ARTICLE
Auteur(s) : Joe
Nkansah-Poku1, René Philippe2, Robert
Nketsia Quaicoe1, Sylvester Kuuna Dery1,
Arthur Ransford3
1CSIR – OPRI, Coconut Programme, P.O. Box 245,
Sekondi, Ghana
2UPR 29, CIRAD – BIOS, Campus Int de Baillarguet,
34398 Montpellier, France
3CSDP, Ministry of Food & Agriculture, P.O. Box 245,
Sekondi, Ghana
The Cape Saint Paul Wilt Disease (CSPWD), a lethal-yellowing
type disease of coconut has been in Ghana since 1932. The disease
is caused by a phytoplasma and is found in Africa and the
Caribbean. The symptoms of the disease are premature nut drop with
or without yellowing of fronds and blackening of immature
inflorescences. This is followed by progressive yellowing or in
some instances browning of the crown from the older leaves upwards.
Eventually, the crown turns yellow, dries up and then falls off,
leaving a bare trunk or “telephone pole”.
Two types of spread of CSPWD have been observed. In one type, a
local centre of infection appears in one or two palms; this is
followed by new cases appearing at random around the initial
centre. The second is a “jump spread” whereby the disease appears
at a spot remote from a known focus. This is then followed by a
local spread in all directions.
The disease epidemic which began around Cape Saint Paul in Woe
near Keta destroyed thousands of coconut palms and caused the
collapse of the coconut industry in the Volta region by the
mid-1950s [1]. The disease appeared in the Western region at Cape
Three Points in 1964 and in the Central region at Ayensudo in 1983.
The history, occurrence, epidemiology and spread of the disease in
Ghana have been reported by Johnson and Harries [2], Ofori and
Nkansah-Poku [3] and Dery et al. [4].
McCoy et al. [5] pointed out that most secondary spread of
lethal yellowing disease occurs within 100 m of a new focus
and eradication could be useful if practised rigorously in the
early stages of the outbreak. Philippe et al. [6] and
Nkansah-Poku et al. [7] observed that cutting out regularly
all diseased palms slows down the rate of spread of CSPWD.
Since 2000 the damage and spread of the disease have been
monitored through disease surveys. In 2006 and 2007, an aerial
surveillance was conducted to have a quick view of the extent of
the disease spread in the Western region. These were the only
occasions ever of monitoring the CSPWD spread with an aircraft in
the country. The disease has also been managed by removal of
diseased palms in a focus at Ampain to reduce the disease spread
westwards to protect the large healthy plantings beyond Ampain.
The hybrid between the “Malayan Yellow Dwarf” and “Vanuatu Tall”
(MYD × VTT) was used in a rehabilitation programme in 1999
following a recommendation by Dery et al. [8]. Between 1999
and 2004, 1,300 ha of devastated areas were replanted with
this hybrid in the Western and Central regions.
This paper reports and discusses the aerial surveillance,
current disease distribution in the Volta, Central and Western
regions, containment of the disease at Ampain as well as the
disease situation on new replanting plots with the hybrid, MYD ×
VTT.
Materials and methods
CSPWD survey
Aerial surveillance
A small two-seater ULM aircraft was used in the survey. The
aircraft moved at an average speed of about 35 m/s and at a
height of approximately 300 m. Waypoints of suspected diseased
coconut palms/foci spotted from the aircraft were recorded with a
"Global Positioning System 60" (Garmin Ltd). Recorded waypoints
were estimated in metres to the left, centre or right of the
aircraft from the spotted diseased palms/foci and managed with
MapSource software for assessment and ground verification.
Photographs were also taken of suspected CSPWD infection spots. The
aircraft flew along designed flight patterns worked out by the
research team and the pilot.
Ground surveillance
Two types of ground survey were conducted. The first was an
extensive survey aimed at covering all the coconut-growing regions.
In this type, we made use of accessible footpaths and motorable
routes in an area to inspect coconut plantations. Village enquiries
and interviews of farmers and agriculture extension agents were
carried out to gain information on the presence or absence of the
disease in an area. The second was an intensive survey aimed at
identifying diseased spots and or individual diseased palms in an
infected plantation or nearby healthy farms, which are at risk of
getting infected. The “systematic walk through” method was used to
locate palms with visible CSPWD symptoms in an area. Samples of
infected palms were taken for polymerase chain reaction (PCR)
analysis to confirm disease incidence. Known diseased foci were
monitored monthly in this way. GPS points captured during aerial
surveillance were also verified by this method.
