Comparative studies on lethal diseases of coconuts
Phytoplasmas associated with lethal diseases of palms are found in several regions of the world. Palm lethal yellowing phytoplasma (EPPO A1 quarantine pest) is present in several countries in the Caribbean, in Mexico, Belize and Honduras. Similar diseases are reported from West Africa (Cape St Paul wilt disease in Ghana, Kribi disease in Cameroon, Kaincopé in Togo and Awka disease in Nigeria) and East Africa (in Tanzania, Kenya, Mozambique). Similarities between the disease syndromes have led to the assumption that palm lethal yellowing observed in the Caribbean region and the African lethal diseases were similar. However, important differences in epidemiology and varietal susceptibility suggest differences between pathogens on each continent, and also between east and west Africa.
In a previous study (see EPPO RS 94/223), it was found that the palm lethal yellowing phytoplasma, although related, is different from coconut lethal disease phytoplasmas found in east Africa. The present studies were carried out to compare west African phytoplasmas with palm lethal yellowing phytoplasma and the pathogens found in east Africa, by using molecular techniques (cloned palm lethal yellowing phytoplasma probes in DNA dot hybridization plus PCR amplification and RFLP analysis of 16S rRNA genes). Genetic similarities were found between the phytoplasmas, as the DNA probes (cloned from palm lethal yellowing phytoplasma) hybridize to infected palms from various regions of east and west Africa. However, studies using PCR and RFLP showed that African isolates are different from palm lethal yellowing phytoplasma, and that differences are observed between isolates from east and west Africa. However, it was observed that the phytoplasmas found in Ghana and Nigeria (respectively causing Cape St Paul wilt disease and Awka disease) are probably the same pathogen. The authors concluded that their studies suggested that coconut phytoplasmas found in the Caribbean, east and west Africa, although genetically related, are different, indicating the presence of three distinct isolates. In addition, they felt that the use of PCR with RFLP will be a useful tool for diagnosis of the pathogen and epidemiological studies.
Sources
Tymon, A.M.; Jones, P.; Harrison, N.A. (1997) Detection and differentiation of African coconut phytoplasmas: RFLP analysis of PCR-amplified 16S rDNA and DNA hybridisation.
Annals of applied Biology, 31(1), 91-102.