The Vanuatu coconut disease: Coconut foliar decay (CFD)

By R. Bourdeix, JP Labouisse and T. Sileye

Coconut foliar decay (CFD) is a disease of coconut (Cocos nucifera L.) associated with infection by coconut foliar decay virus (CFDV),which is endemic in Vanuatu, South Pacific. The local cultivar 'Vanuatu Tall' (VTT) is the only cultivar that is fully tolerant to CFD, whereas introduced cultivars and hybrids are affected to different degrees.

Foliar decay on Malayan dwarf seeding.
Source: Pestnet.org

The first symptom on palms in the field is yellowing of a few leaflets on any of the fronds between position seven and 11 from the spear leaf. The yellowing spreads along the fronds, and the fronds break near the base so that they hang down through the still green lower leaves. As the younger leaves age, reaching positions seven to 11, they, too, turn yellow, break and hang down. In time diseased palms have a few green fronds at the top, and broken mid-section fronds hanging through the lower, still green fronds below. As the disease progresses, the trunks narrow towards the top, and the palms die after 1-2 years, except for those that are tolerant to the disease and show remission of symptoms. In experimental tests with the susceptible Malayan Red Dwarf, symptoms occur 7-10 months after infection.

Hibiscus tiliaceous, by By Vincenzo Defifoot
 (Own work, ccby-sa 3.0)

The virus that causes the disease is small and found at low concentrations in coconut palms; because of this, it is difficult to see in sap viewed by electron microscopy. Spread is by Myndus taffini, a planthopper, which breeds on the roots of Hibiscus tiliaceous. The adults of this insect feed on coconuts. In Vanuatu, Hibiscus tiliaceous is used as a living fence around blocks of coconuts, and those palms at the margins of the blocks are often the first to show symptoms. Although Myndus taffini is known to transmit the virus, there is little known about its relationship with the virus. For instance, it is not known if the virus multiplies in the insect, or whether the nymphs or the adults, or both, acquire the virus.

From 1967 to 2008 a conventional breeding programme was conducted with the aim of creating hybrid planting material combining tolerance to CFD with improved copra yield and high copra weight per nut. This objective was achieved by crossing the progeny of selfed trees of 'Rennell Island Tall' (RIT) cultivar, selected for their low susceptibility to CFD in field screening tests, with VTT, improved by mass selection and intercrossing. An improved VTTi x RIT hybrid was identified with a high degree of tolerance to CFD (less than 1% of diseased trees after 11 years of exposure to high disease pressure). The annual production of the improved VTTi x RIT hybrid ranged from 21.9 to 28.6 kg of copra per tree, depending on the RIT parent, and was, on average, 34% higher than that of 'VTT Elite' an advanced cultivar obtained after four selection cycles of local VTT. However, the production of the hybrid in Vanuatu involves constraints such as frequent replanting and isolation of the seed garden and CFD control for the RIT parents. Conducting research on the genetic determinism and the mechanism of tolerance to CFD for better control of the disease is of great importance, in the event that it spreads outside Vanuatu.

In August 2015, discussion was conducted between Mike Foale and Roland Bourdeix, initiated by Hugh Harries.

Mike, 28 August 2015

Dear Roland

I have adopted a cautious attitude to the use of coconut germplasm in locations remote from the place of origin and would like to have your opinion about the following examples.

• In Vanuatu there is an endemic virus that does not cause any symptoms of pathology in the indigenous tall population, yet exotic germplasm is destroyed by it.
• In Solomon Islands in 1963 I planted imported seed from Rangiroa atoll but the seedlings died due to severe leaf spot (Drechslera sp) damage yet the local tall was not affected.
• The leaf beetle Brontispa longissimia causes very little damage to the local tall in Solomon Islands yet when it reached Vietnam recently mature palms were destroyed by it.
• The hybrid Malayan orange dwarf by Rennell Tall has given a 30% increase in yield of copra in plantations in Solomons yet in New Britain (PNG) it was severely attacked by Scapanes australis – a local rhinoceros beetle.
• PB121 which yielded very well in Cote d’Ivoire was attacked by Phytophthera sp in Indonesia and I understand that many died.

These examples suggest to me that coconut populations have existed for perhaps millions of years in different regions of the world and undergone natural selection for local biohazards while remaining vulnerable to species and varieties of such hazards in other locations. I understand that West African Tall has not been present in West Africa for more than 500 years yet perhaps the source population (in India?) possessed adequate tolerance to varieties of Phytophthora in West Africa but not Indonesia.

I would appreciate very much your comments on my hypothesis.

Sincerely, Mike Foale, Honorary Research Fellow, University of Queensland

Roland 31 August 2015

Dear Mike and Hugh

In my opinion, they are already so many reasons (such as LYDs) for people to not conduct coconut germplasm exchanges; so I definitely would not add additional reasons to limit the coconut germplasm movements.

