The following points highlight the top eight management processes of post-harvest diseases. The management processes are: 1. Pre-Existing Defence Mechanism 2. Induced Defence Mechanism 3. Disease Resistance 4. Means for Sustaining Harvested Produce 5. Biological Control 6. Chemical Control 7. Natural Chemical Compounds 8. Other Safe Compounds.
Management Process # 1. Pre-Existing Defence Mechanism:
(i) Cuticle as a Barrier against Invasion:
The cuticle provides physical barrier to pathogen penetration and also act as a chemical barrier by releasing antagonistic substances which either toxic to pathogen or inhibit the growth of pathogen in the plants. Wounding and treatments that disrupt or dissolve the cuticle result in more rapid infection by various pathogens.
The cuticle thickness has been correlated with the resistance of tomato fruits to Botrytis cinerea or peach cultivars to Monilinia fructicola. Elad and Evenson (1995) described several ways by which a thicker cuticle may enhance host resistance i.e. by providing mechanical resistance to cracking, penetration, inhibition of spore germination and infection process of wound pathogens.
(ii) Inhibitors of Cell Wall Degrading Enzymes:
Pathogen development within the host is correlated in many cases with the activity of cell wall degrading enzymes which are responsible for cell death and liberation of nutrients become available to the pathogen. Liberation of nutrients results in stimulation of pathogen growth and accelerated disease development.
This may explain the great importance attributed to compounds that are capable of suppressing or preventing enzymatic activity. Experiments with various pathogens have emphasized that sugars in the culture medium serves as available nutrients for pathogen and stimulate its growth may inhibit pectolytic and cellulolytic enzyme production and activity.
It was found that the presence of glucose in the culture medium, either as a sole source of carbon or in addition of malic or citric acid, inhibited pectinlyase activity of Penicillium expansum while resulting in enhanced fungal growth.
(iii) Preformed Inhibitory Compounds:
Preformed compounds are produced in response to ingress of pathogens may occur constitutively preformed or passively and their appearance is considered as part of an active defence response. Preformed resistance regarded as constitutive antimicrobial barriers, involves the presence of biologically active and low molecular weight compounds in healthy tissues which provides protection against infections.
Phenolic compounds contribute to resistance through their antimicrobial properties which elicit direct effects on the pathogen or by affecting pathogenicity factors of the pathogen.
They also enhance resistance by contributing to the healing of wounds through lignifications of cell walls around wound zones. Evidence strongly suggests that esterification of phenols to cell-wall materials is a common aspect in expression of resistance.
Phenolic compounds have long been implicated in disease resistance in many horticultural crops. The antifungal properties of phenolic compounds and their derivatives are frequently found in young fruit at concentrations higher than in the ripe fruit show the way to the hypothesis that these compounds play an important role in the maintenance of resistance in unripe fruit.
The concentrations of these phenols decline as fruits mature with a corresponding increase in fruit susceptibility to Post-harvest diseases pathogens. In vitro assays have shown that the phenolic compounds like chlorogenic acid and ferulic acid directly inhibited Fusarium oxysporum and Sclerotinia sclerotiorium, respectively.
The principal phenols in the peach fruit epidermis and subtending cell layers are chlorogenic and caffeic acids and their level is higher in resistant peach cultivars than susceptible genotypes. Several antifungal compounds viz., citral, limetin, 5-geranoxy-7-methoxycoumarin and isopimpenellin have been isolated and identified from lemon peel in concentrations sufficient to inhibit decay development.
The presence of antifungal compounds mainly citral in young lemon fruits may elucidate that citrus fruits in their developmental stages on the trees are resistant to decay, in spite of their constant contact with Penicillium spores and the injuries that inevitably occur in the grove and provide points for penetration.
In addition, several other natural compounds have been identified like tannins in young banana fruits, benzylisothiocyanate in unripe papaya fruits, saponin, tomatidine and tomatine in green tomatoes, monoene and diene from unripe avocado fruit are additional examples of in-fruit toxic compounds.
Since the concentration of these compounds decreases with fruit ripening, it has been considered that they have a role in resistance to decay in young fruits.
Management Process # 2. Induced Defence Mechanism:
(i) Phytoalexins:
Phytoalexins are low molecular weight toxic compounds produced by healthy cells adjacent to localized damaged and necrotic cells in response to initial infection of microorganisms or to an attempt of infection. In other words, in order to overcome an attack by the pathogen, the host is induced by the pathogen to produce antifungal compounds that would prevent pathogen development.
