THE OCCURRENCE AND BIOCONTROL OF THE POSTHARVEST FUNGI RESPONSIBLE FOR SWEET POTATO SPOILAGE
CHAPTER ONE INTRODUCTION
Background of the Study
Sweet potato (Ipomoea batatas) is a tuberous rooted perennial crop that is usually grown as annual crop. It originated from Central America (Ecocrop, 2010) and is widely grown as an important staple food in most parts of the world. There are over 403 varieties of sweet potato of which the flesh can be white, yellow, red, purple, pink, violet and orange or brown while the skin colour varies among yellow, red, orange and brown (Ecocrop, 2010). The crop has great food and health values. Many parts of the plant including the leaves, root and vines are edible. The roots are widely used as carbohydrate food; the tender leaves commonly eaten by man while the leafy stems are fed to livestock (Woolfer, 1992). Beside these, the crop has been noted to provide surprising health benefits including fighting cancer, diabetes, vitamin A deficiency, and inflammation; preventing arteriosclerosis, heart disease, depression, emphysema, arthritis, stroke, muscle cramp and stomach ulcers; reducing arthritis and inflammation and curing bronchitis and stomach ulcers (Alum et al., 2013).
The population of Nigeria relies on sweet potato as a food security crop (Adeyonu et al., 2016). Unfortunately, sweet potato roots are susceptible to many microbial infections at different stages including field, harvest and storage (if they are not properly harvested and stored) and marketing stages. This type of spoilage commonly associated with sweet potatoes is a major constraint to the potential of sweet potato as food and health security crop (Echerenwa and Umechuruba, 2004). This result in many detrimental effects including deterioration in food quality
characteristics, great loss in storage roots, unavailability of food produce during off-season and a
waste of farm inputs and scarce resources such as water. It also saps human effort and investments and adversely affects the people’s economic access to crop produce. Microbial spoilage also compromises food (sweet potato) safety; posing a serious health concern (Walsh et al., 2004; Jain et al., 2011; Esnakula et al., 2013; Esnakula et al., 2013 and Georgiadou et al., 2014) .
Due to the negative economic importance of fungal pathogens, control strategies are needed. Several postharvest pathogen control methods used include fungicide treatment, gamma irradiation and hydro-warming. These methods, though reported to have intermediate impacts in controlling spoilage and enhancing shelf life of sweet potato tubers (Ray and Ravi, 2005), have some drawbacks including unavailability to Nigerian farmers, unfriendly to environmental sustainability, phytotoxic to man and a great propensity to trigger resistance in the targeted pathogens (Okigbo and Nmeka, 2005). Given these drawbacks associated with the orthodox fungi and rot control approaches, focus hasin recent times, shifted toward exploitation of plant extracts as novel fungicides in plant protection (Okigbo and Nmeka, 2005; Okigbo and Omodamiro, 2006). Many botanicals have been extensively researched on and proved to possess antimicrobial properties; hence myriads of reports have been documented stating the uses of plant extracts to control plant diseases. Some plants tested for antimicrobial properties include Chromolena odorata (Siam weed), Ocimum gratissimum (wild basil), Moringa oleifera (moringa) and Zingiber officinale (Ginger) (Okigbo and Nmeka, 2005; Okigbo et al., 2009a).
Statement of the Problem
Microbial pathogens affecting crops tend to vary in occurrence and distribution depending on the environment, crop physiology, harvesting and storage, thus the incidence of postharvest rot disease, the frequency of occurrence of different pathogens and their importance as primary pathogens of decay may change with reference to location. Therefore, to develop an effective disease
control programme for the sweet potato sector, it is important to know the fungal pathogens responsible for the disease within a location. Moreover, the development of potent and drawback-free decay control measures is critical to prevention of microbial spoilage and reduction of food losses to microbial attack. In spite of these recognitions, the occurrence and control of fungi associated with sweet potato spoilage in Ebonyi State notable for sweet potato production in South-Eastern Nigeria seem not to have received attention; yet there is apparently a steady increase in post-harvest microbial spoilage of sweet potato observed and reported by some farmers in the area.
Attention to postharvest rot control has been focused on the use of single plant extracts with antimicrobial activities that were always far less potent than those of synthetic chemicals employed as treatment checks in several investigations. Moreover, the antimicrobial activity of plant extracts that is observed in in-vitro conditions is quite different from its effect in complex food systems. In most cases antimicrobial activity is decreased due to interactions with food components. This could be a challenge in utilizing plant antimicrobials, as a higher concentration could result in unfavorable changes to the taste and aroma of food (Havelaar et al., 2010). Combinations of extracts can lead to additive or synergistic effects on postharvest pathogens. Despite these recognitions, literature in Nigeria still lacks sufficient data on the potency of combined plant extracts against microbial spoilage pathogens for use in sweet potato preservation.