Of course you gave some examples of local adaptation, but please consider also these one:

• The Rennell island Tall was widely used with great success in the Pacific region; of course in some places, there is the scapanes problem, but in many other, it was OK: Papua New Guinea mainland, Fiji, Samoa, etc….
• The Sri Lanka Green dwarf and the Vanuatu Tall are the only varieties tolerant to LYD disease in Ghana – and there is no LYD in Sri Lanka and Vanuatu
• The PB121 hybrids was mainly a success, even if some problems were encountered in some places of Indonesia - the real problem was not phytophthora except in a few zones, but the small size of the fruit was not appreciated by the farmers – Many of them were very happy to turn their PB121 to heavy Toddy production.
• The case of Vanuatu Tall and the DFMT disease is very strange, and definitely we did not conduct enough research of this question. The Malayan Red Dwarf is the variety most sensitive to the DFMT disease. But MRD can be “vaccinated” in the nursery, and then it does not die anymore from the disease. Nobody tried to take seednuts from these “vaccinated” MRD, plant them and see if the progeny is sensitive of not to the DFMT disease; may be the DNA of the virus integrate the genome of the coconut palm, or something like that. May be the Vanuatu tall is not at all resistant to the DFMT, but is simply “vaccinated”. May be this kind of “Vaccination” help the Vanuatu tall to tolerate the Lyd disease in Ghana….

So I continue to believe that conducting coconut germplasm exchange is efficient and useful, even if a high level of precaution is needed.

Kind regards, Roland

Mike Foale to John Randles and us, 20th October 2016
Hi John

The lack of symptoms of Foliar Decay in coconut populations in Vanuatu seems to me to be a quite remarkable thing. I just read from the summary of your 1992 paper (Localisation of coconut foliar decay virus in coconut palm): Viral DNA was detected in symptomatic and asymptomatic palms of both high and low susceptibility, in disease-free tolerant cultivars, and in palms in remission from disease.

I have the impression that populations from pretty well all the islands of Vanuatu are "disease free tolerant". What could be the evolutionary history behind the wide spread of this disease-free status over such a scattering of habitats? Added to that I gather that the population on Rennell, "down-stream" from Vanuatu with respect to oceanic current direction, has a degree of tolerance.

I plan to put together, with the collaboration of willing co-authors, a description of this and other examples where there is clear "local" tolerance or resistance of a particular bio-hazard.

These include the Solomons populations which are little troubled by Brontispa leaf beetle which currently is playing havoc in southeast Asia; also the Solomons populations which are not much troubled by Cercospora (Drechslera) fungal leaf spot whereas a population from the Tuamotus was wiped out in the nursery in 1964; also the population of Hainan Island which tolerates short periods of temperature just a few degrees above zero while all introduced germ-plasm perishes.

It appears to me that the coconut has been evolving in these habitats for long enough to develop remarkable adaptations to local physical or bio-hazards. Would you be able to hazard a guess as to how long would have been needed for a coconut population scattered around the dozens of Vanuatu islands to develop tolerance to a virus injected by a local insect?

Or do you know of anyone else who might hazard such a guess? My guess is some millions of years, but I have no idea how to support that, except that there must have been some remarkable chemical roulette going on and getting a winner must surely have taken a fair while.

Best wishes

Mike

Roland, 20th October 2016
Dear all,

This is a fascinating topic because it seems that, in Vanuatu, the coconut palms can become "vaccinated" to the Foliar Decay Virus (FDV). I saw a small experiment in Santo where Malayan Red Dwarfs, the most sensitive variety to FDV, where "vaccinated" in the nursery and grow very well during decades. This was initially done by Jean Paul Morin. So this subject is crucial, and I suggest to study further this vaccination and to test these three strange hypothesis:

• The Vanuatu Tall is not at all tolerant to FDV, he is only "vaccinated", and this vaccination can be transmitted by seeds
• As Vanuatu Tall is also tolerant to Lethal Yellowing Disease (LYD) in Ghana, this vaccination against FDV is also efficient for LYD
• If the Vanuatu Tall is not vaccinated but really tolerant to FDV, there is a unique physiological and genetic reason why Vanuatu tall is tolerant to Both LYD and FDV

So this subject is of great importance, because it deals not only with FDV but also with LYD.

Kind regards
Roland

References
Pacific Pests and Pathogens - Fact Sheets - Coconut foliar decay (231)

Labouisse, J. P., Sileye, T., Bonnot, F., & Baudouin, L. (2011). Achievements in breeding coconut hybrids for tolerance to coconut foliar decay disease in Vanuatu, South Pacific. Euphytica, 177(1), 1-13.

Rohde, W., Randles, J. W., Langridge, P., & Hanold, D. (1990). Nucleotide sequence of a circular single-stranded DNA associated with coconut foliar decay virus. Virology, 176(2), 648-651.