Genetic diversity in coconut lethal yellow disease phytoplasmas in East Africa
Lethal diseases of palms occur in several parts of the world. Palm lethal yellowing phytoplasma (EPPO A1 quarantine pest) occurs in the Caribbean, Mexico, Belize and Honduras. Similar phytoplasma diseases are observed in West Africa (Cameroon, Ghana, Nigeria, Togo) and in East Africa (Kenya, Mozambique, Tanzania). Similarities between diseases have led to the assumption that palm lethal yellowing in the Americas and the African lethal diseases were similar. However, important difference in epidemiology and varietal susceptibility suggested differences between pathogens on each continent and also between East and West Africa. Genetic comparisons have previously shown that phytoplasma isolates from East Africa were related but distinct from those in West Africa and the Caribbean (see EPPO RS 97/222, 94/223).
Further genetic comparison have been made between isolates from East Africa. In Tanzania, 22 million coconut trees (Cocus nucifera) are planted along the mainland coast and in the islands of Zanzibar, Pemba and Mafia. The lethal disease was first reported in coconut trees near Bagamoyo (region of Dar es Salaam) at the beginning of this century. The disease has caused extensive damage on the mainland for the last 30 years and now also occurs in Mafia island. However, disease incidence is not identical in all affected regions. In southern regions, it is estimated that 56% palm trees have been killed since 1965 whereas only 8.5% have been affected in northern regions. Using PCR, RFLP and rDNA sequencing, genetic comparison were made between several Tanzanian isolates from regions with low, medium and high disease incidence. Phytoplasma isolates from Kenya (neighbouring region, north of Tanzania) and from Mozambique (neighbouring region, south of Tanzania) were also studied. Phytoplasmas were detected in all diseased samples. Results showed that phytoplasma isolates from Kenya and Tanzania are similar. But they are both distinct from Mozambique isolates, the later being related to West African isolates. No distinction could be made between Tanzanian isolates despite the fact that differences are observed in disease incidence. Differences in disease severity observed in Tanzania cannot be explained by the occurrence of different pathogens, but perhaps by genetic variability in palm tree populations or different insect vectors but more studies are needed.
Sources
Mpunami, A.A.; Tymon, A.; Jones, P.; Dickinson, M.J. (1999) Genetic diversity in the coconut lethal yellowing disease phytoplasmas of East Africa.
Plant Pathology, 48(1), 109-114.