Justification for the Study
The developing nations of the world have always been in short supply of food. Around 1 billion people are being faced by severe hunger in these nations of which 10% actually die from hunger-related complications. This problem is further compounded by the accelerated increase in human population, which creates pressure on every form of food supply (Urom, 2014). Today, one of the main global challenges is how to ensure food security for a world growing population whilst
ensuring long-term sustainable development. According to the Food and Agricultural Organization, food production will need to grow by 70% to feed world population which will reach 9.3 billion by 2050. Worse still, in the meantime, while the number of food insecure population remains unacceptably high (FAO, 2010), each year and worldwide, massive quantities of food including sweet potatoes are lost due to spoilage and infestations from farm to folk. This problem arises due to inadequate agricultural storage and microbe-induced spoilages (Kana et al., 2012).
Studies that will generate baseline information on the occurrence of postharvest spoilage fungi of sweet potato in Ebonyi state and a potent drawback-free strategy for controlling the rot pathogens are critical to reducing sweet potato yield losses at postharvest. The potential benefits of reducing postharvest losses of sweet potato to mycodeterioration are large. It is critical to alleviation of poverty while reducing pressure on ecosystems, climate and water. It is also a strategy for contributing to food security enhancement and closing the food gap between food available today and food needed in 2050 to adequately feed the planet’s projected 9.3 billion people.
1.4. Aim of the Study:
The study was aimed at investigating the occurrence and biocontrol of the postharvest fungi responsible for sweet potato spoilage in Ebonyi State
The objectives of the study were to:
1. isolate and characterize the fungi associated with sweet potato rots from Ebonyi State,
2. determine the seasonal percentage occurrence and pathogenicity of the isolated fungi,
3. determine the severity of the pathogenic fungi and susceptibility of Ebonyi farmers’ most commonly grown sweet potato cultivars to rot,
4. evaluate the effect of fungi infection on the nutritional quality of sweet potato cultivars most commonly grown by Ebonyi farmers,
5. determine the in vitro antimicrobial activity of single and combined extracts of Zingiber officinale (ginger), Garcinia kola (bitter kola), Allium sativum (garlic) and Moringa oleifera (moringa) on the fungal pathogens
6. determine the in vivo antimicrobial activity of selected plant extract combinations using sweet potatoes as a food model
CHAPTER FIVE DISCUSSION
The present study investigated the occurrence and biocontrol of postharvest fungi responsible for sweet potato spoilage in Ebonyi State, South Eastern Nigeria. In this study, 352 fungal isolates were obtained from 200 rotted sweet potato roots, suggesting that postharvest rots of sweet potatoes in Ebonyi State occur together as a complex rot involving many fungi. Results indicated that postharvest rots were more prevalent in the dry season than in the wet season. Such evidence of decreased percentage occurrence of isolated fungi during the rainy season indicates an improved environmental status, less favourable to fungi proliferation.
