**3. Plants contain naturally fungicides**

Plants possess a range of tools for combating fungal infections. Defence mechanisms include physical barriers and chemical defence includes many naturally occurring compounds and systems to prevent microbial attacks by using several types of pre-formed compounds and induced response plant defence compounds.

Plants have defensive response by means of some formed or pre-formed substances belong to the commonly called plant secondary compounds (secondary metabolism). Secondary metabolism is known as all those compounds that they are not involved in primary metabolic processes such as respiration and photosynthesis. However, many compounds do have roles intermediate between primary and secondary metabolism. For instance, some plant secondary compounds like plant growth substances are associated with plant growth and development (Seigler, 1998). In fact, secondary metabolites are compounds with a restricted occurrence in taxonomic groups that are not necessary for a cell (organism) to live with its environment, ensuring the survival of the organism in its ecosystem (Verpoorte, 2000).

There has been much speculation over many years about why organisms take the trouble to produce secondary metabolites or natural products which often have considerable structural complexity. All such structures serve the producing organisms by improving their survival fitness (Williams et al., 1989). In this case, it seems likely that the fungi and bacteria which produce strobilurins, oudemansins, myxothiazols and/or melithiazols, which are substances with antifungal properties synthesized by fungi or bacteria give themselves an advantage by doing so, presumably by suppressing other organisms which compete for nutrients in the same environment (Williams et al., 1989).

Secondary metabolites are now thought to mediate chemical defence mechanisms by providing chemical barriers against animal and microbial predators. Plants produce numerous chemicals for defence and communication, and can elicit their own form of offensive chemical warfare by targeting the proliferation of pathogens. These chemicals may have general or specific activity against key target sites in bacteria, fungi and viruses.

These substances are classified into different ways: based on chemical characteristics, plant origin or biosynthetic origin. From a chemical point of view, the compounds can be divided into a number of groups based on typical characteristics such as alkaloids and phenolic compounds, principally, Based on biosynthetic origin, there are terpenoids, phenylpropanoids, polyketides and other compounds belong to other minor groups must be considered. Therefore, the total number of these compounds is very elevated. A great number of those are used to prevent animal consumption. However, many of these defensive substances are toxic for the plants themselves and they are inactivated as glycosides or by polymeration or they are confined in intercellular spaces.

Natural Fungicides Obtained from Plants 7

in those substances and they have been used in foods as flavouring agents (Davidson, 1997). Most of these compounds are terpenes and have fungicide properties are described below. Plant extracts can be directly used or substances responsible for the antimicrobial properties can be isolated. A new extract with potent antifungal properties is the extract obtained from *Aloe vera*, which has been found to have antifungal activity against four common postharvest pathogens: *Penicillium digitatum*, *P*. *expansum*, *Botrytis cinerea* and *Alternaria* 

Among the compounds belonging to plant extracts, some important examples have been selected and explained below. They can be classified in other groups, mainly as phenolic

Few studies have focused on the mechanism by which plant extracts and their essential oils inhibit microorganisms. The terpenes present of the essential oils are the primary antimicrobials. Many of the most active terpenes, for example, eugenol, thymol and carvacrol, are phenolic in nature. Therefore, it would seem reasonable that their modes of action might be related to those of other phenolic compounds. Essential oils may inhibit enzyme systems in yeasts, including those involved in energy production in cells and

Eugenol [2-methoxy-4-(2-propenyl)-phenol] is a substance obtained from clove (*Syzygium aromaticum* L.), which contain 95% of eugenol as the main volatile oils. Cinnamic aldehyde [3-phenyl-2-propenal] is another antimicrobial substance obtained from cinnamon (*Cinnamomum zeylanicum* J Presl.). This plant also contains eugenol (only 8% of volatile oil and 75% of the first one). Both eugenol and cinnamon inhibits spores of *Bacillus anthracis*, while eugenol and aqueous clove infusions inhibited outgrowth of germinated spores of *B*. *subtilis* in nutrient agar (Davidson, 1997). In addition, clove extracts has antifungal properties. Thus, it inhibited growth initiation for over 21 days at 25ºC of *Aspergillus* and *Penicillium* species. Cinnamon was the next most effective spice, inhibiting three *Penicillium*

The genus *Allium* contains various compounds with resistance to fungal disease. Some are constitutive inhibitors, such as the phenolic compounds catechol that is present in the outer bulb layers of pigmented *Allium cepa* (onion) cultivars were it confers resistance to *Colletotrichum* (Link & Walker, 1933). Onion also produces a class of cyclopentane phytoalexin upon pathogen infection, designated tsibulins (Dmitriev et al., 1990), which accumulate in bulb scales at infection sites during incompatible interactions with *B*. *cinerea*. Tsibulins inhibited spore germination and germ tube elongation of *B*. *cinerea in vitro*. The ED50 values were lower than the actual phytoalexin content in bulb scale spots, were *B*. *cinerea* lesion formation was restricted (Dmitriev et al., 1990). Only little accumulation of cyclopentane phytoalexins was observed in onion bulb scales during infection by the specialized pathogen *B*. *allii* (Dmitriev et al., 1990). *B*. *allii* seems to either suppress tsibulin accumulation; analogous to the interaction of *B*. *narcissicola* and its host narcissus of actively

Isothiocyanates derive from glucosinolates in cells of plants of the Cruciferae, or mustard family (cabbage, kohirabi, Brussels sprout, cauliflower, broccoli, kale, horseradish, mustard, turnips, rutabaga) are potent antifungal and antimicrobial agents (Davidson, 1997). These compounds are formed from the action of the enzyme myrosinase (thioglucoside glucohydrolase; EC 3.2.3.1) on the glucosinolates when plant tissues are injured or

degrade tsibulins, as discussed for other antifungal compounds above.

*alternata* (Barkai-Golan, 2001).

compounds or essential oils.

synthesis of their structural components.

species for over 21 days (Davidson, 1997).
