Studies on the Antimicrobial Potency of Five Crude Plant Extracts and Chemical Fungicide in in vitro Control of Aspergillus flavus, Causal Agent of White Yam (Dioscorea rotundata) Tuber Rot

Gwa VI1,2* and Akombo RA3

1Department of Crop and Environmental Protection, Federal University of Agriculture, PMB 2373 Makurdi, Nigeria

2Department of Crop Production and Protection, Faculty of Agriculture and Agricultural Technology, Federal University, Dutsin-Ma, PMB 5001, Katsina State, Nigeria

3Department of Crop Production Technology, Akperan Orshi College of Agriculture, Yandev, PMB 181 Gboko, Nigeria

*Corresponding Author:
Iorungwa Gwa
Department of Crop and Environmental Protection
Federal University of Agriculture
PMB 2373 Makurdi, Nigeria
Tel: +234 818 657 0255
E-mail: [email protected]

Received date: November 11, 2016; Accepted date: November 11, 2016 Published date: December 23, 2016

Copyright: © 2016 Gwa VI, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

 
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Abstract

Studies on the antimicrobial potency was carried out on five crude plant extracts (Piper nigrum, Zingiber officinale, Azadirachta indica, Carica papya and Nicotiana tabacum) and a chemical fungicide (mancozeb) using three concentrations of hot aqueous plant extracts (30, 60 and 90 g/l) and mancozeb (4, 8 and 12 g/l). The extracts and chemical were separately amended in potato dextrose agar (PDA) in in vitro control of Aspergillus flavus; causal agent of yam tuber rot. Rotten white yam tubers were collected from farmers’ barns at various locations in the study area and taken to Advanced Plant Pathology Laboratory, Federal University of Agriculture, Makurdi, Nigeria for analyses. A. flavus was isolated and identified from the rotten white yam tubers; pathogenicity test was carried out to confirm the actual organism associated with the rot in yam. The test revealed that A. flavus was able to incite rot in the healthy yam tubers. The results further showed that all the test plants were able to significantly (p<0.05) inhibit the mycelia growth of A. flavus in culture. Zingiber officinale was consistently observed to be more potent irrespective of concentration and duration of incubation; this was closely followed by Piper nigrum while Azadirachta indica was third in effectiveness. The least effective plant extracts were Carica papaya and Nicotiana tabacum, respectively. The synthetic chemical, mancozeb exceedingly gave 100% inhibition of the test fungus at all the levels of concentrations and throughout the period of incubation and was statistically significant with the extracts. The findings have shown the potential of plants in the control of yam tuber rot caused by Aspergillus flavus. The use of plant products will therefore, reduce over dependence on the use of synthetic chemicals by farmers in controlling pathogens of yams as well as reducing the cost of management and environmental pollution.

Keywords

Antimicrobial; Potency; Plant extract; Fungicide; Inhibition; Yam; Aspergillus flavus

Introduction

Yam is an important crop in many parts of the tropical and subtropical regions where it is a major source of staple food for millions of people [1,2]. More than 90% of the global yam production (40 million tons fresh tubers/year) is produced in West Africa with Nigeria accounting for about 35.02 million metric tons [1]. In addition to its nutritional value, yam has considerable social and cultural significance especially among the Tivs in middle belt of Nigeria [3] and the people of South Eastern Nigeria [4]. Yams are affected by array of diseases including tuber rot caused by Aspergillus flavus in storage in yam growing areas of Nigeria.

