Plants are the sources of natural pesticides that make excellent leads for new pesticide development (Bobbarala et al., 2009).
Plant extracts have played significant roles in the inhibition of seed-borne pathogens, in the improvement of seed quality and field emergence of plant seeds. Contrary to the synthetic drugs, antimicrobials of plant origin are not associated with many side effects and have an enormous, therapeutic potential to heal many infectious diseases (Patel et al., 2007).
Plants are rich in a wide variety of secondary metabolites such as, essential oils, tannins, terpenoides, alkaloids, flavonoid, saponins and phenolic compounds which have been found in vitro to have antimicrobial properties extracts of many plants are now known and exhibit antimicrobial activity (Dewanje et al., 2007 and Rani et al., 2008). Although a plant based pesticides are cheap, locally available, non-toxic and easily biodegradable, limited efforts have been made to screen plants that are suspected to possess antimicrobial properties for effect against pathogenic microorganisms. Higher plants may contain secondary compounds that could effectively control plant diseases, but which are yet to be exploited and used as pesticides. Although there is a growing interest in the use of medicinal plants to control plant diseases, only about 2,400 plant species among more than 250,000 higher plants have been screened for phytoactivity (Nduagu et al., 2008). In recent years, antifungal agents such as plant-based essential oils and extracts were focused on attention to control phytopathogens in agriculture. Historically, many plant oils and extracts have been reported to have antimicrobial properties (Joseph et al., 2008). It is important to investigate scientifically those plants which have been used in traditional medicines as potential sources of novel antimicrobial compounds.
The resurgence of interest in natural control of plant pathogens and increasing consumer demand for effective, safe, natural products mean that quantitative data on plant oils and extracts are required (Hadizadeh, et al., 2009). Ethanol and water extracts of Hypericum triquetrifolium, Vitex agnus castus, Eryngium billardieri, Chrozophors tinctoria, Heliotropium circinatum and Prosopis fracta were more effective against Ascochyta rabiei and Fusarium oxysporum fungi compared to other plant extracts (Boskani, 2008). The biological activity of total extract of twelve Iraqi wild plants such as Hypericum triquetrifolium, Cleome quinquenerria, Euphorbia tinctoria were tested against two plant pathogenic fungi, Pythium aphanidermatum and Altarnaria alternata. The test showed a growth inhibition effect of some plant extracts especially extract of H. triquetrifolium against the two fungi (Mahmoud, 1985 and Al-Anni et al., 2003).
Alcoholic extract of six wild plants Vitex agnus – castus, Cyperus rotundus, Hypericum triquetrifolium, Quercus aegilops, Quercus infectoria and Cupressus sempervirens, also the oils of both flowers of V. agnus-castus and the tubers of Cyprus rotundus were found to have antifungal activities against the growth of Aspergillus niger (Hamawandi, 2006). The essential oils of Cupresses sempervirens, Eucalyptus citriodoraund and Taxodium disticum have been found to be more effective against Aspergillus sp, Penicillium sp, Fusarium sp. and Mucor sp, (Al -Fazairy et al., 2004).
Trichoderma spp. as bio-control agent
The success of Trichoderma spp. as a bio-control agent is believed to involve various modes of action, including antibiotic production, secretion of lytic-enzymes, mycoparasitism, competition for space and nutrients, and induction of systemic resistance (Cortes et al., 1998; Rocco and Perez, 2001 and Harman et al., 2004).
Number of Trichoderma harzianum isolates produces a wide variety of fungal cell wall degrading enzymes, such as pectinases, cellulases, chitinase, and proteases. Chitin and 1,3-glucan are the main skeletal polysaccharides (Jayalakshmi et al., 2009). Several reports have demonstrated positive relationships between the production of cellulase, chitinase and protease and the ability to control plant diseases, these enzymes are induced in Trichoderma during the parasitic interaction and can inhibit the growth of several fungal plant pathogens by degrading cell walls (Saber et al., 2009).
The basic role of Trichoderma is ascribed to fast mycelial growth, sporulation and the production of antibiotics and extracellular enzymes (Benitez et al., 2004 and Papvizas, 1985).
T. harzianum is considered as a saprophytic fungus, it’s habitate in the soil which is generally used practically as bio-agent against a wide range of soil borne plant pathogens (Papavizas, 1985), also it is known for it’s potentially effectiveness (Papavizas, 1992). However, some studies have also shown that it can stimulate the growth of a number of vegetable and bedding plant crops (Baker, 1989). Lewis et al., 1998 pointed to the antagonistic activity of Trichoderma spp. to F.solani and R.solani.
Application of fungicides
Fungicides have been used widely to control these pathogens in vitro (Reuveni, 2006) and in vivo (Errampalli, 2004). Fungicide bavastin inhibited conidial germination and sporulation of Fusarium oxysporum (El-Abyad et al., 1983), as well as fungal growth (Sarhan et al., 1999). Vitavax200B increased germination of wheat seeds and reduced seedling infection by Cochliobolus sativus (Sharma-Poudyal et al., 2005). Benomyl, captafol, imazalil, thiram have been used for controlling of Fusarium crown and root rot on tomatoes leaving residues in fruit tissues (Marois and Mitchell, 1981and Jarvis, 1988). Also application of methylbromide and chloropicrin reduced Fusarium crown and root rot of tomato (Mc Govern and Vavrina, 1998).
It has been found that vitavax (caboxin) – thiram and vitavax – captan achieved good disease control ( El-Shami et. al. 1993). Also, it was demonstrated that thiram and topsin – M were effective in reducing F. oxysporum f.sp. lycopersici population 83.4% when used at a rate of 800 meg/g soil (Dwivedi et al., 1995).