Phytochemical Content and In Vitro Antimycelial Efficacy of Colocasia esculenta (L), Manihot esculenta (Crantz) and Dioscorea rotundata (Poir) Leaf Extracts on Aspergillus niger and Botryodiplodia theobromae

The in vitro efficacy of aqueous leaf extracts of three common root crops (Colocasia esculenta, Manihot esculenta and Dioscorea rotundata) were investigated against Aspergillus niger and Botryodiplodia theobromae. The pathogenic fungi were isolated from rotten tubers of yam using antibiotic-amended potatoes dextrose agar. Graded aqueous leaf extract concentrations (25 g/L, 50 g/L and 75 g/L) of each plant were applied against each test fungi on PDA plates and their mycelial growth inhibition were monitored at 28 ± 2 C for 144 hrs. Camazeb (Alderelm, UK), a commercial fungicide, at concentrations of 5 and 10 g/L was used as positive control agent. The phytochemical constituents of each extract were also analyzed using standard techniques. The antifungal activities of the extracts were concentration dependents. Colocasia esculenta extracts (75 g/L) demonstrated the most significant (P < 0.05) antifungal activities with mycelial growth inhibition of 92.2 % and 85.9 % against A. niger and B. theobromae, respectively. These effects were followed by the activities of D. rotundata extracts with 82.8 % and 80.9 % inhibitions, respectively. Though, M. esculenta leaf extracts generally demonstrated the least antifungal activities against both pathogens, their activities were significant at 75 g/L. The antifungal potency of both D. rotundata and C. esculenta were comparable to Camabez which demonstrated 100 % inhibition at both 5 and 10 g/L concentrations. Higher concentrations of saponins (5.64 6.71 mg/100g), tannins (4.42 – 7.23 mg/100g) and flavonoids (3.64 – 5.25 mg/100g) were detected in the extracts, and could probably be linked to the observed antifungal activities. Findings from this study suggest that D. esculenta, C. esculenta and M. esculenta leaf extracts posses bioactive molecules that could be exploited in the control of phytopathogens of tubers crops, which are the main staple food in most tropical countries. Introduction Plants serve as the major sources of food for man and other animals. One of the vital challenges to food security in developing countries is colossal loss of agricultural produce due to inadequate storage facilities and microbial-induced wastages [1]. Microbes, particularly fungal infections, have been found to be the leading spoilage pathogens of preand post-harvest crops because of their abilities to produce diverse hydrolytic enzymes and mycotoxins [1]. Therefore, the control of fungal pathogens of economic crops is crucial for sustainable food production and distribution. Over the years, conventional cultural control practices and use of fungicides have been exploited by the government and private organizations in the management of phytopathogens. Currently, some potent fungicides are still be used for the management of field and post-harvest rot diseases of crops with great successes [2, 3]. Though relatively effective and quick at actions, the major drawbacks with some of the chemicals agents have been issues of risks of nonbiodegradability, bioaccumulation, biomagnifications of the chemical residues along the food chains, making them environmentally unfriendly agents. Besides, the frequent and indiscriminate application of these fungicides has led to the emergence of more resistance strains of crop Journal of Horticulture and Plant Research Submitted: 2017-11-26 ISSN: 2624-814X, Vol. 1, pp 9-18 Revised: 2017-12-11 doi:10.18052/www.scipress. com/JHPR.1. 9 Accepted: 2018-02-07 CC BY 4.0. Published by SciPress Ltd, Switzerland, 2018 Online: 2018-03-12 This paper is an open access paper published under the terms and conditions of the Creative Commons Attribution license (CC BY) (https://creativecommons.org/licenses/by/4.0) pathogens [4]. To address the issues, researchers, have advocated the shift of attention to natural, biodegradable plants materials with potent phytochemical molecules [5]. Natural agents, such as plants and their by-products, are currently being considered as possible future alternative based on the outcomes of recent reports on their potent bioactive constituents against public health pathogens of plants, animals and humans [6-9]. Hence, there is need for environmentalists to intensify the search for more plants with antimicrobial properties against phytopathogens of economic significance. Colocasia esculenta (L.) Schott (Family: Araceae) is a perennial herbaceous plants that is commonly cultivated for its edible starchy roots (corm) and leaves in tropical and sub-tropical countries [10]. Previous reports have shown that the plant possess essential minerals, vitamins and medicinal properties [10-12]. Manihot esculenta (Crantz) (Family: Euphorbiaceae) is an annual, woody shrub that is massively grown in crop in tropical and subtropical regions for its edible starchy tuberous root. The leaves also serve as vegetable in most part of the county. Apart from the nutritional benefits of this plant, their leaves, seed oil, tuber peels have been reported to possess medicinal properties against diverse human and animal ailments of microbial origins [13, 14]. Dioscorea rotundata (L) [Poir] is an annual climbing herbaceous plant with cylindrical stem and dark glossy green leaves [15]. They belong to the family Dioscoreaceae and are native to African countries, where they serve as major sources of carbohydrate, minerals, vitamins and dietary fibres [16]. Various species of Dioscorea plants have been exploited as ancient medicinal plant for the management of diverse human ailments ranging from superficial to deep-seated infections [15, 17]. Considering the profound nutraceutical and pharmacological potentials exhibited by C. esculenta, D. rotundata and M. esculenta plants, it is pertinent to investigate their antimicrobial potentials against common phytopathogenic pathogens of African staple crops. Consequently, the leaf of the three tuberous vegetables (Colocasia esculenta, Manihot esculenta and Dioscorea rotundata) were investigated for their phytochemical contents and antifungal efficacy against two (Aspergillus niger and Botryodiplodia theobromae) of the most versatile post-harvest spoilage pathogens of economic crops in tropical countries, with a view to providing additional information on potential natural phytochemicals needed for the management of these pathogens. Materials and Methods Source of plant materials Fresh leaves of Colocasia esculenta (cocoyam), Manihot esculenta (cassava) and Dioscorea rotundata (white yam) (Fig. 1) were sourced from farm lands within Amai town, Ukwuani Local Government Area of Delta State, Nigeria. Their leaves are manually plucked and transported in black polyethylene bag to the laboratory for further processing. Discorea rotundata (Poir) Manihot esculenta (Crantz) Colocosia esculenta (L.) Figure 1. Leaves of the three selected medicinal plants 10 JHPR Volume 1