Furthermore, results showed that five genera (Aspergillus, Fusarium, Botryodipladea, Rhizopus and Penicillium) and seven species (A. niger, A. flavus, A. awamori, F. solani, B. theobromae, R. oryzae and Penicillium expansum) were the postharvest fungi responsible for the spoilage of sweet potato in Ebonyi State. A similar study in Anambra State by Agu et al. (2015) examined fungi associated with the post-harvest loss of sweet potato using a total of ten tubers obtained from Eke-Awka market, Awka South Local Government Area, Anambra State. The spoilage molds they identified were three species: Aspergillus fumigatus, Aspergillus niger and Rhizopus stolonifer. Though Anambra and Ebonyi States are both in South East of Nigeria, the reason for the variation in occurrence of up to seven different species of fungi in the present study and those (3 species) of Agu et al (2015) may be due to several factors such as sample size and sampling location. A different report buttressing this point can be seen with the findings of Amienyo and Ataga (2007) who analyzed sweet potato samples collected from different markets
in Port Harcourt and identified six fungi comprising four of the fungi genera (Fusarium, Rhizopus, Aspergillus and Botryodiplodea) and species (Aspergillus flavus, Aspergillus niger, Botryodiplodia theobromae, Fusarium solani). In a similar vein, four of the genera of fungi (Fusarium, Rhizopus, Aspergillus and Penicillium) reported in this work were also implicated sweet potato rot by Salami (2007) though with different species composition with the exception of A. niger (Fusarium roselens, Rhizopus stolonifer, Aspergillus fumigatus, Penicillium, and Aspergillus niger)
Botryodplodia theobromae was the mostprevalent fungi (99 in number with percentage mean of 28.45%) isolated from the rotten sweet potato rootsin this study. This finding is similar to observations made in Portharcot, Rivers State by Amienyo and Ataga (2007). Out of six storage fungi (B. theobromae, R. stolonifer, A. niger, A. flavus, F. oxysporum and F. solani) that were isolated from the rot infested sweet potato roots, the authors reported Botryodplodia theobromae as most abundant fungi. Rhizopus oryzae was the second most frequently isolated fungi (78 in number with mean percentage of 22.56 %) from sweet potato rot in both seasons. Agu et al (2015) reported Rhizopus spp (though R. stolonifer) as one of the most frequently isolated fungus from spoilt sweet potato tubers in Anambra State. Similar scenario was found between two studies done by Amienyo and Ataga (2006) and Salami (2007) where Amionye reported that Rhizopus Spp (R stolonifer) was the most frequently isolated fungus from spoilt sweet potato tubers in South western, Nigeria. Soft, Rot, caused by Rhizopus stolonifer (Ehr. ex Fr.) Lind and several other species of Rhizopus, principally affects edible roots. Soft rot probably is widely distributed where ever sweet potatoes are grown, but apparently causes greater losses in more temperate areas (Brooke et al., 2003). The pathogen is soil and air borne and harvested sweet potato roots usually get contaminated with fungal spores, development of which requires
wounds for penetration and establishment of the fungus. Therefore, control measures are based on prevention of wounding sweet potatoes (to avoid creating portals of entry for the fungus), and proper curing of roots before storage. Aspergillus niger was the third most isolated fungi (70 in number for both seasons, mean frequency of 35 with percentage mean of 19.68%) from the rotted sweet potato roots. This lends credence to report by Person et al. (2010) that possessing the ability to grow on a wide variety of substances. A. niger is a common contaminant of food, soil and indoor environment. A. Flavus was the fourth most isolated fungi (46 in number with percentage mean of 13.03%) from the decaying sweet potato roots. Oyewole (2006) too reported
A. flavus as one of the fungi associated with postharvest fungal rots. F. solani was the fifth most isolated fungi (39 in number with percentage mean of 10.99%). Amienyo and Ataga (2007) also isolated F. solani from rotting sweet potatoes. In addition, F. solani was also reported to have been recovered from corn (Nur et al., 2011). Penicillium expansum was the sixth most isolated fungi in this present study. This is in keeping with the finding by Oladoye et al. (2016). In their study titled Biomolecular characterization, identification, enzyme activities of molds and physiological changes in sweet potatoes (Ipomea batatas) stored under controlled atmospheric conditions, the authors isolated Penicillium expansum from sweet potato and implicated it with causation of sweet potato rot. Our findings regarding fungi responsible for rot causation in sweet potato is also in accord with that of Onuegbu (2002) who also implicated Penicillium species, Aspergillus niger and Aspergillus flavus as fungi responsible for decay of sweet potato roots. Aspergillus awamori was the seventh most isolated fungi in this study.
Several other fungi (Monilochaetes infuscans, Fusarium oxysporum, Ceratocysts fimbriata, Rhizopus stolonifer, Macrophomina phaseolina, Diaporthe batatalis, Mortierella ramanniana, Mucor pusillus, Botrytis cinerea and Erysiphe polygoni) implicated with Sweet
potato rotting by other investigators (Oyewole, 2006) were not isolated in this study. This buttressed the report that complexity of postharvest fungi occurrence vary from place to place. High moisture content of sweet potato makes storage difficult and roots vulnerable to microbial attacks, resulting in high losses (Agu et al., 2015). Several other reports (Kihurani, 2004; Eleazu and Ironua, 2013) also have it that moisture content and nutritional compositions of sweet potato roots make the roots susceptible to infection by fungi.
