Plant secondary metabolites as source of postharvest disease management : An overview

Postharvest losses of stored products are enormous due to fungal deteriorations. Although, there are a number of synthetic fungicides available in the market for checking these deteriorations, they have several side effects such as high toxicity, long degradation periods, their residues in food chain, chronic poisoning through the continuous intake of small quantities, development of new races of pathogens, non-biodegradable nature and exhaustible source. Therefore, we must search the new sources of pesticides, which in addition to their efficiency, must be safe and selective to target specific pathogen. Plants are very rich sources of bioactive chemicals such as phenolics, polyphenols, quinones, flavones, flavonoids, flavonols, tannins, cumarins, terpenoids, lectins and polypeptides. Some plants yield fraction of essential oils, which have inhibitory effects on microorganisms. They are highly enriched with terpenoids. They are volatile, biodegradable, eco-friendly and are easily available in local environment. Several studies have been conducted on the use of such botanicals for controlling postharvest diseases, and hence, the present communication reviews the work done on investigating the fungitoxic potential of essential oils and extracts from higher plants in fungal deterioration of stored products.


INTRODUCTION
It is estimated that between 60 to 80% of all grains produced in the tropics is stored by farmers themselves.For small farmers, the main purpose of storing grains is to ensure household food supplies.It also provides a form of saving to cover future cash needs through sale or for barter or gift exchange.Small quantities of grains are also stored for seed purpose.Farmers who produce surplus may also store grains for sale later to take advantage of seasonal price rise.
Traditional storage systems must provide maximum protection against deterioration of commodity by inclement weather and pests.Farm storage systems have been developed to satisfy these requirements.Most of them are well adapted to their environment and losses are generally low, often below 5% of grain weight over a storage season (Tyler and Boxall, 1984).However, for resource, poor farmers even losses of this magnitude have important implications for food security.In addition to storage of food grains at farmer's level, thegovernments of different countries also procure and store them for reasons of food security for its growing population.There was spiral increase in populations of several countries including India.In case of India, it has crossed one billion marks in 2000A.D. and it will give India the dubious distinction of being the most populated country in the world by 2025A.D. Therefore, the challenge of feeding ever growing population shall be a very difficult task.We cannot meet such a challenge with the increase in food production alone but their protection from deterioration caused by fungi and other pests during storage have to be given due emphasis (Shukla, 1997).
In developing countries, the greatest losses during

DETERIORATION IN STORED PRODUCTS BY PESTS
There are several storage fungal pests that cause deteriorations in stored commodities.The most common among them are -Aspergillus flavus, A. niger, A. clavatus, A. terreus, A. versicolor, A. candidus (Shukla, 1997;Shukla et al., 2000;Pandey, 2008;Shukla, 2010).Several of these fungal pests such as species of Aspergillus, Penicillium, Fusarium, Alternaria and Cladosporium etc. not only bring about deterioration in the quality and quantity of agricultural produce in storage and transit but they also create health hazards in animals and human beings by producing toxic metabolites in the form of mycotoxins in the stored commodities (Samson et al., 1995;Shaaya et al., 1997;Pandey, 2008).
These organisms are capable of growing under diverse conditions of moisture, pH and temperature.If the mould growth occurs, there is always the concomitant possibility of mycotoxin production (Zohri and Abdel-Gawad, 1993).Mycotoxins are dangerous chemicals that cause several complications in the body.They are carcinogenic, hepatoxic, nephrotoxic and teratogenic (Samson et al., 1995;Pandey, 2008).Extreme toxicity of mycotoxins lies in the fact that they are extremely stable and dangerous in minute quantities.Further, once formed, they cannot be removed from the commodity concerned by processing or removal of visible mould growth.They are heat stable, so they cannot be destroyed by cooking.Since mycotoxins are extremely toxic, regulatory and industry guidelines limit are set at very low levels.In developing countries, often the good quality products are exported while substandard produce unacceptable to foreign buyers (because they exceed regulatory limits for mycotoxin content) is sold to the domestic market (Dawson, 1991).Therefore, the mycotoxin contamination of food and feeds is not a particular problem to the developed world, although heavy economic costs are incurred in ensuring low concentrations of mycotoxins (Mannon and Johanson, 1985).In poorer developing countries, such contaminations have more serious consequences, affecting agricultural economies, reducing annual production and good quality exports and seriously affecting the health of the population.Therefore, the control measures for checking deterioration and mycotoxin production should be such that which occur naturally in the local environment; less toxic to environment, animal and human being and cost effective.