Disease management/containment
To reduce the rate of spread of the disease, eradication of
diseased palms was undertaken at the Ampain focus. There is a
continuous stretch of coconut planting from this focus westwards to
Cote d’Ivoire. The focus was monitored periodically and monthly in
times of sufficient logistics. During each visit, palms were
inspected and those showing disease symptom were felled with a
chain saw machine, fronds pruned off and trunk cut into pieces of a
metre long to facilitate quick drying.
Monitoring of Coconut Sector Development Project (CSDP)
replanted plots
The CSDP of the Ministry of Food and Agriculture (MoFA) started a
rehabilitation programme with the MYD × VTT hybrid from 1999, and
1300 ha was replanted by 2004 under a project funded by Agence
Francaise de Developpement (AFD). All fields replanted with the
hybrid were monitored between March 2006 and May 2007 through
monthly inspections. All cases of disease development based on
visual symptoms were recorded. First infection cases in each plot
were verified and then confirmed by PCR analysis.
Results
Aerial surveillance
Two surveys were conducted, the first from August 28 to August 31,
2006 and the second, March 25-March 30, 2007. The first survey was
conducted in the area denoted as “established infection area” (figure 1). Two
incipient foci were revealed by that survey, one to the north and
the other to the west of Salman 15.2 km and 11.2 km away
from the coast, respectively, following ground verification of
points captured with GPS. These foci were beyond the previously
known disease front. In the second survey, the area covered
extended close to the frontier with Cote d’Ivoire but 2 km
away from the border for security reasons. The disease appeared to
have established itself in the approximate pattern shown in figure 1. The extent
of the survey is denoted by the mauve line and the areas of
possible and established infection delimited in red. The green
delimited area was not surveyed because it appeared to be a
“coconut-free zone”. The ‘possible new infection area’ had coconuts
with a general yellowing tinge to the lower leaves over a large
area, which runs down towards the frontier with Cote d’Ivoire.
Isolated cases of yellowing were also found in other areas.
Ground verification survey
Using the data captured on the GPS and the aerial photographs (figure 2) taken
during the flights, ground survey for the verification of the GPS
captured disease symptoms was conducted in the area marked
“possible new infection area” and beyond Half Assini all the way to
the frontier. A thorough ground search identified GPS points
041 (southeast of Tikobo 2) and 167 (near Half Assini). Samples
from representative CSPWD suspected palms within the designated
area were taken for PCR analysis. A total of six samples from
six locations were analysed. Morphology of the sampled palms ranged
from symptoms of pronounced yellowing reminiscent of nitrogen
deficiency, yellowed lower crown with dark spotting in the lamina
resembling symptoms of potassium deficiency and pale crowns with
tapering trunks as in senile palms on poor soil. All the palms
tested negative, thus confirming the absence of the disease in
these palms.
A critical observation of these suspected disease outbreak
points showed that apart from two locations, where there were
intercropping with maize, cassava and vegetables, all the palms had
been left to grow without any agronomic intervention and were
associated with bushy undergrowths. All the target palms were West
Africa Tall (WAT) ecotypes above 40 years and showing poor nut
loads. Apart from yellowing of lower leaves, no other symptom, for
example, nut drop or necrosis of opened inflorescences, was
observed in any of the suspected palms.
Limit of spread of CSPWD
The disease zone stretches from Ampain in the west to Keta in the
east along the coast. However, there are pockets of healthy groves
remaining in between and further inland. The figures 2 and 3 show
the current distribution of the CSPWD incidence in Ghana.
Disease distribution in the Western region
The worst affected district in the region is the Ahanta West, which
has lost about 70% of its coconut plantings. Few healthy groves now
exist and the disease appeared less active in existing foci.
The most active foci are located in the Nzema East district. The
Asanta focus appeared in 1992 and by 2001 had almost devastated all
the coconuts in the area (90% loss) crossing the river Ankobra
lying in the east to merge with the Awuku focus established in
1990, which had also spread westwards through Axim. The Asanta
focus has spread as far north as Nyamebekyere by 2006. Another
focus at Ampain since its identification in 1995 has expanded the
least in the district, despite the loss of 70% of the original
25 ha planting. About 20-30% of the district’s 12,000 ha
of coconuts are lost. Two other active foci in the region are
located at Botogyina and Daboase (figure 3) in the
Shama and Mpohor Wassa East districts, respectively. The spread
after 1995 has mainly been by expansion of existing foci rather
than jump spread.
