Characterisation of the Causal Agent of Mango Anthracnose Disease in Ghana
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Date
2014-05
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University of Ghana
Abstract
This research work was carried out to update information on the nature, the identity of the
causal agent and the importance of mango anthracnose disease in Ghana. It was also to
determine the effect of the disease on the juice quality of fruits and come up with
appropriate control measures in the country. A field survey was carried out in 12
administrative districts of Ghana in 2010 and 2011 to assess the disease incidence and
severity. The effect of the disease on yield/fruit quality was assessed in a commercial
farm by determining the percentage of fruits that dropped or could not be marketed due to
the disease. The pathogen causing the disease was isolated from the diseased lesions and
characterised using cultural, morphological, biochemical and molecular approaches. The
total soluble solids and acidity content of infected fruits were measured as means of
determining the effect of the disease on juice quality of the fruits. The susceptibility of
the different strains of the pathogen to fungicides available in Ghana was assessed using
PDA amended with the fungicides after which efficacy of fungicides was evaluated in the
field. The results show that two different symptoms, a sunken dark lesion and cracked
skins were observable in Ghana. The disease was not found in the field in 5 out of the 12
districts surveyed and the incidence ranged from 0% in the Hohoe, Berekum, Kintampo,
Savelungu/Nanton and Tolon/Kumbungu districts to 100% in the Kwaebibrem and
Kumasi metro districts in both the 2010 and 2011 major mango growing seasons. The
severity index of the disease on a scale of 0-5 ranged from 0 to 3.8 in 2010 and 0 to 3.7 in
2011. The disease was found to cause shriveling of fruit panicles and blemishes on skin
of fruits resulting in yield loss of 4.5% in the major season and 29.9% in the minor
season in a mango orchard in the Yilo Krobo district. Colletotrichum gloeosporioides
sensu lato was confirmed as the causal agent of the disease. From a total of 45 isolates,
16 (35%) were identified as Colletotricum asianum while 29 (65%) were identified as
Colletotrichum species. Artificial inoculations confirmed the pathogenicity of isolates of
the pathogen on mango and induced similar disease level on Haden, Irwin, Julie, Keitt,
Kent, Palmer and Tommy Atkins cultivars of mango. Cross-infection studies showed 32
(32%) of the isolates were the mango bio-type of the pathogen while 68 (68%) were very
virulent on all the three types of fruit and were isolates that may have cross-infected
mango in the field. Analysis of the secondary metabolites of the two types of strains of
the pathogen indicated they may be producing the same kind of toxin. Analysis of the
total soluble solid and titratable acidity content of the fruit showed that the disease does
not significantly affect the juice quality of mango fruits (p>0.05). The pathogen was
found to be highly susceptible to 8 different fungicides available on the Ghanaian market.
The fungicides were able to reduce the incidence and severity of the disease significantly
(p<0.05) in the field eve. At the postharvest phase dipping of fruits in prochloraz solution
both at ambient temperature and at 53oC were found to eradicate pathogen on harvested
fruits even if the fruits did not receive any preharvest treatment on the field. To minimize
cost, the prochloraz dip at ambient temperature is recommended for postharvest control
of the disease.
Description
Thesis (PhD) - University of Ghana, 2014