MANAGEMENT OF DETERIORATION CAUSED BY PESTS AND MYCOTOXIN PRODUCTION
To control fungal deterioration of agricultural produce, many organic and inorganic fungicides have been developed and used.The use of many of these has, however, been restricted due to their undesirable side effects such as a high and acute toxicity, the long degradation periods, their concentration in food chain, the suspected dangers of chronic poisoning through the continuous intake of small quantities (Samson et al., 1995;Kumar et al., 2007).Besides, due to development of new races of pathogens, many of these fungicides are gradually becoming out of date (Dikshit, 1980).
As such the development of new effective and harmless fungicides is needed on an increasing scale.According to Brandes (1967) much of our efforts are being wasted in routine testing of the standard fungicides, when there is a pressing need to investigate new sources of effective fungicides (Brandies, 1967).
Furthermore, the sources of these synthetic fungicides are largely petrochemicals which are exhaustible.Therefore, haunt for inexhaustible sources of such chemotherapeutants is highly desirable.Green plants appear to be the reservoir of effective chemotherapeutants and can provide reversible source of useful pesticides (Swaminathan, 1978).Tropical floras, in contrast to their temperate zone counterparts, have developed a more efficient and varied defense mechanism because of the far severe conditions for survival.
They, thus provide a rich and intriguing source for isolating natural secondary plant metabolites, which exhibit interesting antimicrobial properties.Although only some 15,000 secondary plant metabolites have been chemically identified, their total number may exceed 4,000,000 (Saxena, 1993).
They are vast cornucopia of defense chemicals.Recent reports on the possibility of use of higher plants and their constituents have indicated their usefulness in providing fungicides, which are largely non-phytotoxic, more systemic and easily biodegradable (Fawcett and Spencer, 1969;Beye, 1978).They are sustainable and can be continuously propagated year-after-year and do not have any negative impact on the environment as long as care is taken to avoid the propagation of plants from foreign ecosystems which might, therefore, become established as weeds.
Further, where plants are used as storage protectants, they are almost always applied to control insect pests.This is reflected in volumes of research directed to identifying insecticidal or insect repellent plants and plant extracts.Nevertheless, some work has been undertaken to determine whether plants can control storage fungi.Most workers have investigated the properties of spices as inhibiting agents of mycelial growth of Aspergillus species and of its toxin production.
These include Ricinus communis, Arnebia nobilis and Nicotiana plumbaginifolia (Bigrami et al., 1980).Other fungi, such as, Fusarium solani, F. phaseoli and Verticillium albo-atrum, have been shown to be susceptible to tannins extracted from bark of various trees, including chestnut and wattle (Lewis and Papavizas, 1967).In addition to above, several other fungi have been shown to be susceptible to essential oils extracted from higher plants (angiosperms and gymnosperms) (Table 1).