The accumulation of phytoalexins does not only depend on infection. Such compounds may be elicited by fungal, bacterial or viral metabolites, mechanical damage, plant constituents released after injury, by a wide diversity of chemical compounds or by low temperature, irradiation and other stress conditions.
Phytoalexins are thus considered to be general stress response compounds significant in disease resistance, produced after biotic or abiotic stress. Potato tubers could be protected against disease caused by a compatible race of Phytophthora infestans on inoculation with an incompatible race.
Rishitin has also been found to be induced in potato tubers after 24 hour inoculation with Fusarium sambucinum which causes dry rot in stored potatoes.
Several phytoalexin compounds such as umbelliferone, scopoletin and esculetin are produced in sweet potato roots infected by the fungus Ceratocystis fimbriata and it was noted that these compounds accumulate more rapidly in roots resistant to fungus than in susceptible roots.
Benzoic acid is a phytoalexin produced in apples as a result of infection by Nectria galligena and other pathogens. The elicitor of benzoic acid synthesis was found to be a protease produced by the pathogens.
Benzoic acid has proved to be toxic molecule expressed only at low pH values in unripe apples where the initial development of the fungus was certainly halted. Benzoic acid derivatives have been shown to be the best inhibitors of some major Post-harvest pathogens such as Alternaria spp., Botrytis cinerea, Penicillium digitatum, Sclerotinia sclerotiorium and Fusarium oxysporum.
(ii) Pathogenesis-Related Proteins:
Pathogenesis related proteins known as PR proteins are diverse group of plant protein, toxic to invading pathogens. They are present in plants in trace amount but produced in much greater concentration following pathogen attack or stress.
The better known PR proteins are PR1 protein, B- 1,3-glucanases, chitinases, lysozymes, PR4 proteins, cysteine or glycine rich protein, proteinase inhibitors, proteinases, chitosanases and peroxidises.
Glucanohydrolases such as chitinases and B-1,3-glucanases are stimulated in response to elicitors produced by infections, considered to play a major role in constitutive and inducible resistance against pathogens.
They diffuse towards cell wall and break downs the chitin supported structure of pathogenic fungi, whereas lysozymes degrade the glucosamine and muramic acid component of bacterial cell wall. The glucanases are abundant and widely distributed in plant species able to catalyze endo-type hydrolytic cleavage of glucosidic linkages in B-1, 3-glucans.
Chitosan, a B-1,4-glucosamine polymer found as a natural constituent in cell walls of many fungi capable of both directly interfering with fungal growth and eliciting defence mechanisms in the plant tissue. Peroxidases are another group of PR proteins whose activity also has been correlated with plant resistance against pathogens.
Plant peroxidases are glycoproteins that catalyze the oxidation by peroxide of many organic and inorganic substrates have been implicated in a wide range of physiological processes such as ethylene biosynthesis, auxin metabolism, respiration, lignin formation, suberization, growth and senescence.
Correlations between deposition of cell wall strengthening materials such as lignin, suberin and extensin, and peroxidase activities are consistent with a role in defence mechanism through wall-strengthening processes.
(iii) Active Oxygen:
Active oxygen produced by plant cells during interactions with potential pathogens in response to elicitors involved in pathogenesis. Active oxygen species including superoxide, hydrogen peroxide and hydroxyl radical can affect many cellular processes involved in plant-pathogen interactions.
They are induced in affected cells in very short period on contact of fungus or its elicitors and reach to a maximum activity within hours. Active oxygen species play a role in various defence mechanisms including lignin production, lipid hydroperoxidation, phytoalexin production and hypersensitive responses.
A first report on the production of active oxygen in potato tubers enduring a hypersensitive response was given by Doke (1983) demonstrated that O2 production occurred in potato tissues upon inoculation with an incompatible race of Phytophthora infestans (i.e., a race causing a hypersensitive response) but not after inoculations with a compatible race (i.e., a disease-causing race).
In a recent study Beno- Moualem and Prusky (2000) found higher reactive oxygen content on inoculation of resistant avocado fruits with Colletotrichum gloeosporioides, whereas inoculation of susceptible fruits had no such effect.
Management Process # 3. Disease Resistance:
Very little work has been done in scenario of resistance to pathogens of Post-harvest commodities. In fact, in breeding and selecting fruits and vegetables for certain desirable horticultural characteristics developed a variety popular for human consumption makes them more susceptible to pathogen.