This rotting is a major factor limiting the postharvest life of yams [5] Tuber rot organisms produce a variety of extracellular enzymes and a host of metabolites that degrade cell wall polymers, resulting in maceration of parenchymatous tissues [6]. Studies conducted in several part of the country have shown that an average of over 25% of the yield is lost annually to diseases and pests in storage [1]. Bonire estimated microbial postharvest losses in yam at 40% [7] while Okigbo and Ikediugwu indicated that between 20% and 39.5% of stored tubers may be lost to rot causing organisms [2]. Studies conducted by Arinze indicated about 50% reduction of the total stored tubers lost to rot organisms within the first 6 months of storage [8]. These make microbial tuber rot an important constraint to yam production. Most of these rots of yam tubers are caused by pathogenic fungi organisms such as Aspergillus flavus, Aspergillus niger, Botryodiplodia theobromae, Fusarium oxysporum, Fusarium solani, Penicillium chrysogenum, Rhizoctonia spp., Penicillium oxalicum, Trichoderma viride and Rhizopus nodosus [9-11]. These disease causing agents reduce both the quantity and quality of yam produced and also make them unappealing to the consumers [12]. Several control measures have been adopted for the control of yam fungal tuber rots. These include use of synthetic chemicals, biological control method and curative method as well as uses of natural plant extracts [12]. Synthetic chemicals such as sodium orthiophenylphenate, borax, captan, thiobendazole, forcelet, benomyl, nordox, bleach (sodium hypochlorite), mancozeb have been found to significantly reduce storage rot in yam [9,13,14]. However, the obvious pollution problems in the environment and the toxic effects of synthetic chemicals on non-target organisms have prompted investigations on exploiting pesticides of plant origin for control of fungal pathogens [15]. These pesticides of plant origin are specific, biodegradable, cheap, readily available and environmentally safe [16]. It is against this backdrop that the study focuses on the antimicrobial potency of some selected plant extracts against the in vitro mycelia growth of Aspergillus flavus isolated from rotten yam tubers in storage.

Material and Methods

Experimental site

The study was carried out at the Advanced Plant Pathology Laboratory, Federal University of Agriculture, Makurdi, Nigeria.

Collection of rotten yam tubers

Rotten yam tubers of white yam varieties (Dioscorea rotundata) showing various disease symptoms of dry rots were obtained from yam farmers from various storage barns in Tor- Donga, Katsina-Ala, local government area of Benue State, Nigeria which lies between longitudes 9°20' and 9°23'E, and latitude 7°17ʹ and 7°20ʹN, respectively. The rotten yam tubers were packaged in sterile polyethylene bags, taken to the laboratory for isolation and identification of pathogens. The samples were protected using wire mesh to prevent rodent attack [17]. Potato Dextrose Agar (PDA) was the medium used.

Isolation of Aspergillus flavus from rotted Yam tubers

The white yam tubers (Dioscorea rotundata) were cut from diseased and healthy parts. The cut pieces were soaked in 5% sodium hypochlorite solution for 2 minutes for surface sterilization. The pieces were then rinsed in four successive changes of sterile distilled water [18]. The yam pieces were placed on sterile paper towels in the laminar Air flow cabinet (Environmental Air control Inc. USA) to dry for 2 minutes.

Inoculation

The infected tissues were later picked onto sterile filter paper using a sterile forceps and then wrapped with filter paper for 2-3 minutes. The dried infected tissues were aseptically plated on Petri dishes containing acidified sterile potato dextrose agar (PDA) and the plates were incubated at ambient room temperature (3°C ± 5°C) for 192 hours.

Characterization and identification

Fungal colonies that grew on the incubated plates were subcultured into fresh separate sterile acidified PDA plates and incubated to obtain pure cultures of pathogens. The purified isolates were kept in slants and stored for characterization and pathogenicity test. Microscopic examination and morphological characteristics were noted and compared with existing authorities [19,20]. Aspergillus flavus which was one of the most frequently isolated organisms was selected as the test fungus.

Pathogenicity test

Healthy yam tubers were washed with running tap water, rinsed in four successive changes of sterile distilled water, Thereafter; the tubers were disinfected with 5% sodium hypochlorite solution for 30 seconds and again rinsed with sterile distilled water. The tubers were allowed to air dry. A flamed 5 mm cork borer was used to bore hole into the healthy white yam tubers [2], a 5 mm diameter disc from the pure culture of Aspergillus flavus was cut and replaced in the holes created in the healthy Dioscorea rotundata tubers..

