Characterisation of the Causal Agent of Mango Anthracnose Disease in 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.