Abstract
Phytoplasmas are associated with the lethal disease of coconut palms in Tanzania (LDT). It is a destructive lethal yellowing-type syndrome (LYTS) exhibiting differences in losses between the southern districts and the northern districts. To explain these differences, the existence of variable pathogenic strains of the LDT phytoplasma was investigated using ribosomal RNA gene PCR primers. A total of 84 samples were collected from 67 palms in 14 coastal districts of Tanzania, including the low, moderate, and high incidence areas. Of these, 38 samples were studied in detail. Detected phytoplasma rDNA was characterized by either sequencing of the PCR products and/or restriction fragment length polymorphism (RFLP). Sequence analysis of the P1/P7-primed PCR products revealed several positions of variability, making it possible to distinguish two main geographical clusters. The northern cluster included samples from Tanga region only and is associated with low/moderate disease incidence. A second larger cluster included samples from the rest of the coastline between Bagamoyo and Mtwara. Five genotypes could be identified based on mutations/deletions in the P1/P7 PCR product, two within the northern cluster, and three within the southern cluster. The geographical distribution of the two clusters and the genotypes could be related to the history of coconut introductions in Tanzania. The sequences obtained also confirm that the phytoplasmas associated with LDT are significantly different from all of the other phytoplasmas associated with coconut lethal yellowing-type syndromes worldwide, and it is proposed that these phytoplasmas should be classified into their own 16Sr group.
Highlights
Coconut palm (Cocos nucifera L.) is the most important perennial oil crop that supports the livelihood of most farmers in the coastal areas of Tanzania
Products obtained with Rohde's primers from 12 East African Tall (EAT) coconuts were sequenced
This study is the first to show a regional distribution of different LDT phytoplasma genotypes
Summary
Coconut palm (Cocos nucifera L.) is the most important perennial oil crop that supports the livelihood of most farmers in the coastal areas of Tanzania. 25 million palms are grown on 262,000 ha along the coastal belt of mainland Tanzania and the islands of Zanzibar, Pemba and Mafia, where more than 95% of palms are cultivated by smallholder farmers (Kullaya and Mpunami 2008). The crop is threatened by a destructive Lethal Yellowing-Type Syndrome (LYTS) known as "Lethal Disease” or “Lethal Disease Tanzania" (LD or LDT) or even “Tanzanian lethal decline” (Schuiling et al 1992a) (Figure 1). Since 1965, LDT has killed more than nine million palms or 40% of the groves (Schuiling et al 1992b). LDT was first reported in the country in 1905 near Bagamoyo, in 1912 in the south close to the Mozambique border, and at Kunduchi, close to Dar-es-Salaam (Schuiling et al 1992b). The most serious outbreaks occurred independently in the 1940s in the districts of Kilwa, Rufiji (south of the Pwani region) (Figure 2) and in the 2000s, in Mkuranga (centre of Pwani)
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