History and etiology of the lethal disease of coconut palm ( cocos nucifera l. ) in Tanzania
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Date
1987
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Sokoine University of Agriculture
Abstract
In the Bagamoyo District,large coconut plantations were
established as early as 1870 and the industry received a further
impulse from German settlers in the 1930's. The first report on
the occurrence of the lethal disease was in 1911. fit present the
incidence of the disease is low. Coconut palms were planted in the Klsarawe and Rufiji
Districts during three main periods. The first during the Arab
rule mainly between 1070 and 1888, the second during the German
administration (1891 - 1914 ) and the third and most important
after the first World Wer. Although the lethal disease occurred
probably as early as 1890, it made little impact on the two first
plantings. However, serious outbreaks occurred in the 1940's
when the post 192B groves came into bearing.
The most important outbreak in the Kisaj»owe District was
near Sanga-Sanga in 1941. The disease spread in eastern, then
southern directions to Msonga and Magawa. It moved as well to the
north across the* Luhute valley to Sotele and Binge. In both bisec
tions the disease spread over a distance of approximately 40
kilometers during a period of 35 - 40 years. While this general
direction of movement over an extended period of years was
revealed by the survey, the spread did not occiir along an extended
front and was often unpredictable. There was no apparent relation
between the direction of spread of the disease and the prevailing
winds although it spread faster through groves planted along open
spaces like valleys or swamps.
Similarly rampant spread of the lethal disease was reported
from the Rufiji and Kilwa Districts. Outbreaks of the lethal
disease were reported at various places along the coast of
mainland Tanzania at various times from the 19th century onwards.
Sometimes independent outbreaks occurred simultaneously. It is
therefore impossible to recognize one or more sources from which
the lethal disease originated. The disease has obviously been
indigenous in the coastal areas of mainland Tanzania for a long time.
Injection of both bearing and non-bearing diseased coconut
palms with tetracycline-hydrochloride antibiotic resulted in
remission of symptoms in test palms but only when injection was
started in sufficiently early stages of disease.
Total remission of symptoms was obtained in only one
bearing palm, fill the other remitting palms showed delayed
symptom development compared with their controls and succumbed
later, The temporary symptom remitting bearing palms often produ
ced new healthy inflorescences which blackened later. In non
bearing palms, those which were injected in early stages of dise
ase showed completely rnaffected spear leaves for a long time but
nonetheless the palms collapsed later. Taken in conduction with
the finding of MLO in diseased palms, these results support the
concept of a MLC etiology for the lethal disease.
Insect population studies at seven different lethal
disease affected sites revealed that the insects occurring on ce
conut palms in Tanzania are almost exclusively planthoppers
(Fulgoroidea). Twenty-nine different species were identified at
CAB Intitute of Entomology of which only seven were common at all
sites.These seven species are the most likely candidates as the
vectors of the agent causing the lethal disease.
Seasonal peaks in the populations of several species were
observed but they were neither clear nor consistent. No variety
or hybrid was visited by consistently higher numbers of insects
than any other. Removal of shade appeared to result in an incre-
ase of insects on the palms*
In transmission trials to find the vector of the lethal
disease, only one test palm showed symptoms similar to those of
the disease. However, the palm was observed in an advanced stage •
and not sampled. This possible case of transmission needs
confirmation.
Description
Dissertation
Keywords
Etiology history, Coconut palm, Lethal disease, Large coconut plantations, Tanzania