ABSTRACT

Field, greenhouse and laboratory studies were carried out at the Department of Crop Science, University of Nigeria, Nsukka in order to evaluate the leaf spot disease of eggplant and its management with some botanicals. Field survey of diseased plants was conducted on eggplant farm. Solanum aethiopicum L. plants were sampled on every 1 m distance along the diagonal transects for disease incidence and severity. Pathogen isolation from severely infected leaves was carried out in the laboratory where the diseased leaves were plated on fresh Potato Dextrose Agar. Identification of the isolated fungi was carried out with the aid of identification scheme based on their cultural characteristics. Nursery preparation for raising eggplants used in the green house was also carried out on a sterilized soil. The five most abundant pathogens isolated were inoculated separately on the seedlings of eggplant in five replicates. The experiment was arranged in a completely randomized design. Phytochemical contents of the four plant extracts were determined in the laboratory. An in vitro control of the organism responsible for the leaf spots was carried out using plant extracts at 0.030 g/ml, 0.060 g/ml, and 0.120 g/ml and 0.250 g/ml concentrations. The experimental design was a 9x4 factorial in a completely randomized design (CRD). The data were analyzed using analysis of variance (ANOVA). Means were later separated using Fisher’s least significant difference (F-LSD). Helminthosporium infestans Dur. & Mont, Cladophialophora carrionii Trejos, Aspergillus niger van Tieghem, Rhizopus nigricans Ehrenb and Neurospora africana Huang & Backus were isolated from the diseased eggplant leaves. H. infestans recorded the highest percentage frequency (61.11 %) while A. niger had the lowest percentage frequency (5.56 %). H. infestans was pathogenic to eggplant seedlings. The Koch postulate test confirmed H. infestans as the causal organism of the spots symptoms. The qualitative phytochemical analysis on the test plants revealed the presence of tannin, soluble carbohydrate, hydrogen cyanide, steroids, flavonoids, alkaloids as well as glucosides in all the extracts analyzed. The effect of ethanolic extracts of the test plants significantly (P<0.05) reduced the incidence and severity of the pathogen. Anti-fungal activity of seed extracts of G. kola on H. infestans was highest at 0.0120 g/ml concentrations. G. kola could be used as fungicide to manage leaf spot in eggplant because of its availability and eco-friendliness.

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INTRODUCTION

The African eggplant (Solanum aethiopicum L.), known as garden egg, añara, aubergine in Europe and brinjal in India or guinea squash is one of the important vegetable crops grown worldwide. The name eggplant is derived from the shape of the fruits of some varieties which are white and have the shape of chicken eggs. Eggplant is essentially a warm weather crop which extensively grows in Eastern and Southern Asia, including India, USA, Bangladesh, Pakistan, China, Japan, and the Philippines. It is also popular in Egypt, France, Italy (Tindall, 1983). According to FAO 1994 Production Year Book, the world eggplant production land area was 556,000 ha, and the total production was 8,979,000 metric tons. Gill and Tomar (1991) reported 299,770 ha of eggplant production area in India, and 29,150 ha in Bangladesh in 1992-93. Eggplant can be grown in all parts of Nigeria all the year round. It is grown commercially as an annual crop; it is a short- lived perennial branching herb with a height of 0.5-1.5m. The fruit can be eaten in various forms without the need for an elaborate preparation. It is eaten raw, cooked or used to season other foods. Eggplant supplements starchy foods. It is also a cheap source of protein, minerals and vitamins (Lombin and Yayock, 1988). The tender green leaves of some species are also used as vegetables or eaten raw in African salads, ugba. It can be eaten as appetizer or offered to visitors as desert (kola).

The African eggplant (Solanum aethiopicum L.) is affected by several fungal diseases which inflict heavy losses in its production. One of such fungal disease is the leaf spot disease. Severely infected leaves drop off prematurely resulting in the reduction of yield. Due to environmental concerns, great emphasis has been laid on alternative measures other than chemicals, to control this fungal disease. The use of botanicals and antimicrobial agents of plant origin is a time –honored practice for control of plant diseases and pests. The necessity to develop a non-toxic, safe and biodegradable alternative to synthetic fungicides has in recent years led to a concerted effort at developing new control measures from plant parts.