Disease distribution in the Central region
In the Central, the disease was spotted at Ayensudo near Komenda in
1983. By 1995, there were several foci dotted in the KEEA district.
These foci merged in 1998 and devastated all the plantings around
Abrobiano, Duakyimase and Kafodzidzi. The foci at Dompoase,
Ayensudo and Ataabadze lying north of Komenda also merged and
spread to Hemang and Dwabor further north by 2003. The recent
survey showed only scattered infected fields with varied damage
levels (1-70%) in the northern part of the district where coconut
plantations are not continuous – coconut plots are separated by
food crop farms.
The Eduma focus established in 1989 had expanded north to merge
with the Abakrampa focus engulfing the plantings at Asebu and its
environs, reaching Asuansi in 2006. Another active disease centre
of interest in the region is the Asaafa-Narkwa corridor. The
disease occurred at Asaafa in 1998 and Narkwa in 1996. These foci
had since merged and devastated about 60% of the plantations as at
April 2008. Beyond Narkwa towards the east, the disease had spread
to Mumford near Apam. The loss in this zone is about 40-60%.
The Agona district which was previously disease free is now
affected. An outbreak occurred in 2006 Obratwawu near Bawjiase and
about 650 palms (less than 1%) of the original stand had succumbed
to the disease by April 2008. This focus is of importance because
it is at the centre of one of the extensive coconut groves in the
region and it seriously threatens the supply of fresh nuts to
Accra, the capital city of Ghana. Another outbreak occurred at
Efutu Breman near Jukwa in 2005 and by 2008 devastated about
1.5 ha.
Disease distribution in the Volta region
In the Volta region, the disease is currently less active with
losses between 0 and 2%. Existing disease foci are shown in figure 4. After the
major epidemic in 1960s and 1970s in the Keta area, the disease
apparently disappeared until 1995 when it resurfaced to attack
replanted fields [3]. However, the disease after resurgence had
been less aggressive and its attack is sporadic and mainly centred
around the Woe-Tegbi-Dzelukope corridor. It has remained endemic at
the Cape Saint Paul area in Woe. Losses are very low and the
economic impact appears negligible. Only very few diseased palms
were observed at the corridor during the last five surveys
conducted between 1997 and 2008. A new outbreak was identified
at Glidzi-Agbosome in 2006 with nine infected palms. Thus, the
presence of the disease since 1995 had been in few locations around
the Keta lagoon with minimal losses. Less than 1% loss had been
recorded of the estimated 1,500 ha of coconut currently in the
southern portion of the region.
Disease management by removal of diseased palms
The CSPWD focus at Ampain is the western front of the disease zone
along the coast. An estimated 3800 coconut palms covering an area
of 25 ha surrounded the main focus identified in 1995. Seventy
percentage of the original stand has succumbed to the disease
leaving about 30% (1,130 palms) in existence as at March 2008. Of
the percentage that was infected, 1,331 palms were removed by
cutting out through containment exercise undertaken since 1996.
However, due to funding problems, cutting was not done regularly
through the period except 2003-2004 and from August 2007 to March
2008. However, field observation shows a slow spread of the disease
at this focus.
Disease spread in CSDP replanted plots
Between 1999 and 2004, 1300 ha of devastated areas were
replanted by CSDP with the MYD × VTT hybrid. Approximately 1,000
farmers were involved in the programme and had plot sizes of
1-3 ha in the Western and Central regions. In all,
1288 ha monitored for CSPWD infection (929 and 359 ha
respectively in Western and Central regions) and 62 ha (4.8%)
(29 and 33 ha, respectively, in the Western and Central
regions) were observed and confirmed by PCR analysis to be infected
by the disease. None of the plantings of 1999 covering 35 ha
(22 in Western and 13 in Central) and of 2004 covering 196 ha
(176 in Western and 20 in Central) was affected by the disease as
at May 2007. When the programme began in 1999, planting sites were
carefully selected unlike the subsequent years. The affected plots
were mostly planted in 2000, 2001 and in the mixed year plantings.
The disease incidence was slightly higher in the Central region
(33 ha) than in the Western region (29 ha) (figure 5).
Discussion
Coconut varieties in plantations in Ghana are mostly local WAT
ecotypes. Their susceptibility to the CSPWD is very high. The
pattern of the disease spread appears erratic occurring in “leaps”
or “jumps” characteristic of insect transmission as in Caribbean
lethal yellowing disease [9]. However, some healthy palms can be
observed even in heavily diseased farms.