The same procedure was used for the control except that sterile agar discs were used instead of the inoculum in the holes created in the tubers. Petroleum jelly was used to completely seal the holes [21]. The inoculated yam tubers were placed in three replications at ambient room temperature (30°C ± 5°C) under sterile condition. The plates were incubated for 14 days after which the tubers were examined for infection and disease development. The infected tubers were compared with the initially decayed tubers.

Preparation of plant extracts

Plant parts were prepared according to the methods described by [22] and [23] with some modifications. Seeds of Piper nigrum (Black Pepper), Rhizomes of Zingiber officinale (Ginger), leaves of Azadirachta indica (Neem), leaves of Carica papaya (Pawpaw) and leaves of Nicotiana tabacum (Tobacco) were washed thoroughly with cold running tab water and airdried. The dried parts of each plant species were pounded into powder separately using a sterilized mortar and pestle. Hot water (100°C) extraction was obtained by adding 30 g, 60 g and 90 g of the powder of each plant extracts to 1litre of sterile distilled water separately in 1000 ml Pyrex flask.

These were left for 24 hours and subsequently filtered through four fold of sterile cheese cloth. The filtrates obtained were used as the plant extracts in the experiment. Mancozeb was prepared in sterile distilled water at 4 g/l, 8 g/l and 12 g/l concentrations respectively. The efficacies of the aqueous plant extracts and chemical fungicide were tested in vitro for their fungicidal activity against tuber dry rot of white yam (Dioscorea rotundata) caused by Aspergillus flavus.

In vitro assay of plant extracts against Aspergillus flavus

The method of Amadioha and Obi [24] was used to determine the fungitoxic effect of different plant extracts and chemical fungicide in vitro on mycelia extension of A. flavus by creating four equal sections on each plate. This involves drawing two perpendicular lines at the bottom of the plate. The point of intersection indicates the centre of the plates. These were done before dispensing PDA into each of the plates. Then 15 ml of the prepared medium was poured into sterilized Petri dishes and 5 ml of each plant extracts and chemical fungicide at the different The method of Amadioha and Obi [24] was used to determine the fungitoxic effect of different plant extracts and chemical fungicide in vitro on mycelia extension of A. flavus by creating four equal sections on each plate. This involves drawing two perpendicular lines at the bottom of the plate. The point of intersection indicates the centre of the plates. These were done before dispensing PDA into each of the plates. Then 15 ml of the prepared medium was poured into sterilized Petri dishes and 5 ml of each plant extracts and chemical fungicide at the different levels of concentrations were poured into Petri dishes containing the media separately [25], mixed well and allowed to solidify, the solidified medium was inoculated centrally at the point of intersection of the two perpendicular lines drawn at the bottom of the plate. Five mm diameter mycelia discs retrieved from oneweek- old fresh cultures of test fungus grown on PDA plates served as inoculum [26].

The control experiments had 5 ml of sterile distilled water added to PDA in place of the plant extracts and chemical fungicide, respectively. The inoculated Petri dishes were sealed with masking tape and incubated for 120 hours at ambient room temperature (30°C ± 5°C). Radial growth of A. flavus was then recorded after every 24 hours for up to 120 hours after inoculation by measuring mycelia growth diameters along two diagonal lines previously drawn on the reverse side of each Petri dish to serve as a reference using a transparent ruler. The absence of growth in any of the plates was indicative of the potency of the extract and the chemical fungicide against the test fungus. Fungitoxicity was determined in form of percentage growth inhibition (PGI) according to the method described by Korsten and De Jager [27].

PGI (%)=(R-R1)/R*100

Where,

PGI=Percent Growth Inhibition,

R=the distance (measured in mm) from the point of inoculation to the colony margin in control plate,

R1=the distance of fungal growth from the point of inoculation to the colony margin in treated plate.

Experimental design and data analysis

The design used was Completely Randomized Design (CRD) with three replicates as described by Gomez and Gomez [28]. Test of variance was calculated using Analysis of variance (ANOVA) and statistical F-tests were evaluated at P ≤ 0.05. Differences among treatment means for each measured parameter were further separated using fishers least significance difference (LSD) to determine levels of significance according to Cochran and Cox [29].

