AETIOLOGY OF THE FUNGAL ROT OF CUCUMBER FRUIT (CUCUMIS SATIVUS L.) AND THE EFFICACY OF ITS CONTROL WITH PHYTOCHEMICALS FROM CRUDE EXTRACTS OF TWO PLANTS

ABSTRACT

The aetiology of the fungal rot of cucumber fruit (Cucumis sativus L.) in Enugu State was investigated. Five fungal pathogens were isolated from diseased cucumber fruits. They were identified as Fusarium oxysporum Schlecht, Aspergillus niger Van Tiegh, Rhizopus stolonifer Ehrenb. ex.Fr, Geotrichum candidum Link and Mucor micheli ex Staint – Amans. The pathogenicity tests proved them as the causal agents of the rot.  The effects of ethanolic extracts of Chromolaena odorata and Moringa oleifera and benlate solution on the isolates were also determined. The ethanolic extracts of the test plants inhibited mycelial growth of the pathogens in vitro as well as in vivo. The inhibitory effects of the extracts increased with increase in concentration. All concentrations of 20 mg/ml to 100 mg/ml reduced the mycelial growth of the pathogens to some significant (P > 0.05) level. Benlate solution also reduced the mycelial growth of the pathogens. Phytochemical analysis of C. odorata revealed the presence of tannins, saponins, phenols, flavonoids, terpenoids, glycosides and alkaloids while M. oleifera contained all the phytochemicals except alkaloids. The presence of these active ingredients in the extracts may have contributed to their inhibitory properties. These findings suggest that plant extracts could serve as potential sources of bio-fungicides.

CHAPTER ONE

INTRODUCTION

Fruits, due to their high moisture content and nutrient composition are very susceptible to attack by pathogenic fungi which in addition to causing rots may also produce mycotoxins which make them unfit for human consumption (Stinson et al., 1981; Philips, 1984; Moss, 2002). Postharvest diseases account for about 50% losses in fruits stored in poor storage conditions especially under high humidity. They pose a major problem to the agricultural industries (Agrios,

2005). Cucumber fruits are among the fruits susceptible to postharvest diseases caused by fungi under poor storage conditions.

Cucumber (Cucumis sativus L) is a member of Cucurbitaceae family (Saltveit, 1997). It is borne on indeterminate, tendril-bearing vines of subtropical and tropical origin (Robinson and Decker-Walters, 1997).  Cucumis sativus, originated from Northern India.  It is a quick growing vine crop that produces mature fruits within 2 months of planting. Healthy vines continue to produce marketable fruits for at least 4 weeks. The plant has large leaves that form a canopy over the fruit.  The fruits are round to oblong or roughly cylindrical, with tapered ends, and may be as long as 60 cm in length and 10 cm in diameter (Miller and Wehner, 1989). Valenzuela et al, (2005) showed that the crop is widely grown because of its nutritional value.

1.1 Horticultural Maturity Indices

Cucumber fruits are harvested at various stages of development but when they are generally immature, at sizes ranging from 5 – 15 cm long before the seeds are fully enlarged and hardened (Tan, 1997). Firmness with external glossiness and formation of jelly-like material around the seeds are indicators of proper harvest maturity (Saltveit, 1997; National Agricultural Research Institute (NARI), 2004). Saltveit and McFeeters (1980) stated that straight, uniformly

cylindrical fruits that are slightly tapered at both ends are of highest quality. Cucumbers contain over 90% water (NARI, 2004).

1.2 Taste of Cucumber

There appears to be variability in the human olfactory response to cucumbers, with the majority of people reporting a mild, almost watery flavour or a light melon taste, while a small but vocal minority having the opinion of a highly repugnant taste, some said almost perfume-like (Miller and Wehner, 1989).

Various approaches have been employed with regard to remove bitterness from cucumbers. Among these, are some very common and popular practices in India which include cutting off the ends of cucumber, sprinkling some salt, and rubbing the now-exposed ends of the cucumber with the cut-off ends until they frothed. Haung (2009) stated that one ought to peel a cucumber from the end that was once attached to the vine, in order not to risk spreading the bitterness throughout the cucumber.

1.3 Uses of Cucumber and it’s nutritional value

Cucumber is one of the most popular salad vegetables. It is served and eaten at home or in restaurants especially in fast food establishments. Fresh cucumbers are sliced and served in salads or as garnishes to add colour to the meal. The young fruits, usually of small-fruited cultivars, are pickled as gherkins, the smallest used for mixed pickles and small to medium sized fruits for dill pickles. Aboloma et al. (2009) stated that the seed kernels are occasionally eaten and yield edible oil. The young leaves are cooked as spinach in Indonesia and Malaysia (Anonymous, 2010). Cucumber gives relief from heart burn, acid stomach, gastritis and even ulcer. Daily consumption of cucumber juice helps control cases of eczema, arthritis and gout. Cucumber has been found to be beneficial for those suffering from lung, stomach and chest

problems (Bates et al., 1990).  Cucumber is an excellent source of silica, which is a trace mineral that contributes to the strength of connective tissue in the human body (Brown, 2008).

