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Mode of transmission
Lethal yellowing disease is not transmitted by seednuts (ripe coconuts in the husk) or by commercial dry nuts (with or without husk) used to make desiccated coconut or to process to coconut oil.
Seed transmission of LY has never been demonstrated. Experiments have been made where seednuts that have been collected from susceptible palms in diseased areas (and picked up from the ground beneath dying palms) have been germinated and grown in disease free areas and monitored regularly. These experiements have never given rise to diseased seedlings or plants.
Nevertheless, the commercial movement of seednuts from areas where LY is epidemic to areas free of the disease is not permitted by any national or international plant quarantine regulations. Individuals should never be tempted to take a single nut from a diseased area to plant in their garden in a disease free area. The statistical probability of seed transmission may be infinitessimally small but it is not zero.
Although phytoplasma have been identified in embryos it has not (yet) been shown that those embryos can germinate normally, or if they can be germinated in culture, that the resulting plantlets will contain the phytoplasma and be infectious sources to allow acquisition by uninfected vectors.
Artifical transmission has been attempted, using carborundum incoculation, pressure inoculation or paristic plants such as Cassytha filiformis, but without success.
Transmission by insect vectors collected from diseased palms or in diseased areas has been reported from Jamaica and Florida but has not been successful elsewhere. The Jamaican transmission test failed to identify the vector, whereas in Florida, Myndus crudus was implicated (Tsai, 1975 & 1977; Howard & Thomas, 1980).
It is not known if a single infection by a single insect of a single phytoplasma is sufficient to cause infection or if multiple infections by numerous vectors (possibly of more than one genus and perhaps of different pathogenic strains of the phytoplasma) are necessary,
Radioactive markers indicated that other phloem feeders in Jamaica were potential vectors (Eskafi, 1982) but once M. crudus accused was these have never been tested.
Transmission tests using caged palms in Ghana and Tanzania have also been unsuccessful.
Alternative methods of caging potential vectors in small cages on leaflets have also failed to effect transmission.
All these methods have relied on symptom development, however, modern PCR techniques might be able to show that infection occurs in the locality of a local ised feeding site even if a small event does not lead to infection of the whole palm.
Arrow ewaf infection site
Stem tissue
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