Results

The fungus that was isolated from rotten white yam (Dioscorea rotundata) tubers resulting from the sampling survey above was identified as Aspergillus flavus. Colony characteristics growth on PDA was described as orange green in colour surrounded by a clear white zone; growth was rapid covering the entire plate within 120 hours of incubation (Figure 1).

plant-sciences-agricultural-research-Pure-culture-Aspergillus-flavus

Figure 1: Pure culture of Aspergillus flavus growing on Potato Dextrose Agar and Macro conidia borne by phialides arising from a terminal swelling on the conidiophores.

Microscopic examination of the mycelia of the test fungus showed non-septate conidiophores arising from thick-walled foot cells. Each conidiophores ends in a terminal enlarged spherical swellings. Conidia were borne by phialides arising from a terminal swelling on the conidiophores (Figure 1). The result of Microscopic examination of the mycelia of the test fungus showed non-septate conidiophores arising from thick-walled foot cells. Each conidiophores ends in a terminal enlarged spherical swellings. Conidia were borne by phialides arising from a terminal swelling on the conidiophores (Figure 1). The result of the pathogenicity test established the susceptibility of the healthy yam tubers and invasion by Aspergillus flavus. The control experiment however, showed no rot indicating absent of the pathogen (Figure 2).

plant-sciences-agricultural-research-Pathogenicity-test-rot-caused

Figure 2: Pathogenicity test showing rot caused by A. flavus and pathogenicity test control showing no rot.

Results of the study indicated that the concentrations of the tested plant extracts against A. flavus had a positive effect in inhibiting mycelia growth in vitro. The results of A. flavus growth on PDA amended with plant extracts showed that Piper nigrum, Zingiber officinale, Azadirachta indica, Carica papaya and Nicotiana tabacum had antifungal properties against A. flavus at high concentrations (60 g/l and 90 g/l) more than at low concentration (30 g/l) in vitro (Table 1).

Plant Extract Concentration (g/L) Period of Incubation (Hours) LSD
24 48 72 96 120
Piper nigrum Conc I (30) 100.00 ± 0.00a ns79.44 ± 2.42b b62.69 ± 4.17c 48.79 ± 3.52d 54.40 ± 3.29cd 9.60
Conc II (60) 100.00 ± 0.00a ns85.00 ± 7.64ab ab70.56 ± 2.42bc 54.45 ± 6.66d 57.13 ± 2.72ccd 15.17
Conc III (90) 100.00 ± 0.00a ns91.67 ± 8.33ab a81.76 ± 2.94b 66.48 ± 2.94c 63.88 ± 2.68c 13.66
  LSD - 23.10 11.29 16.15 10.07  
Zingiberofficinale Conc I (30) 100.00 ± 0.00a 79.44 ± 2.42b 73.43 ± 5.50bc 64.73 ± 2.95cd 61.15 ± 2.57d 10.11
Conc II (60) 100.00 ± 0.00a 79.44 ± 2.42b 73.89 ± 3.89b 62.79 ± 4.96c 62.60 ± 1.41c 9.72
Conc III (90) 100.00 ± 0.00a 94.44 ± 8.56a 81.30 ± 4.06b 74.44 ± 4.08b 69.01 ± 4.64b 13.03
  LSD - 13.04 15.72 14.12 10.95  
Azadiracta
indica
Conc I (30) ns72.20 ± 14.70a ns65.56 ± 8.68ab ns55.56 ± 5.56ab ns50.87 ± 2.49ab a45.26 ± 0.89b 25.57
Conc II (60) ns100.00 ± 0.00a ns79.44 ± 2.42b ns62.22 ± 6.19c ns56.75 ± 4.29c ab53.12 ± 4.26c 12.65
Conc III (90) ns100.00 ± 0.00a ns86.11 ± 7.35b ns70.09 ± 4.41c ns58.84 ± 2.94c as58.58 ± 1.52c 12.95
  LSD 29.37 23.23 18.80 11.52 9.20  
Carica
papaya
Conc I (30) ns55.56 ± 5.56ns ns52.22 ± 7.78ns ns59.72 ± 5.01ns ns52.71 ± 4.58ns ns48.94 ± 4.91ns 17.91
Conc II (60) ns72.20 ± 14.70ns ns63.30 ± 18.60ns ns59.26 ± 9.26ns ns54.67 ± 6.07ns ns50.39 ± 3.46ns 37.15
Conc III (90) ns83.30 ± 16.70ns ns72.78 ± 6.83ns ns77.22 ± 6.83ns ns62.43 ± 4.95ns ns62.43 ± 2.95ns 28.33
  LSD 45.76 42.45 27.05 18.12 13.36  
Nicotianatabacum Conc I (30) ns83.30 ± 16.70ns ns67.22 ± 4.34ns ns59.72 ± 5.01ns ns54.68 ± 3.32ns ns55.69 ± 4.08ns 26.34
Conc II (60) ns83.30 ± 16.70ns ns67.22 ± 4.34ns ns59.72 ± 5.01ns ns54.68 ± 3.32ns ns57.13 ± 2.72ns 26.34
Conc III (90) ns83.30 ± 16.70ns ns72.78 ± 6.83ns ns63.06 ± 1.94ns ns60.69 ± 2.45ns ns63.88 ± 2.68ns 26.35
  LSD 57.67 18.24 14.67 10.59 13.95  
Mancozeb® Conc I (4) 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 -
Conc II (8) 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 -
Conc III (12) 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 100.00 ± 0.00 -
  LSD - - - - -  
Means on the same row (for each Plant Extract) with the different superscript are statistically significant (p<0.05) by period of incubation.
Means on the same column (for each Plant Extract) with the different subscript are statistically significant (p<0.05) by concentration, ns=not significant.

