Background

The cashew apple, also called cashew fruit, is the fleshy part of the cashew fruit that is attached to the cashew nut. The fruits of the numerous cultivated types of cashew are fiery scarlet, orange red, or yellow colour and are sometimes peripheral lobed in appearance (Spooner et al., 2005). The cashew fruit is softer in texture compared to other fruits (Engindeniz, 2008). Cashew is widely consumed. It is eaten on its own, used in recipes, or processed into cashew cheese or cashew butter. Cashew has pulp that can be processed into a sweet, astringent fruit drink or distilled into liquor. The shell of the cashew seed yields derivatives that can be used in many applications from lubricants to paints (FAO, 2015). Nevertheless, cashew fruit is a very perishable fruit with a short shelf life. During storage, cashew is susceptible to post harvest diseases caused by various pathogenic fungi and bacteria (Coursey, 1983). Inflorescence dieback is a serious disease of cashew caused by Gibbertella persicaria, Aspergillus spp, Geotrichum spp well as Botrytis spp. This reduces the fruit bearing of cashew in Nigeria. Over 50 pathogens such as Lasiodiplodia theobromae, Aspergillus niger, Trichoderma viridae and Cephalosporium sp have been reported on cashew throughout the world (Ukeh and Chiejina, 2012). Attack by most organisms follows physical injury or physiological breakdown of the commodity (Mujib et al., 2007). It has been estimated that 30-50% of cashew fruits harvested for human consumption are lost through rough handling during loading and unloading (Mujib et al., 2007). Due to the activities of these pathogens on post-harvest cashew fruits, there is need for postharvest control of the commodity. Fungicides are still the primary means of controlling postharvest diseases on fruits. However, fungicides have lost their effectiveness due to resistance by plant pathogens (Mortuza and IIag, 1999). Besides, concerns about environmental contamination and human health risks associated with fungicide residues periodically led to regulatory reviews and restrictions or cancellations, and export markets are increasingly more sensitive to the use of chemicals for disease control. Therefore, there is need to find methods alternative to conventional fungicides for the control of postharvest pathogens of cashew fruits. The use of selected plant extract was therefore investigated as an alternative control measure against postharvest pathogens of cashew fruits.

1 Materials and Methods

1.1 Source of fruit

Mature, firm and healthy cashew fruits were harvested from an orchard in Igbara Oke (7°24’16.6N and 5°0’46.96E), Ondo State, Nigeria and transported to the Biology Research Laboratory of the Federal University of Technology Akure. Fruits of uniform firmness, size and colour were selected before treatment, the fruits were washed with sunlight detergent and rinsed with clean water, disinfected for 1 minute in 10% sodium hypochlorite and allow to air-dry at room temperature for 15 minutes.

1.2 Isolation of fungi from infected fruits

Isolation of associated fungi from the spoilt cashew fruits was made by cutting out the interface between the healthy and diseased tissue and placing pieces of the affected fruit rind on plates of solidified malt extract agar. The plates were incubated using Gallenkamp incubator (1997 model) at 28±2°C for 5 days. Subcultures of the isolate were prepared by transferring agar cut with distinct mycelium to solidified MEA and incubated at room 28±2°C until pure cultures were obtained. The resulting pure culture was used for morphological characterizations of each isolate.

1.3 Morphological identification of fungal isolates

After incubation, identification of isolates was based on the structural features as seen in the culture plates and microscopic characteristics. Cotton -in- blue lactophenol method was used and examination was done with x 40 objective for the presence and type of hyphae, mycelium whether clear or dark and spore morphology using the text of (Alexopoulous et al., 1996).

1.4 Preparation of cashew leaf extract

Detached cashew leaves were detached from the cashew tree and air dried for 5 days. The dried leaves were pulverized with a blender to a smooth powdery form. Varying concentrations (0.5%, 1%, 1.5%, 2% and 2.5%) of the pulverized cashew leaves were prepared with appropriate volumes of water. Twenty (20 g) of the cashew leaf powder was dissolved in 380 mls of water to make 0.5% concentration of the extract while 40 g of the cashew leaf powder was dissolved in 360 mls of water to make 1% concentration of the extract. Also, 60 g of the plant powder was dissolved in 340 mls of water to make 1.5% concentration of the extract and 80 g of the powder dissolved in 320 mls of water to make 2.0% concentration of the extract while 100 g of the plant powder was dissolved in 300 mls of water to make 2.5% concentration of the extract. Conical mouth was sealed with cotton wool wrapped with aluminium foil. Solutions were kept for 72 hours at room temperature.

1.5 Treatment of cashew fruits with cashew leaf extract

After 72 hours, each prepared concentration of the cashew leaf was filtered using Whatman filter paper with a sterilized glass funnel inside a sterilized conical flask. The pseudo cashew fruit was dipped separately into each concentration of the prepared extract for 1 minute while untreated cashew fruits served as control. Fifteen cashew fruits were dipped in each concentration. The treated fruits were air dried and stored in desiccators at 28±2°C and 45-50% relative humidity which was measured using thermo hygrometer.

1.6 Assessment of disease incidence

Cashew fruits were assessed daily for evidence of fungal infections. Infection was recognized by flavedo discolouration or appearance of mycelium on the fruit surface.

1.7 Phytochemical screening of cashew leaf extract

Aqueous extracts were prepared from the powdered sample according to the method of Trease and Evans (2004) and analysed for various phytochemical constituents.