A test of pathogenicity confirmed that all the isolated fungi (Aspergillus flavus, Aspergillus niger, Fusarium solani, Rhizopus oryzae, Penicillium expansum, Botrydiplodiae theobromae and Aspergillus awamori) were responsible for rot induction in sweet potatoes evidenced by typical rot symptoms shown by roots artificially inoculated with the test fungi within 7 days of incubation and the fact that the isolates on re-isolation, exhibited morphological characteristics and growth patterns similar to those earlier observed on axenic cultures. The isolated fungi have been found to incite different categories of rot, with dry rot being the most prevalent amongst java rot caused by B. theobromae, soft rot caused by R. oryzae and dry rot observed with root infection by A. niger, A. flavus, A. awamori, F. solani and P. expansum. Though the mechanisms of fungi action were not experimentally elucidated in the present study, many researchers have discussed the mechanisms of some of the fungi implicated with rot causation in Ebonyi Satae the present study. In a study conducted by Oladoye et al. (2016) tittled “Biomolecular characterization, identification, enzyme activities of molds and physiological changes in sweet potatoes (Ipomea batatas) stored under controlled atmospheric conditions”, fungi were isolated from both the surface peels and the deep tissue cuts of stored sweet potatoes by which virtue the authors proffered the suggestion that the spoilage fungi, once they successfully colonize the surface of the tuber, that the infections can easily proceed into the deep
tissue to cause tissue spoilage. Several reports show that the Phytopathogenic microorganisms are assisted by the enzymes they secrete. Ray (2004) has also reported the production of extracellular hydrolytic enzymes and in particular, cellulolytic and pectinolytic enzymes capable of breaking down storage tubers and that the cellulase degrades cell walls during pathogenesis and inhibition of this enzyme ultimately affects the disease development.
Findings on fungi severity showed that the rot severity exhibited by the fungi ranged from very moderate severity to very high severity. All fungal pathogens were more severe in the dry season than in the wet season. R. oryzae exhibited the highest percentage severity of rot in both cultivars and seasons while P. expansum displayed the lowest severity. The highest percentage severity recorded by R. oryzae in this study lends credence to report by Clark et al. (2009) that soft caused by Rhizopus species rot is internationally considered one of sweet potato’s most important postharvest diseases. As reported by Scot (2009), pectolytic and other enzymes produced by Rhizopus spp quickly cause host discoloration and liquefy host tissues. Uncured sweet potatoes are more prone to damage by all the postharvest fungi than are cured sweet potatoes. Therefore there is the need to educate the farmers on the need for adequate and proper curing of their sweet potato produce before storage.
The relative susceptibility of two most commonly grown sweet potato cultivars by Ebonyi farmers was determined and results revealed that the twosweet potato varieties known as ‘Tupiaochi’ and ‘Oyorima’ were susceptible to the phytopathogens prevalent in the area, cultivars showing a significant range of responses. Sweet potato cultivar Tupiaochi was considered more susceptible to fungal infection in the dry season than in the rainy season. Several factors can affect root susceptibility to infection. Sweet potato root susceptibility to some fungal diseases may change during storage and with wound type. Sweet potatoes stored for
long periods of time after harvest are more susceptible to rot, especially soft rot disease. Other causes and implications of variation in susceptibility to pathogenic infections have been suggested and discussed by several authors (Laine et al., 2011 and Arash et al., 2013). According to May and Anderson reported by Arash et al. (2013), variation in susceptibility to pathogenic infections is a function of the host genetic structure. Arash et al. (2013) added that constant genotype-by-environment interactions can influence the degree of disease expression in host plants; they gave examples of the environmental variables including, but not limited to, ambient incubation temperature, host nutrient status and host age. The cultivar variability in susceptibility to the postharvest fungi exhibited by the tested sweet potato cultivars in this study also showed that it is possible to pursue host resistance as a means of controlling postharvest rot in some sweet potato cultivars. It will be rational to infer that because of their susceptibility and consequent poor storability, most farmers sell their produce just after harvest to avoid losses, resulting in low income or reduced profits; a practice that also affects farmers’ food security particularly in the lean season. The results show that all the cultivars were very susceptible to infection by both pathogens in the first trial (dry season) than in the second trial (wet season). This might be due to differences in the prevailing temperature during incubation. During the first trial, the ambient temperature ranged from 22 to 280C and from 20 to 250C in the second trial, thus the temperature during the first trial may have provided a more favourable environment for pathogen activity, thus enhancing infection.
The nutritional composition of the sweet potato cultivars commonly grown by Ebonyi farmers - Tupiaochi and Oyorima were moisture (water), crude fibre, protein, ash, fat and carbohydrate. The moisture content of any food is an index of its water activity and is used as a measure of the stability and susceptibility to microbial contamination..