Breeders generally select fruits and vegetables with thin skin, low tannin and high sugar content and all these factors favour susceptibility to pathogens.
The vegetative plants, fruits evolved resistance mechanisms under different selection pressures which are primarily directed toward microorganisms and insects. The resistance to vegetative pathogens may not impart resistance to Post- harvest diseases, therefore, resistance of fruits and vegetables in the field may not relates to Post-harvest resistance.
Daubeny and Pepin (1977) studied 116 genetically diverse strawberry clones and found that field resistance to Botrytis cinerea was not necessarily related to Post-harvest resistance against B. cinerea or Rhizopus. Plant breeders need to recognize that resistance to Post-harvest diseases may be distinct from field resistance in developing breeding programs for harvested produce.
Such resistance resides in plants that are genetically incompatible with selected varieties. Austin (1988) screened tubers of somatic hybrids produced by protoplast fusion between Solanum brevidell, a diploid, non-tuber-bearing wild species and a tetraploid potato (S. tuberosum) for resistance to bacterial soft rot caused by Erwinia sp.
They were successful in moving this resistance into S. tuberosum where it is now sexually transferable.
Certain resistance responses in plants have to be “turned on” or induced type. Very rudimentary understanding of inducible resistance responses and their use for Post-harvest disease control is present. Harvested commodities are often thought of as physiologically nonresponsive and we fail to realize that they have active as well as passive resistance mechanisms.
Recently, Systemic resistance induced in cucumber, muskmelon and watermelon by inoculating the foliage with pathogens was actually transferable to the fruit. Attenuated strains of plant pathogens could possibly be used to impart resistance to both the vegetative plant and the harvested fruit.
There is need to investigate these ways whereby preharvest treatments may make harvested fruits and vegetables more resistant. A variety of elicitors have been found that will induce resistance responses in fruit. Stevens (1990) have found that low doses of UV irradiation induced resistance in sweet potatoes and onions to Post-harvest rots.
Swinburne (1978) found that proteases from Nectria galligena were able to elicit benzoic acid in apple fruit that was antibiotic to the pathogen. So it necessitate to explore the multiplicity of chemical and physical elicitors that can induce resistance in harvested commodities and attempt to make use of them for biological control.
Management Process # 4. Means for Sustaining Harvested Produce:
There is increasing public awareness that many of the chemical treatments applied to fresh horticultural products to control decay are potentially harmful to consumers. This fact, along with the possible consequence that a number of chemicals withdrawn from the use, has revived and increased interest in physical treatments that could serve as alternatives to fungicides.
Fruits and vegetables are subjected to a wide range of storage conditions to delay their microbial and physiological deterioration. A variety of environments are created and maintained to accomplish this including sanitation, heating, refrigeration, controlled and modified atmospheres, ionizing radiation and ultraviolet illumination.
(i) Sanitation:
Fruits and vegetables that have been injured or wounded during harvest or shipping liable to come in contact with the pathogens can accomplish infection by wound pathogens. Since disease development requires the presence of a given pathogen along with an available wound for penetration. A reduction in either of these factors will lead to the suppression of disease development.
Wounding can be minimized by careful harvesting, sorting, packaging and transportation, including preventing the fruit from falling at all stages. Regarding the avoidance of wounds one should remember that physiological injuries caused by cold, heat, oxygen deficiency and other environmental stresses also predispose the commodity to attack by wound pathogens.
The level of inoculum may also be reduced by careful and strict sanitation procedures. The air of the packinghouse permanently carries an abundance of pathogenic spores which may originate from infected fruits and vegetables covered with spores or from plant remnants in the packinghouse or its surroundings which serve as substrates for many pathogenic fungi.
During the citrus packing season, spores of Penicillium digitatum and Penicillium italicum form the most conspicuous components of the spore population in packing houses and their vicinity.
Fruit containers, the equipment in the packinghouse, as well as the workers hands and tools, all bear pathogenic fungal spores during the season. Fresh fruits arriving at the packinghouse may, therefore, come into contact with pathogenic spores from any of these sources.
Many fruits and vegetables have to be cleaned and washed on arrival at the packinghouse in order to remove soil particles, dust or other contaminants. Water that does not contain disinfectants becomes heavily contaminated by fungal spores and bacterial cells and may infect products treated with non-recirculated water. Chlorine is the principal disinfectant used to sanitize wash water in packinghouses.