1.8 Statistical analysis

Data obtained for disease incidence were subjected to Analysis of Variance (ANOVA) and where significant, the means were compared at 5% level of probability using Duncan’s Multiple Range Test (SPSS version 21).

2 Rseults

2.1 Effect of different concentrations of cashew leaf extracts on disease incidence of cashew fruits after treatment

On day 1 of storage after treatment, both the control and all treated cashew fruits irrespective of treating concentrations had no disease incidence (0%) which implied that all the cashew fruits were disease free. The same trend of results was also observed on day two of storage except that the control and fruits treated with 0.5% leaf extract had 45% and 20% as their respective disease incidence (Figure 1). On day 3 of storage, all the cashew fruits treated with 2.0% and 2.5% leaf extract had no disease incidence (0%) which implied that the fruits were disease free. However, control and fruits treated with 0.5%, 1.0% and 1.5% leaf extracts had 80%, 40%, 30% and 30% as their respective disease incidence (Figure 2). On day 4 of storage, only the cashew fruits treated with 2.0% leaf extract had no disease incidence (0%) when compared with other treated fruits and the control except the fruits treated with 0.5%, 1.0%, 1.5% and 2.5% leaf extracts had disease incidence of 100%, 75%, 75%, 45% and 15% respectively while control fruits also had 100% disease incidence (Figure 3).

On day 5 of storage, both the control and all treated cashew fruits had 100% disease incidence except those treated with 2.0% and 2.5% leaf extracts which had 100% and 50% as their respective disease incidence (Figure 4). On day 6 of storage, control and all cashew fruits treated with 0.5%, 0.5%, 1.5% leaf extracts had 100% as their respective disease incidence except fruits treated with 2.0% and 2.5% leaf extracts having 15% and 75% as their respective disease incidence (Figure 5). On day 7 of storage, the control and the cashew fruits treated with 0.5%, 0.5%, 1.5% and 2.5 leaf extract had 100% as their respective disease incidence except the fruits treated with 2.0% leaf extract had 30% as its disease incidence (Figure 6). Cashew leaf extract of 2.0% concentration was adjudged the best among all the treatments investigated.

2.2 Phytochemical constituents of aqueous leaf extracts of cashew (Anarcadium occidentale)

The results of phytochemical constituents of aqueous extract of the cashew leaf showed that saponin, alkaloid, steroid, tannin, cardiac glycoside, flavonoid were present in the cashew leaf extract while phlobatannin was absent (Table 1).

3 Discussions

This study showed that cashew leaf extracts were effective in controlling decay of cashew fruits in storage. All the concentrations used (1%, 1.5%, 2.0% and 2.5%) aside 0.5% had 100% effectiveness against postharvest decay of cashew fruits on day 2 of storage. The antimicrobial activity of extracts obtained from the fruits, pseudofruits, leaves and bark of Anacardium occidentale have been reported (Akinpelu, 2001). However, as storage duration progressed to Day 3, there was a decline in the efficacy of these concentrations except 2% and 2.5% cashew leaf extract that still maintained 100% effectiveness against the spoilage pathogens. Aqueous extract of Anacardium occidentale leaf had been reported to be 5.93%-15.35% effective on mycelial growth of Botryodiplodia theobromae of yam tuber rot after two days of incubation in vitro and there was a decrease in the efficacy as incubation period increased indicating that the efficacy of the active compounds of the plant species were not persistent in culture medium or depreciated after two days of culture (Schmourlo et al., 2005). The same trend of results was obtained on day 4 of storage except that the 2.5% cashew leaf extract had recorded 15% disease incidence while 2% cashew leaf extract still showed 100% efficacy.

However, on day 5 of storage, the same trend of result was obtained except that the 2.5% cashew leaf extract had recorded 50% disease incidence while 2% cashew leaf extract had recorded 10% disease incidence. On day six of storage, there was a decline in the efficacy of all the concentrations except that the 2.5% cashew leaf extract had recorded 75% disease incidence while 2% cashew leaf extract recorded 15% disease incidence. All the concentrations of cashew leaf extract on day 7 had 100% disease incidence except 2% that had disease incidence of 30%.

The antifungal potency of the cashew leaf extract may be as a result of the presence of active water solution antifungal principles associated with the plant. Antimicrobial properties of plant extract had been attributed to the presence of alkaloids and flavonoids (Kubo et al., 2003). Antifungal activity of cashew fruits had been attributed mainly to the presence of flavonoids, tannins, organic acids, alkaloids, saponins, terpenes, and especially to the presence of phenolic compounds, including anacardic acids, cardol and cardanol (Kubo et al., 2003). These observed compounds detected in the aqueous extract of Anacardium occidentale leaves probably enhanced the control of post-harvest decay of the treated cashew fruits.

4 Conclusions

Several workers have reported antifungal activities of different plant species and stressed the importance of plants as possible source of natural fungicides. Results obtained showed that fruits dipped in 2% aqueous extract for 1 minute had extended storage life of 7 days as against the typical shelf life of cashew fruits which is less than 48 hours and thus proved most effective among all the concentrations used.

Author’s contributions

Oladele O.O. carried out the treatments of cashew fruits with the aqueous extract of the cashew leaf, assessment of disease incidence and morphological identification of observed fungal isolates on the diseased cashew fruits while Fatukasi O.I. did the preparation of the cashew extracts, phytochemical screening of the extracts and statistical analysis. All authors read and approved the final manuscript.

Acknowledgements

The authors are grateful to Mrs Adeniran who assisted in the phytochemical screening of the cashew extracts.

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