Resistance & Susceptibility

of Available Planting Material

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Last modified: 3 December 2005

When the Malayan Dwarf variety was found to be highly resistant to LY in Jamaica it became the basis for a rehabilitation programme (see "The Malayan Dwarf supersedes the Jamaica Tall coconut", published in Oléagineux in 1970-71). Its agronomic shortcomings stimulated the search for alternative planting material and many foreign accessions were screened in Jamaica. Some varieties proved to be as highly susceptible as the Jamaica Tall and none had better resistance to LY than the Malayan Dwarf. Those which showed most promise could trace an origin to southeast Asia. As a result of a breeding programme which started in the early 1960s it was possible to release the first commercial F1 hybrid, known as Maypan, in the mid 1970s (see "Maypan: an F1 hybrid coconut variety for commercial production", published in World Crops in 1974). Unlike hybrids that were developed around the same time in Ivory Coast and the Solomon Islands, the Maypan was bred to combine good disease resistance with both precocity and high yield.

At this point it is worth defining the terms "highly resistant", "highly susceptible" used in the preceding paragraph. Resistance and susceptibility are interchangeable terms in the sense that a variety with intermediate resistance also has intermediate susceptibility. High resistance is never 100% (which would be equivalent to immunity) and even highly susceptible varieties usually leave a few survivors in any area devastated by the disease (Even if there were no such "escapes", the "nut fall" symptom ensures that a next generation of susceptible palms will grow even if all adult palms are destroyed). Sometimes the phrase "more or less resistant" (más o menos resistente; plus ou moins résistant) is used to indicates that a variety that shows good resistance in one location may do less well in another. For example, the Malayan Dwarf does not do as well in East or West Africa as it does in the Caribbean and Latin America. Yet, even in these African situations, it is possible to discern that there are relative levels of susceptibility and that resistance, whilst not as effective, is still present.

As a result of the research carried out in Jamaica it was possible to suggest that the resistance of varieties from the southeast Asian region is accounted for by the domestication of the coconut in that region. Natural selection for resistance (upon which human selection for attributes such as red or yellow fruit colour and dwarf habit would be superimposed) resulted in the varieties that are used today as hybrid seed parents. The natural out-crossing nature of the coconut palm meant that a lower level of resistance occurred in introgressed populations (introgression between susceptible wild types and resistant domestic types). That level of resistance was maintained in southeast Asia however because the disease organism also remained present. This helps to explain why phytoplasmas associated with coconuts in Indonesia or Malyasia result in sporadic local infections rather than epidemics.

For practical purposes replanting in epidemic disease areas is usually restricted to highly resistant dwarf or F1 hybrids. Populations with a natural intermediate level of resistance (such as those on the Pacific coast of America) have already been used as male parents in hybrid production and may yet prove to be sufficiently resistant to prevent epidemic LY outbreaks.

As indicated above, coconut varieties are sub-populations generated by introgression between naturally evolved but susceptible types and resistant types selected during domestication. Depending on the degree of introgression, varieties can be categorised for the degree of resistance or susceptibility to lethal yellowing along a scale from highly resistant to highly susceptible. Growing conditions, infection pressure and (probably) pathotype differences occur so that strong differences in Carib-America are seen as slight differences in East Africa and as overall endemic resistance in Southeast Asia. A knowledge of the source, possible isolation and the history of introductions helps to understand differences.

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LY DISEASE RESISTANCE RANKING

The foillowing scale, based on trials, field observations and theory, is offered for discussion:

Rank 1 – highest resistance – Malayan Dwarf

Rank 2 – high resistance – Maypan

Rank 3 – intermediate resistance – American Pacific Coast Tall and some Southeast Asian varieties, also Mayjam and PB121

Rank 4 – low resistance – Southeast Asian & Pacific Island Tall varieties

Rank 5 – no resistance – American & African Atlantic Coast Tall types – Jamaica Tall, West African Tall etc.

The ranks are not distinctly independent (they merge and overlap) and the actual level of resistance or susceptibility will be strongly influenced by growing conditions, vector populations etc,

See the section on Ideas for further discussion and sub-division of this ranking.

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SOURCES OF PLANTING MATERIAL:

Malayan Dwarf is available almost everywhere because it was widely planted in the 1920s and 1930s because of its early bearing. It found favour for replanting after hurricanes because of this early bearing character .

Pacific (coast) Tall from many locations between Jalisco and Oaxaca have been tested at San Crisanto in Yucatan, Mexico and found to have satisfactory resistance.

Panama Tall is a specific example of the Pacific Coast Tall that was introduced to Jamaica, from the Pacific coast of Panama almost 90 years ago. Two selections, known as Aguadulce and ...

Maypan is the result of the cross between MD and PT and specifically identidfies material coming from Jamaica or material produced from parents that can be identified as having come from selected palms in Jamaica.

Mapan is a cross between MD and Pacific coast Tall produced and named in Costa Rica. Although the pollen parent has not been screened for LY resistance the resulting hybrids are performing weell in diseased areas.

Mayapan is a name that can be used for hybrids produced in Mexico using MD and PcT that have been screened for resistance.

 Mayjam, Majam and Mayajam would be the equivalent crosses between MD and Atlantic Tall. Resistance would be less and there would have to be particualr reasons for recommending planting such material (one reason might be better establishment under difficult growinbg conditions on coral cays for example but this hypothesis remains to be tested (see New Ideas).

Rennell Tall is a particular variety from the South Pacific (the male parent for a hybrid produced in the 1970s) . Although, preliminarilly classified as a Niu vai (domestic) type there is reason to believe that some Rennell populations are introgressed.

On the principle that tall varieties can be identified as predominantly wild or predominantly domestic types by fruit component analysis, it should be possible to predict resistant of crosses made in the absence of disease.

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OTHER PALM SPECIES:

Listing of palm species known to be susceptible to lethal yellowing disease in Florida (after: Harrison et al 1999 Detection & diagnosis of lethal yellowing,pp 183-196. In: Oropeza et al (eds) Current Advances in Coconut Biotechnology. Kluwer.)

Listing of palm species NOT known to be susceptible to lethal yellowing disease in Florida (after Harrison & Howard (undated)) URL

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SAFE MOVEMENT OF PLANTING MATERIAL

The safe movement of coconut germplasm has been the subject of guidelines and pest risk analysis. For further information, consult

Safe movement of coconut germplasm

Pest Risk Analysis FAO (relevant for all pests and diseases, not only LY)

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