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The humid especially the rainforest ecological zones are endowed with abundant flora of families of plants and herbs with untapped pesticides potentials (Amadioha, 2003, 2004). Stoll (2000) listed an array of plant families and genera possessing antimicrobial properties, amongst which were Azadirachta indica, Zingiber officinale, Garcinia kola, Carica papaya, Gongronema latifolium and host of others. Amadioha (2003), Kumar and Pamar (1996) and Prakash and Roa (1997) listed some of the advantages of plant extracts over synthetic chemicals to include possession of low mammalian toxicity, minimal health hazards and environmental pollution. There is practically no reported risk of developing pest resistance to these products when used in their natural forms. Also, no side effect on plant growth, seed viability or food quality has been reported. Botanicals are less expensive and easily available because of their natural occurrence. Synthetic fungicides are expensive and inaccessible to indigenous farmers who are the bulk producers of eggplant in Nigeria (Amadioha, 1998; Onuegbu et al., 2001). A natural plant product with fungicidal properties could be more environmental friendly than synthetic fungicides.

Aqueous extracts of some plants have been used in laboratory bioassays (John and James, 2004). These plants include Allium cepa (onion), a biennial herb of Liliaceae family used commonly as spice for flavoring food. Allium sativum L. (garlic), another biennial herb of Liliaceae family and the second most widely use Allium after A. cepa; it is used as condiments for flavoring foods. Stoll (1998) reported the bactericidal properties of Azadirachta indica A Juss (neem), a fast growing tree of the family Meliaceae and also a medicinal plant with insecticidal, nematicidal, antifungal and bactericidal properties. It occupies a foremost status among all the plants exploited so far for bio-efficacy against pests and diseases (Kumar and Pamar, 1996). The primary antimicrobial constituents are Azadirachtin A and B. In addition, Neem contains a good number of other chemical substances which include Salannin, Meliantriol, Azadirachtannin A, Cinnamoyl,

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Isoazadirohide, Nimbin/Nimbidin, which seem to have anti- viral effects as well as Vilasinim as isolated from the leaf and Azadirone from the seed. Garcinia kola Henkel (bitter cola) is a perennial tree in the family Guttiferae with whorled leathery leaves. The seeds are chewed as stimulants and for other various medicinal values. Traditionally, the seeds are believed to repel snakes. Zingiber officinale Rose (Ginger) is rhizome of the family Zingiberaceae. The rhizome yields essential oil, oleoresin, consisting 1-3% volatile of which serve as the active ingredient against microorganisms and pests (Benjilali et al., 1984).

Medicinal plant materials have been successfully used for the treatment of fungal and bacterial infections in humans (Akinyosoye and Oladummoye, 2000), suggesting that some plant materials may also possess antifungal and antibacterial constituents which are useful in controlling plant diseases (Amadioha, 1998). Previous reports (Akpomedaye and Ejechi, 1998; Ejechi and Ilondu, 1999; Ejechi and Akpomedaye, 1999) show that spices, herbs and other plant materials possess antifungal activity. Akinyosoye and Oladunmoye (2000) have reported the antifungal efficacy of stem and leaf-extracts of Mirabilis jalapa L. in reducing mycelia growth of four different strains of fungi. The legendary medicinal qualities of the neem tree have been known for a long time and the aqueous leaf extract have systemic action (Egunjobi and Onoyemi, 1981; Sowunmi and Akinusi, 1983). The toxic effects of some plant extracts on fungal activities is an indication that such plants could be used as fungicides especially by the peasant farmers who cannot afford the costly synthetic agrochemicals to control fungal diseases that attack their crops.

This research work was therefore aimed at studying the leaf spot disease of African eggplant and its management with extracts of A. indica leaves, Z. officinale stem, C. papaya leaves and G. kola seeds and some synthetic fungicides.