Table 1 In vitro effect of different filtrate concentrations of some plant extracts and chemical fungicide at different concentrations on Percentage Growth Inhibition of Aspergillus flavus after 120 hours of incubation.

There was however, no significant difference in activity of mancozeb between concentrations I, II and III in reducing the mycelia of A. flavus in culture throughout the period of incubation (Table 1). The results also showed that the duration of incubation has a significant (P ≤ 0.05) effect on percentage growth inhibiton of A. flavus and that the potency of the extracts decreased with increase in the period of incubation (Table 2). The result further showed that Z. officinale, P. nigrum and A. indica continue to be more efficacious than C. papaya and N. tabacum at all the levels of the concentrations despite the duration of incubation.

Plant Extract Period of Incubation (Hours)
24 48 72 96 120
Concentration I          
Azadiractaindica 72.20 ± 14.70ab 65.56 ± 8.68bc 55.56 ± 5.56c 50.87 ± 2.49c 45.26 ± 0.89d
Carica papaya 55.56 ± 5.56b 52.22 ± 7.78c 59.72 ± 5.01bc 52.71 ± 4.58c 48.94 ± 4.94cd
Mancozeb® 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a
Nicotianatabacum 83.30 ± 16.70ab 67.22 ± 4.34bc 59.72 ± 5.01bc 54.68 ± 3.32c 55.69 ± 4.08bc
Piper nigrum 100.00 ± 0.00a 79.44 ± 2.42b 62.69 ± 4.17bc 48.79 ± 3.52c 54.40 ± 3.29bcd
Zingiberofficinale 100.00 ± 0.00a 79.44 ± 2.42b 73.43 ± 5.50b 64.73 ± 2.95b 61.15 ± 2.57b
LSD 28.81 16.22 14.27 9.69 9.66
Concentration II          
Azadiractaindica 100.00 ± 0.00 79.44 ± 2.42ab 62.22 ± 6.19b 56.75 ± 4.29b 53.12 ± 4.26c
Carica papaya 72.20 ± 14.70 63.30 ± 18.60b 59.26 ± 9.26b 54.67 ± 6.07b 50.39 ± 3.46c
Mancozeb® 100.00 ± 0.00 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a
Nicotianatabacum 83.30 ± 16.70 67.22 ± 4.34b 59.26 ± 5.01b 54.68 ± 3.32b 55.69 ± 4.08bc
Piper nigrum 100.00 ± 0.00 85.00 ± 7.64ab 70.56 ± 2.42b 54.45 ± 6.66b 57.13 ± 2.72bc
Zingiberofficinale 100.00 ± 0.00 79.44 ± 2.42ab 73.89 ± 3.89b 62.79 ± 4.96b 62.60 ± 1.41b
LSD 27.95 26.19 16.40 14.62 9.42
Concentration III          
Azadiractaindica 100.00 ± 0.00 86.11 ± 7.35ab 70.09 ± 4.41bc 58.84 ± 2.94c 58.58 ± 1.52c
Carica papaya 83.30 ± 16.70 72.78 ± 6.83b 77.22 ± 6.83b 62.43 ± 4.95c 62.43 ± 2.95bc
Mancozeb® 100.00 ± 0.00 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a 100.00 ± 0.00a
Nicotianatabacum 83.30 ± 16.70 72.78 ± 6.83b 63.06 ± 1.94c 60.69 ± 2.45c 59.70 ± 3.92bc
Piper nigrum 100.00 ± 0.00 91.67 ± 8.33ab 81.76 ± 2.94b 66.48 ± 2.94bc 63.88 ± 2.68bc