Solutions of hypochlorous acid and sodium or calcium hypochlorite salts are the most effective and economical agents available for destroying microorganisms in water and they have been applied widely to reduce bacterial contamination of the water used to wash fruits and vegetables.
(ii) Controlled Atmosphere:
Keeping fruits and vegetables in modified atmospheres is an old and still very effective technique. Controlled atmosphere or modified atmosphere around the produce is created by alterations in concentrations of respiratory gases in the storage atmosphere which include elevation of carbon dioxide level and reduction of oxygen or both.
Alterations in the levels of the atmospheric gases were primarily aimed at suppressing the respiration and other metabolic reactions of harvested fruits and vegetables and thus retarding the ripening and senescence processes.
The effect of low O2 levels or high CO2 levels on Post-harvest disease development observed directly by suppressing various stages of the pathogen growth and enzymatic activity or indirectly by maintaining superior physiological conditions i.e. the resistance of the host to infections.
Appreciable reduction of spore germination, mycelial growth and sporulation in many fungal species occur at less than 1 per cent O2 concentrations although spore sensitivity to low O2 does not necessarily match hyphal sensitivity.
Low oxygen tension of 1-3 per cent tolerated by many agricultural commodities in storage that also reduces the growth of various decay causing bacteria such as Erwinia carotovora, Erwinia atroseptica and Pseudomonas fluorescens.
High carbon dioxide and low oxygen status may directly suppress Post-harvest decay in apples under controlled atmospheric condition by retarding various metabolic functions and respiration of the pathogens.
Similarly, growth of Alternaria alternata, Botrytis cinerea and Cochliobolus herbarum were inhibited by 50 per cent in an atmosphere of 20 per cent CO2, whereas growth of Fusarium roseum was inhibited by 50 per cent only when the CO2 level was elevated to 45 per cent.
Growth colonies of the two bacteria E, carotovora and E. atroseptica was also inhibited by high CO2 concentrations. The retardation of yellowing and rot in controlled CO2 and O2 concentrations has been reported for various leafy vegetables during controlled storage.
(iii) Heat Treatment:
Heat treatments may be applied to the commodity by means of hot water dip and sprays, hot vapour, dry air, infrared or microwave radiations. However, practical systems have used mainly hot water or vapour for fungal control and extended to removal of insects from fresh commodities.
Vapours heating used for disinfestations of fruits for stem end rot, anthracnose and other diseases in many countries. Similarly, the development of green mould on grapefruit caused by Penicillium digitatum inhibited by the treatment of moist air at 46°C used to provide quarantine security against the Mexican fruit fly.
Hot humid air has also been useful in controlling decay in crops that would have been injured in hot water. For example, Post-harvest decay of strawberries caused by Botrytis cinerea and Rliizopus stolonifer was controlled by exposure of the fruits to humid air at 44°C for 40-60 min. The likelihood of using hot water dips to control decay in various fruits and vegetables.
Today, with the trend toward less reliance on chemical treatments, the interest in Post-harvest use of heat treatment has revived. Short-term heating, where the fruit or vegetable is dipped in hot water at 44-55°C for a few minutes to 1 hour has been the main heat treatment method continued over the years.
The principle is that the use of high temperatures to inactivate the pathogen without causing significant changes to the host. Early studies had already confirmed that fruits and vegetables commonly tolerate such temperatures for 5-10 minutes and that even shorter exposures to these temperatures are sufficient to control many of the Post-harvest pathogens.
Recently increased interest in long-term heat treatments, in which the commodity is exposed to temperatures usually at 38-46°C for a longer duration 12 hour to 4 days. Both short term and long term heating aimed at suppressing storage decay could act directly by inactivating the pathogen or indirectly via physiological and biochemical changes in the host which enhance the resistance of the tissues to the pathogen.
The mode of action of hot water dips on decay development appears to be interaction with fungal pathogens exhibited by the inhibition of spore germination and germ-tube elongation of B. cinerea and A. alternata, the two main fungi responsible for Post-harvest decay of vegetables. Heat treatments retards of the ripening process which may lead to the maintenance of fruit quality during prolonged storage.
(iv) Radiation:
The utilization of ionizing radiation for extending the shelf life of fresh fruits and vegetables mainly aimed at control of Post-harvest diseases, delay of the ripening and senescence processes and control of insect infestation for quarantine purposes. Irradiation can be used as a physical means for decay control and Post-harvest life extension of fruits and vegetables.
