear that this high productivity will not be maintained in the long term and they have doubts about the longevity of the palms which, in their view, would be shorter than for the local Tall. Experimentally, we lack the hindsight required to judge the validity of those arguments.

Conclusion

In view of the defects of the VRD × VTT hybrid, its production was halted in 1996. Since that date, emphasis has been placed on the second generation of the VTT × RIT hybrid selected for better CFD tolerance. To date, after 5 years in the field, that hybrid has shown no signs of CFD attack, be it on-station or on-farm. Its precocity and yields are highly satisfactory. However, dissemination of this hybrid will always be limited by seedling production and transport costs, due to the need for centralized seed gardens.

Large-scale use of the Elite VTT produced in decentralized seed gardens combined with improved nursery and plantation management techniques are the main ways of increasing coconut productivity in Vanuatu in the short and medium terms.
Table 11 Summary of the main characteristics of the elite VTT, VTT × RIT and VRD × VTT and the constraints for seednut production.

Acknowledgement

References

2 Calvez C-H, Renard J-L, Marty G. La tolérance du cocotier hybride Local × Rennell à la maladie des Nouvelles Hébrides/Tolerance of the hybrid coconut Local × Rennell to New Hebrides disease. Oléagineux 1980; 10: 443-51.

3 HANOLD D, MORIN J-P, LABOUISSE J-P, RANDLES JW. Foliar decay disease in Vanuatu. In: G Loebenstein, G Thottappilly, eds. Virus and virus-like of Major Crops in Developing Countries. The Netherlands Kluwer Academic Publishers, 2003: 583-96.

4 Randles JW, Hanold D, Julia J-F. Small circular single-stranded DNA associated with foliar decay disease of coconut palm in Vanuatu. J Gen Virol 1987: 273-80.

5 Rohde W, Randles JW, Langridge P, Hanold D. Nucleotide sequence of a circular single-stranded DNA associated with coconut foliar decay virus. Virology 1990; 176: 648-51.

6 RANDLES JW, CHU PWG, DALE JL et al. Genus Nanovirus. In: RB Wickner, ed. Classification and Nomenclature of Viruses. San Diego: Academic Press, 2000: 303-9.

7 Julia J-F. Myndus taffini (Homoptera Ciixidae), vecteur du dépérissement foliaire des cocotiers au Vanuatu. Oléagineux 1982; 8-9: 409-14.

8 Morin J-P. Elevage et biologie de Myndus taffini Bonfils (Homopterea: Cixiidae), vecteur du dépérissement foliaire du cocotier au Vanuatu. ANPP Troisième conférence internationale sur les ravageurs en agriculture. Montpellier, 7-9 décembre 1993.

9 Randles JW, Miller DC, Morin J-P, Rohde W, Hanold D. Localisation of coconut foliar decay virus in coconut palm. Ann Appl Biol 1992: 601-17.

10 Hanold D, Langridge P, Randles JW. The use of cloned sequences for the identification of coconut foliar decay disease-associated DNA. J Gen Virol 1988; 69: 1323-9.

11 Julia J-F, Dollet M, Randles J, Calvez C-H. Le dépérissement foliaire du cocotier par Myndus taffini (DFMT): Nouveaux résultats/ Foliar decay of coconut by Myndus taffini (FDMT): New results. Oléagineux 1985; 1: 19-27.

12 Calvez C-H, Julia J-F, De Nuce M. L’amélioration du cocotier au Vanuatu et son intérêt pour la région du Pacifique: rôle de la station de Saraoutou/ Improvement of coconut in Vanuatu and its importance for the Pacific region: role of the Saraoutou Station. Oléagineux 1985; 10: 477-90.

13 Foale MA. Report on a visit to Rennell Island B.S.I.P. to study the coconut population. Unpublished report, 1964.

14 WHITEHEAD RA. Sample survey and collection of coconut germplasm in the Pacific Islands. 30 May-5 Septembre 1964. Her Majesty’s Stationery Office, London, 1966.

15 De Nucé De Lamothe M, Wuidart W. Les cocotiers Grands à Port-Bouët (Côte d’Ivoire). 2- Grand Rennell, Grand Salomon, Grand Thaïlande, Grand Nouvelles-Hébrides. Oléagineux 1981; 7: 353-65.

16 Bourdeix R, Konan J-L, Labouisse J-P. Rennell Island Tall Coconut. Cogent Newsletter 2000; 3: 10-1.

17 MORIN J-P. Le dépérissement foliaire du cocotier: Rapport de fin d’activités. Centre de coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), Paris, France, 1994.

18 CHANT H. Etudes sur la germination des noix de cocotier Nain Rouge du Vanuatu. Mémoire de fin d’études. Institut Supérieur Technique d’Outre-Mer, Le Havre, France, 1985.

19 OLLIVIER J. Vanuatu Coconut Development Project (KDP): Final report by the consultant. 2nd Phase - February 1989-March 1993. Centre de Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), Paris, 1993.

20 LABOUISSE J-P, BULETARE G. Evaluation of selected coconut cultivars planted in farmers’ fields in Vanuatu: COGENT - CGRNAP Project. Final report. Vanuatu Agriculture Research and Training Center, Santo, Vanuatu, 1997.

21 LAHVA J, LABOUISSE J-P. Enhancing farmers income and germplasm conservation through coconut - based farming system and identification of varieties for multipurpose uses in Vanuatu. Final technical report. Department of Agriculture and Rural Development, Port Vila, 2000.

22 BOURDEIX R. La sélection du cocotier Cocos nucifera L. Etude théorique et pratique. Optimisation des stratégies d’amélioration génétique. Université de Paris Sud, Orsay, France, 1989.

23 BAUDOUIN L, LEBRUN P. The development of a microsatellite kit for use with coconuts. Final report for IPGRI. Centre de Coopération Internationale en Recherche Agronomique pour le Développement, Montpellier, France, 2002.

* See Part 1 in: OCL 2004, Vol.11, n° 4–5, p. 354–3611 Data in the text are "mean ± standard deviation".