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.
CHAPTER 1
1.0 INTRODUCTION
Mango (Mangifera indica L.) is one of the most important fruit crops in the tropics, and approximately makes up 50% of tropical fruits produced in the world (Jedele et al, 2003). The world’s largest producer of the crop is India followed by China and Thailand. The largest producer of the crop in Africa is Nigeria followed by Egypt (FAOSTAT, 2010). Countries in West Africa such as Ghana and Burkina Faso produce modest amounts of the crop (FAOSTAT, 2010).
Mango is a specialty crop in most of the international markets and hence is an important source of foreign exchange for most developing countries including Ghana. Currently, Mexico is the world’s largest exporter of the crop followed by India and Brazil. In Africa, none of the producer countries is considered as one of the top ten exporting countries (FAOSTAT, 2010) although South Africa, Kenya, Mali, Burkina Faso and Cote d’Ivoire export modest quantities of the crop. Ghana has also been exporting the crop for the past 20 years but is still not recognized as an important exporting country in international markets (Anon., 1996).
Ghana is made up of six major agro-ecological zones comprising of the coastal savanna, the wet equatorial forest, semi-deciduous forest, transitional zone, Guinea savanna and Sudan savanna. Currently, the commercial production of mango which was previously concentrated in the coastal savanna zone is now spreading to the other agroecological zones and it is believed that if the major problems related to the production of the crop are properly handled,
the mango crop has a potential of being the number one export earner of Ghana thereby replacing cocoa (Anon., 1996).
There are many factors affecting the production of quality mango fruits in Ghana. These range from unfavourable weather conditions to incidence of pests and diseases. Mango anthracnose caused by Colletotrichum gloeosporioides (Oduro, 2000; Offei et al., 2008) is one of the main diseases affecting the crop in Ghana. The disease has been reported as the most important fungal disease of the crop in the country (Oduro, 2000). The disease causes dark sunken lesions on leaves and immature fruits and can sometimes blight flowers (Agrios, 2005; Ploetz, 1998). One of the most significant effects of the disease is the blemishes caused on fruit in storage or in transit which eventually reduce the marketability of the fruits. Elsewhere, the disease is known to be favoured by high humidity or rainfall during the early fruit maturity stage (Ploetz, 1998; Arauz, 2000) and hence it is cultivated in drier areas or areas in where rainfall does not coincide with early fruit maturity.
Several characteristics have been associated with the mango anthracnose disease worldwide. These include variant forms of symptoms (Nelson, 2008), latency of the infection (Simmonds, 1941) and retention of inoculum in the tree canopy (Arauz, 2000). It has also been reported that the disease results in fruit drop thereby causing direct loss in fruit quantity (Ploetz, 1998; Dodd et al., 1992). The knowledge of these disease characteristics is important designing control measure. However, limited research has been done on these aspects of the disease in Ghana. Thus there is the need for the nature of the disease in Ghana to be elucidated.
Three different species belonging to the genus Colletotrichum have been reported as the causal agent of mango anthracnose. These are Colletotrichum gloeosporioides (Dodd et al., 1997), Colletotrichum acutatum (Fitzell, 1979; Ploetz and Prakash, 1997) and Colletotrichum gloeosporioides var. minor (Fitzell and Peak, 1984). C. gloeosporioides var. minor has been described as a variant form of C. gloeosporioides and has so far been reported only in Australia. Currently, it is no more recognized as an aetiological agent of the disease (Ploetz, 1998). C. gloeosporioides and C. acutatum are distinct species but the disease symptoms they elicit on mango are indistinguishable. They both cause dark brown sunken lesions on mango fruits, leaves and stems and may also cause twig die back (Ploetz, 1999). Multiple species of the genus Colletotrichum are known to infect the same host. For example, C. gloeosporioides and C. acutatum are known to cause disease on citrus (Brown et al, 1996), while coffee is attacked by C. asianum, C. fructicola and C. siamense (Prihastuti et al., 2009). In Florida both C. gloeosporioides and C. acutatum were found to be the causal agent of anthracnose on mango (Davis, 1999). In Ghana only Colletotrichum gloeosporioides has been reported as the causal agent of mango anthracnose as well as anthracnose on other fruit crops (Oduro, 2000; Offei et al., 2008) and therefore it would be important to confirm this.
Colletotrichum gloeosporioides is a group species made up of different species including C. musae, C. kahawae, C. asianum and C. gloeosporioides sensu stricto and several other species brought together by similar spore morphology and rDNA-ITS sequences (Damm et al., 2010). In Ghana, the identification of the causal agent of mango anthracnose as C. gloeosporioides is not specific enough since it could refer to any of the distinct species in the
- gloeosporioides complex. It is necessary, therefore to determine which of the distinct species in the C. gloeosporioides complex, is the causal agent of the disease in Ghana.
The strain of C. gloeosporioides infecting the mango crop is generally referred to as the mango bio-type of C. gloeosporioides and is known to be restricted to the crop and has not been found naturally on any other crop (Alahakoon et al., 1994). Apart from being genetically different compared to the strains of the pathogen from other fruit crops, the mango biotype is readily distinguished from the other strains by virtue of its limited infectivity on other fruit crops compared to its virulent nature on mango as assessed using artificial inoculation studies (Hayden et al., 1994). In addition to the mango biotype of the pathogen other strains have been found to infect mango implying that the crop has been cross-infected on the field by these other strains. In Ghana, it is not known whether strains of the pathogen on mango are the mango bio-type or whether other strains are involved in the disease aetiology. This, together with the biochemical basis for the mode of infectivity of the mango bio-type of the pathogen which has largely not been determined was investigated in this study.
In guava, anthracnose caused by C. gloeosporioides was found to reduce the nutritional quality of the fruit (Amusa et al., 2005). In the case of citrus, the black spot disease attributed to Guinardia citricarpa has been reported as not having any effect on the juice quality of the citrus fruit (Brentu et al., 2012). In mango, there is paucity of the information as regards the effect of C. gloeosporioides on the juice quality of the mango fruit and this was addressed in this thesis.
Fungicide application at regular intervals has been generally recognized as the most effective method of controlling mango anthracnose (Ploetz, 1998). Inherent problems of the method include emergence of resistant strains of the pathogen, harmful environmental side effects (Dodd et al., 1989) and accumulation of fungicide residues in the final produce. It is
generally known that poor application methods of these fungicides will result in poor control results. In Ghana, farmers have complained of poor results achieved with the fungicides available in the country (Odzeyem, 1998). What is not known is whether the pathogen has developed resistance to the fungicides or whether the ineffectiveness of the fungicides is as a result of poor application practices. These were investigated in this current study.
The objectives of this research therefore, were to determine the nature and importance of mango anthracnose disease in Ghana, clarify the identity of the causal agent and recommend appropriate methods for the application of fungicides for the control of the disease in the country.
Specific objectives were:
- Determine the types and nature of the disease symptoms, the major sources of inoculum on the field and control practices being instituted in the field in Ghana against the mango anthracnose
- Determine the disease incidence and severity of mango anthracnose in the field and after harvest in the different administrative regions and districts and the different agro-ecological zones of Ghana.
- Estimate the disease incidence, severity and yield loss associated with mango anthracnose in a selected commercial mango farm in Ghana.
- Characterize the pathogen causing mango anthracnose disease in
- Identify the mango bio-type of the pathogen and determine whether other strains of the pathogen from other fruit crops cross-infect mango on the
- Determine the effect of the mango anthracnose disease on the juice quality of the mango fruit
- Develop a protocol for the control of mango anthracnose in
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