DISCUSSION
Several hundred-research papers are published each year on the antimicrobial activity or other functional activity of botanicals from higher plants, and a complete review of all of them is beyond the scope of this article.The most excellent ones are given above.A brief discussion on efficacy and application of antimicrobial botanicals from higher plants is given below.The antimicrobial botanicals which have the potential to be used as storage protectants can be divided into several useful categories, including phenolics, polyphenols, quinones, flavones, flavonoids, flavonols, tannins, cumarins, terpenoids, lectins and polypeptides (Cowan, 1999).Many herbs, such as thyme, contain multiple active compounds which represent different chemical families.The essential oil fraction of botanicals is often the inhibitoriest chemical fraction to growth and survival of microorganisms.Essential oils are highly enriched with terpenoids.Examples of herbs and spices containing terpenoids which have been shown to have antimicrobial activity include allspice, basil, bay, burdock, cinnamon, Shukla 3 paprika, chilli pepper, clove, eucalyptus, dill, gotu kola, grape fruit seed extract, horseradish, lemon verbena, oregano, paod' arco, papaya, peppermint, rosemary, savory, sweet flag, tansy, tarragon, thyme, turmeric, valerian and willow (Duke,1985;Cate, 2000).The other major chemical group found in plants which has been frequently reported to have antimicrobial and antifungal activity is the sulfoxide/ isothiocyanate family, which includes onion, garlic, mustard and members of the Brassica family.Approximately 30% of essential oils which have been examined are inhibitory to bacteria, and more than 60% of essential oil derivatives have been shown to be inhibitory to fungi (Cowan, 1999;Chaurasi and Vyas, 1977;Shaaya et al., 1997;Shukla et al., 2000;Kumar et al., 2007;Pandey, 2008).The mechanism of action for the antimicrobial activity of botanical storage protectants is not fully understood.However, terpenoids and phenolics are thought to exert inhibitory action against microorganisms by membrane disruption (Cichewick and Thorpe, 1996;Lambert et al., 2001;Schultes, 1978).Simple phenols and flavonoids appear to inhibit growth by binding to biochemicals essential for metabolism (Peres et al., 1997).Both coumarins and alkaloids are thought to inhibit growth of microorganisms at the genetic level (Hoult and Paya, 1996;Rahman and Chaudhary, 1995;Shukla, 2010).
Although numerous studies have been done in vivo to evaluate the antimicrobial activity of botanicals, only a few studies have been done with stored products for preventing or controlling mould growth.Inhibition of fungal growth on coriander and fennel seeds dressed with 0.5% concentration of Cedrus oil has been reported (Dikshit, 1980).Seeds of coriander showed good result when fumigated with essential oil from Citrus media and Ocimum canum at their MIC (Dubey et al., 1993).In vivo, application Cymbopogon citratus oil showed that growth of Aspergillus flavus was greatly checked (Mishra and Dubey, 1994).Seeds of wheat and groundnut fumigated with oil from Eucalyptus citriodora showed excellent result (Shukla, 1997).Oil and leaf powder of Cymbopogon citratus significantly reduced deterioration and aflatoxin production in shelled melon seeds inoculated with toxigenic Aspergillus flavus, A. niger, A. tamarii and Penicillium citrinum.Use of Trachyspermum ammi oil inhibits growth of dominant storage fungi such as spp. of Aspergillus, Penicillium, Alternaria etc. in vivo condition (Shahi et al., 2002;Shukla, 2010).
These studies show that some botanicals have the potential to be effective storage protectant although product development to optimize functionality and flavour will be challenging.More studies are needed on applications of botanicals from higher plants in storage protection to fully understand how best to optimize their use.
Use of many plants in storage protection is commonly Generally Recognized As Safe (GRAS) but some plants  All the oils showed wide range of activity (except Campherized oil) and were more active than some synthetic fungicides Singh et al. (1980) The volatile fractions of leaves of 131 species of higher plants were screened.The oil of Peperomia pellucida was found to exhibit the strongest fungitoxicity against Helminthosporium oryzae.
The MIC of the P. pellucida oil against Helminthosporium oryzae was 2000 ppm at which it showed broad fungitoxic spectrum, quick in killing activity,heavy inoculum density themostable, nonphytotoxic, non systemic, and self life up to 150 days.It also prevents the appearance of disease during preliminary in vivo testing Singh (1980) Different parts of 15 angiospermic plants were screened.The volatile oil extracted from the rhizomes of Alpinia galanga showed the highest fungitoxicity.