1.4 Diseases of Cucumber

Cucumber fruits grown for the production and processing industries are seriously affected by several diseases.  For  example,  powdery mildew  and  scab  cause losses  of  45%  in  field cucumbers, and scab reduces both quality and shelf-life (NARI, 2004). Although, an accurate estimate is difficult to obtain, the annual crop loss is probably between 20 and 30%. In addition, the diseases that affect cucumbers also affect other crops as melons, squashes, gourds, and pumpkins  in  the  cucurbit  family. A healthy  high-quality  crop  can  be  produced  only  from cucumber plants that are free from diseases. Some of the diseases include:

1.4.1 Alternaria Rot

Alternaria rot, caused by the fungus Alternaria alternata, is a common soil-borne pathogen that causes post harvest fruit rot of cucumbers. Fruits which have been stored for extended periods and those damaged by chilling injury are highly susceptible to  Alternaria infection.  Infection  typically  begins  in  injured  areas  or  natural  openings  (lenticels)  in  the pericarp. Symptoms are circular to oval lesions on the cucumber surfaces that are bleached or light brown in colour. The lesions soon become sunken and under humid conditions are rapidly covered by a dark mould, identified as A. alternata (NARI, 2004).

1.4.2 Belly Rot

Belly rot, caused by the soil-borne fungus Rhizoctonia solani, is one of the most common fruit rots of cucumbers. The incidence of belly rot is significantly higher during the rainy season. Fruits initially become infected in the field where they make contact with the soil. Typical symptoms of belly rot include a dark brown water-soaked decay on the side of the fruit in contact

with the soil, followed by a yellowish-brown discoloration of the fruit surface.  If the lesion is allowed to dry (just a few hours) the water-soaked areas turn scabby and cracked (NARI, 2004).

1.4.3 Cottony Leak

Cottony  leak,  caused  by  the  soil-borne  fungus  Pythium,  caused  serious  losses  of cucumber in Guyana. Infection occurs in the field and decay progresses rapidly after harvest. Fruit losses are greater during wet weather, especially following relatively dry weather. Wounds are not necessary for infection of the fruit. Infection occurs where the fruit touches the wet ground. Wet conditions promote infection and decay. The first symptoms of cottony leak are soft, dark green, water-soaked lesions of spots on the fruit surface. Later, as the fungus penetrates the fruit tissue, water oozes out in large quantities. A white, cottony fungal growth completely covers the fruit. The cottony growth may become flattened and matted over the fruit. Decay spreads rapidly during transit and storage, with the formation of nests of mouldy fruits exuding water (Holmes, 2000).

1.4.4 Rhizopus Soft Rot

Rhizopus soft rot, caused by the fungus Rhizopus stolonifer, is a common post harvest cucumber disease. The pathogen becomes established in wounded areas of the fruit or through the cut stem. Initially, yellowish-brown water-soaked spots form with a fairly distinct boundary. The spots are irregular in shape and develop into sunken lesions which rot. Grayish-white masses of mould develop over the wounded area, which eventually turns black. Diseased tissue is soft and very wet. High temperature and humidity and fresh wounds promote disease development (Holmes, 2000).

1.4.5 Anthracnose

Anthracnose, caused by the fungus Colletotrichum orbiculare, is characterized by the formation of numerous dark circular spots on the fruit surface. The spots enlarge to form conspicuous lesions that are sunken and sometimes cracked. Under humid conditions, the lesions are covered by salmon or pink coloured spore masses. Anthracnose is usually confined to the pericarp (NARI, 2014).

1.4.6 Gray Mould

Gray mould, caused by the fungus Botrytis cinerea, survives on plant debris in the soil and is typically found on fruits harvested during wet weather. The fungus usually enters the fruit through the blossom end. It attacks the inner or middle petals of the flower through which it penetrates into the fruit. The infected area is soft, water-soaked, yellowish and later becoming covered with a gray furry mould. The optimum temperature for growth of the fungus is 200C, but substantial rotting can also occur at cooler temperatures (Schuster, 2014).

1.4.7 Blue Mould

Blue mould is caused by various species of Penicillium. Symptoms include multiple small circular to oval water-soaked lesions covered with blue-green spores. The infected fruit have a characteristic musty odour. Some strains of the fungus form restricted lesions, while others spread over much of the cucumber surface and cause a wet collapse. Infection is typically via wounds (NARI, 2004).

Economic losses caused by post-harvest diseases are considerably more than realized. Loss of food calories and nutritional elements, mycotoxins and toxic stress metabolites found in fruits colonized by post-harvest pathogens are emerging as a major dietary problem (Mehrotra and Aggarwal, 2003).

Therefore, the aim and objectives of this study were:

i.         To isolate and identify the fungi associated with post-harvest rot of cucumber fruit. ii.        To determine the frequency of occurrence for each of the isolated pathogens.

iii.       To find out the disease severity in the infected cucumber fruit. iv.        To carry out pathogenicity tests with the fungal isolates.

v.         To carry out phytochemical analysis on Chromolaena odorata and Moringa oleifera

leaf  extracts and  finally

vi.       To evaluate the efficacy of the plant extracts and benlate solution on the isolated pathogens.

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