Zingiberofficinale 100.00 ± 0.00 94.44 ± 5.56a 81.30 ± 4.06b 74.44 ± 4.08b 69.01 ± 4.64b
LSD 29.65 19.79 12.26 10.10 9.34
Means on the same column (for each concentration) with different superscript are statistically significant (p<0.05). (Conc I=30g/L of Plant extract, 4 g/l of Mancozeb; Conc II=60g/L of Plant extract, 8 g/l of Mancozeb; Conc III=90g/L of Plant extract, 12 g/l of Mancozeb).

Table 2 In vitro Percentage Growth Inhibition of Aspergillus flavus by some plant extracts and chemical fungicide at different concentrations after 120 hours of incubation.

Though, the potency of each plant extract decreased with prolonged incubation period, N. tabacum was considered more potent at concentration I and II more than C. papaya while at concentration III C. papaya had a higher percentage growth inhibition than N. tabacum. The synthetic fungicide, mancozeb which showed the highest percentage growth inhibition irrespective of the concentration was considered more effective in reducing the mycelia growth of A. flavus than the plant extracts and also showed significant difference (P ≤ 0.05) with the plant extracts at all the levels of concentrations and throughout the period of incubation (Tables 2 and 3). Mean percentage growth inhibition of three concentrations (I, II and III) after 120 hours of incubation showed that Z. officinale, P. nigrum and A. indica were more effective in reducing the mycelia growth of A. flavus in culture while the least effective plant extracts were C. papaya and N. tabacum respectively (Figure 3).

Plant Extract Concentrations
Conc I Conc II Conc III
Azadiractaindica 57.89±4.03d 70.31±4.89bc 74.73±4.59bcd
Carica papaya 53.83±2.36d 59.98±4.89c 71.64±4.05cd
Mancozeb® 100.00±0.00a 100.00±0.00a 100.00±0.00a
Nicotianatabacum 64.13±4.24cd 64.13±4.24bc 67.91±3.97d
Piper nigrum 69.06±5.11bc 73.43±4.94b 80.76±4.08bc
Zingiberofficinale 75.75±3.86b 75.74±3.86b 83.84±3.50b
LSD 10.32 11.76 10.41
Means on the same column with the different superscript are statistically significant (p<0.05). (Conc I=30g/l of Plant extract, 4 g/l of Mancozeb; conc II=60g/l of Plant extract, 8 g/l of Mancozeb; Conc=90g/l of Plant extract, 12 g/l of Mancozeb).

Table 3 Mean percentage growth inhibition of Aspergillus flavus by some plant extracts and chemical fungicide at different concentrations after 120 hours of incubation.

plant-sciences-agricultural-research-percentage-growth-inhibition

Figure 3: Mean percentage growth inhibition of three concentrations (I, II and III) of plant extract and mancozeb on the mycelia growth of A. flavus by period of incubation.