Oil
showed highest fungitoxicity against Helminthosporium oryzae.The MIC was 0.4% of the medium.The oil was as fungitoxic as quintozene and Zeneb and gave more inhibition of H. oryzae than dinocap and Copper oxychloride.The oil was also fungitoxic against Alternaria alternata, Aspergillus flavus, A. fumigatus, A. niger and Pestilotia spp.Tripathi (1980) Essential oils and extracts from seeds of Putranjiva roxburghi.
Oil was effective against broad spectrum of storage fungal pests.It was thermostable and remained toxic for at least 150 days Saxena (1980) Oil from leaves of Ocimum canum The oil at 3000 ppm exhibited broad range of activity inhibiting all the 31 fungi tested  The oil of Putranjiva roxburghii exhibited the greatest toxicity The oil was found to be fungicidal and thermostable against A. flavus and A.niger, at its minimum inhibitory concentration (MIC) of 400 ppm Tripathi and Kumar (2007) The essential oils of oregano (Origanum vulgare), thyme (Thymus vulgaris) and clove (Syzygium aromaticum) Oregano essential oil showed the highest inhibition of mold growth, followed by clove and thyme.A. flavus was more sensitive to thyme essential oil than A. niger.Clove essential oil was a stronger inhibitor against A. niger than against A. flavus.

Viuda et al. (2007)
The essential oil of Citrus medica L.
The oil exhibited a wide spectrum of fungitoxicity, inhibiting all 14 fungus species of Arachis hypogea Pandey (2008) Table 1.Contd.
The essential oil of Cymbopogon flexuosus, Trachyspermum ammi and their active constituents Oil of C. flexuosus and its major constituents Citral 38% and Geraniol 24.56% as well as oil of T. ammi and its constituents Thymol 80.7%, ρ-cymene 11.4% and αpinene 7.9% were found effective against A. flavus and Penecillium italicum Shukla (2010) contain noxious compounds, which may render them unsafe for both animals and humans to consume.Toxicology axiom."The dose makes the poison" also apply in some cases.It means that a substance that is safely consumed in the diet at low levels may be unsafe if consumed at a higher level in the diet.Therefore, the data demonstrating that the botanical is safe when consumed at the higher level is needed.And it is virtually difficult to find typical toxicological data such as Acceptable Daily Intake and No Effect Level.Further, here may be unusual sensitivities of some parts of the population to specific herbal compounds or strong aromatic ingredients.Therefore, while using botanicals in storage protection, these points should be duly emphasized to avoid negative nutritional or health consequences.

Conclusion
A small number of antimicrobial agents have been used for many years with little expansion, and there is a real need to expand the list of storage protectants which can be used to ensure safety and quality of stored products.These systems may have synergistic or additive uses with one another or may also be used with conventional antimicrobial compounds.The future of naturally occuring antimicrobial system seems be sure, as new storage protection systems are being rapidly developed and used in a variety of storage products.

Table 1 .
Effect of essential oils and extracts on stored pests and diseases.

species and plant part used for extraction
Nigella sativa, leaves of Psidium guajava and galls of Thuja orientalis All the oils except Carum bulbocastanum and Psidium guajava were found active against nine fungi and six bacteria Antifungal activity of the oil was investigated against Curvularia lunata, Rhizopus spp., Aspergillus spp., and Penicillium spp.Oil from M. arvensis inhibited the growth of all fungi.Oils from P. nigrum and A. triplinerve were inactive against A. fumigatus and P. decumbens Chaturvedi (1979)f Adinocalymma allicea Effective against Helminthosporium oryzae at 500 ppm, killed 12 fungi out of 21 tested and proved to be nonphytotoxic to host; and much more active than someChaturvedi (1979)

Table 1 .
Contd.Oil at 1000 ppm showed complete inhibition of Penicillium italicum.The oil of E. dalarympleana and E. laveopinea showed fungistatic activity against the test fungus at 3000 ppm; but the oil of E. laveopinea showed partial inhibition at 3000 ppm.The oil of E. citriodora at 1000 ppm exhibited fungicidal nature and withstood heavy inoculum Essential oils from leaves of Melaleuca alternifolia and Monarda citriodora var.citriodora.

Table 1 .
Contd.Among the products that evidenced the antimould activity citral and eugenol showed the lowest minimum inhibitory concentration which was 1% and 4% respectively.The mould strains assayed are Fusarium spp.Rhizopus spp.Aspergillus flavus, A. niger and Penicillium spp.