The yellow patches in the “possible infection area” (figure 1) observed in the
aerial survey were not CSPWD. It is a known fact that most of the
coconut palms in the south western part of the Western region
suffer from nutrient deficiencies, notably potassium, phosphorus
and magnesium [10]. Most of the palms in the area are almost
senile. Both nutrient deficiency and senescence result in some
yellowing of the crown of the palms. These phenomena coupled with a
high rate of lightning strikes prevalent in the area present an
extensive area of yellowing of coconut palms, a feature which could
easily be mistaken for CSPWD.
The aerial surveillance provides a quick view of the condition
of coconut palms over a large area at a very short time. It is able
to spot incipient foci as palms showing yellowing symptom singly or
collectively can be spotted from the air and captured on GPS. This
is dependent on the speed and height at which the aircraft travels.
Captured data on GPS were detected within the radius of 300 m
on the ground. As indicated above, not all palms showing yellowing
symptoms are necessarily CSPWD infected. Thus, the method is not
foolproof in identifying all CSPWD infections. To achieve that it
has to be supported by ground surveillance to confirm disease
infection. Another major drawback is that it is very expensive and
cannot be done frequently without adequate resources. Aerial
surveillance would be appropriate for early detection of incipient
foci and half-yearly surveys are recommended to be conducted in the
large healthy zone lying west of Ampain.
The rate of spread of the disease at Ampain is slow. From field
observations, the estimated rate of spread per year of CSPWD is
0.33 km (about 1 km per 3 years). However, the
spread around the disease focus at Ampain is less than 1 km
for the past 14 years. Comparatively, the Asanta focus which
had similar features (both foci established almost at the same time
in coconut groves near swampy mangrove area, on outskirts of
settlements less than 1 km to the coast, original coconut
stands mainly of WAT subpopulations, dominant putative vector
species encountered frequently at both sites were of the families
of Derbidae, Cicadellidae, Diaspinae, Achilidae and Cixiidae)
except the periodic cutting (done once in 1994 at Asanta) had
expanded more than 3 km to the east crossing the river
Ankobra, 3.3 km to the west reaching Kikam and more than
4 km to the north. The slow spread at Ampain is attributed
mainly to the containment exercises that have been undertaken at
the focus since 1996. Although not rigorously practised, the
containment exercise had slowed down the spread of the disease in
this area. This agrees with the observations of Philippe
et al. [6] and Nkansah-Poku et al. [7] that cutting out
regularly all diseased palms slow down the rate of spread of the
disease. Felling reduces accumulation of disease inocula in a given
plot. Eradication must be part of an integrated control strategy of
CSPWD. Its contribution will be to reduce the amount of inoculum
present at any point in time for vector acquisition and
transmission. A programme based on early detection and prompt
removal of diseased palms to control disease spread is recommended
to cover a wider area.
The behaviour of the disease in the southeastern portion of the
country presents a unique situation. The disease disappeared after
the major epidemic in the 1960s and 1970s and recurred in 1995.
Since then its attack has been sporadic and less aggressive, and
losses very low compared to that in the Western and Central
regions. The low incidence may be attributed to the following field
conditions. There is a relatively drier condition prevalent here.
Coconut mainly grows in sandy dry soils around the Keta lagoon and
the narrow coastal strip of land between the sea and the lagoon.
Due to intense pressure on land development of settlements under
the plantations is a common feature; hence, there is little ground
cover. Flora diversity is low and insects’ vector population and
activity have been observed in the field to be sparse. Planting
materials used for replanting the devastated fields after the
epidemic have come from diverse sources and thus, presence of
heterogeneous coconut subpopulations. These factors probably are
influencing the variation in disease spread in the region. It would
be interesting to begin comparative studies into the disease spread
in the Volta and Western regions.
The disease attack on the MYD × VTT hybrid in replanted fields
appears low for now. However, it showed the susceptibility of the
hybrid to high pressure of CSPWD as found on the fields in the
Central region. In some fields, palms were killed even before
flowering. However, as suggested by Dery et al. [11] the
hybrid MYD × VTT should be given the preference in areas where the
risk of disease attack is low due to its favourable agronomic
performances and the fact that supplies of a superior hybrid are
currently limited.
Acknowledgements
We are grateful to the Agence Francaise de Developpement (AFD), the
French Embassy and the Government of Ghana for providing funds for
the work. We acknowledge the services of Mr Jonathan Porter, the
pilot who flew the team safely.
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