Discussion

The study has shown that Aspergillus flavus is a major rot pathogen of yam in Tor-Donga which corroborates the results earlier on reported by Ogunleye and Ayansola [10] and Okigbo et al. [11] in other parts of Nigeria. The study revealed that fungitoxic compounds were present in Z. officinale, P. nigrum, A. indica, C. papaya and N. tabacum since they were able to suppress the growth of A. flavus tested. This agrees with earlier reports by some workers on the effect of these plants on pathogens of some crops [30,31]. Onifade also reported the control of C. lindemuthianum using A. indica seed; leaf, bark and root extract, recording a 100% inhibition of spore germination and mycelial growth [32]. Ejale and Abdullah indicated that the dried leaf powder of A. indica was used to control the postharvest rot of tomato [33]. On the other hand, Oluma and Elaigwe observed that extracts of A. indica had no inbibitory effect on the mycelial growth and sclerotial formation of Macrophomina phaseolina [34]. Biu et al., in his investigation observed the presence of anti-nutrients like saponins, tannins, glycosides, alkaloids, terpenes and flavenoids in the aqueous extracts of the leaves of Azadirachta indica [35]. Hycenth reported the antifungal effect of Azardirachta indica against yam rot pathogens (Rhizopus stolonifer) [36]. Taiga et al. showed that N. tabacum cold extract inhibited the Mycelia of F. oxysporum yam rot organism [22]. Amienyo and Ataga used Z. officinale, Annona muricata, Gacinia cola, Alehornea cordifolia, Allium sativum to control wet rot on sweet potatoes caused by rot fungal pathogens [21]. The antifungal properties of Z. officinalis on Aspergilus flavus, Aspergilus niger, Fusarium solani and Fusarium oxysporum on post-harvest yam (Dioscorea alata, Poir) has been reported by Yeni [37]. Shiva et al. evaluated the antimicrobial activity of piperine against Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, Alternaria alternata, Aspergillus niger, Aspergillus flavus and Fusarium oxysporum and found out that the active ingredient in Piper nigrum has inhibitory effect on these pathogens [31]. Ebele used C. papaya; C. adorata and Acalypha ciliata to control pawpaw fruit rot fungi [38]. Ogwulumba et al. used Carica papaya leaf extracts to control incidence of foliar mycopathogens of groundnut (Arachis hypogea) [39] while Suleiman, inhibited mycelia growth of Alternaria solani, causal organism of yam rot using leaf extracts of Carica papaya [30]. Assessment of the effect of mancozeb on Aspergillus flavus mycelia showed that increase in the concentration of mancozeb and duration of incubation does not affect growth inhibitions as the test fungus had already attained the highest level of inhibition (100%) at the lowest concentration (4 g/l). This result agreed with the report of Emua and Fajola, who found out that mancozeb consistently gave 100% inhibition (at concentrations of 250 ppm, 500 ppm and 1000 ppm) of germination of conidia of Cercospora contraria and Didymosphaeria donacina which caused leaf spot diseases of cluster yam (Dioscorea dumetorum) [40].

On the other hand, Amadioha and Markson observed that increase in the concentration of the chemicals and duration of incubation positively correlated with the growth inhibitions [13,41]. The variation noted in the antimycotic effects of the extracts may be as a result of solubility of the active substances in water or the presence of inhibitor against fungicidal principles. This is in agreement with the investigations of Amadioha [42].

Conclusion

The findings have shown the potential of plants in the control of yam rotting fungus caused by Aspergillus flavus. The result revealed that plant extracts could be an alternative to toxic fungicides for controlling plant pathogens since they are composed of various bioactive compounds such as alkaloids, flavonoids, glycosides, phenols, saponins, sterols, etc. Lakshmeesha et al. [43]. The use of plant products will therefore, reduce over dependence on the use of synthetic chemicals by farmers in controlling yam fungal pathogens as well as reducing cost of management and environmental pollution.

References

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