Ionizing radiation may directly harm the genetic material of the living cell, leading to mutagenesis and eventually to cell death. The nuclear DNA, which plays a central role in the cell, is the most important target molecule in microorganism irradiation, although radiation lesions in other components of the cell may also contribute to cell injury or even result in cell death.
An important advantage of gamma radiation over most chemical treatments is its short wavelength and ability to penetrate into the tissues. This enables irradiation to reach at microorganisms in wounds located within the host tissues as quiescent or active infections. Thus, irradiation is also referred as a therapeutic means; effective after infection has already started.
However, the use of irradiation for decay suppression is basically determined by the tolerance of the host to radiation, rather than the fungicidal dose required for pathogen suppression, different host species, and even different cultivars of a given species, may differ in their tolerance to radiation.
Furthermore, dose tolerance may.be influenced by the state of fruit ripeness at the time of treatment and by the subsequent storage conditions.Relatively low radiation doses exposure to several fruits reduces in the incidence of fungal diseases, retarding pathogen growth, delay the ripening and senescence of the commodity.
(v) Ultraviolet Illumination:
The low doses of Ultraviolet light (wavelength of 190- 280 nm) induce disease resistance in a wide range of fruits and vegetables. UV illumination damage plant DNA, affect several physiological processes, retard the ripening of several harvested commodities and reduce the susceptibility to infection.
In UV illuminated apples, tomato and peaches induced resistance to decay has been attributed to inhibition of ripening and maintenance of the natural resistance of the young fruits to infection.
Changes have been found in UV treated (254 nm) citrus fruits indicated that induced resistance occurs in concomitance with the induced activity of enzymes like phenylalanine ammonia lyase (PAL) and peroxidise. These findings led to the hypothesis that UV illumination induced the activity of phytoalexins, enzymes that plays a role in the enhancing resistance to decay.
(vi) Cold Storage:
Storage at low temperature is the main method for reducing deterioration of harvested fruits and vegetables. The importance of cold storage in decay suppression is so great that all other control methods are frequently considered as supplement to refrigeration.
Low temperatures affect both host and pathogen by decreasing metabolic activity and delay physiological changes that lead to ripening and senescence, simultaneously prevent the moisture loss from the host tissues and consequent shrivelling.
The metabolic action, growth ability and enzymatic activity of the pathogen is also directly influenced by environmental temperature and greatly retarded by low temperatures ir storage. Low temperatures delays Post-harvest disease development directly, by inhibition of pathogen development and indirectly, by inhibition of ripening and senescence of the host.
The effect of refrigeration as a means for disease suppression is manifested mainly in systems in which the host is tolerant of near-freezing temperatures while the pathogen requires higher temperatures for growth. Rhizopus stolonifer do not develop at temperatures lower than 5°C and it cannot be a serious problem for strawberries or carrots stored at temperatures below 5°C.
(vii) Calcium Application:
Calcium is an essential element influences the growth and fruiting of plants and contributes to preserving the structural integrity and functionality of cell wall membranes during fruit ripening and senescence. It is a means of suppressing storage diseases by maintaining or enhancing the natural resistance of fruits and vegetables to pathogens via boosting the calcium content of the various plant organs.
Calcium treatment is useful to improve fruit and vegetable quality have primarily deal with the association of low calcium content in plant tissue and development of physiological disorders.
Calcium treatment may reduce storage disorders such as bitter pit and internal breakdown in apples or the internal brown spot in potato tubers. However, many reports have indicated that an increase in tissue calcium content also led to reductions in fungal and bacterial decay.
It was thus found that pre-harvest calcium sprays reduced the rate of storage losses caused by Gloeosporium spp. in apples or Botrytis and Geotrichum rots in stored grapes, while Post-harvest calcium treatments reduced the rate of the blue mould disease caused by Penicillium expansum in fruit. Various methods for increasing the calcium concentration in storage organs have been investigated.
Applying Ca to harvested fruit seems to be the best method for increasing the calcium content in apples; an increase in the calcium content of potato tubers could have been achieved by calcium fertilization during the growth period.
Calcium enters in the tissues through lenticels, cracks in the cuticle and epidermis and the extent of cracking play a significant role in calcium intake and affect the efficiency of the treatment. The amount of calcium taken in during treatment depends on the growing conditions, environmental factors, cultivar and sate of maturity of a cultivar.
Management Process # 5. Biological Control:
Biological control is an alternative and non-chemical means of decay suppression, refers to the use of naturally found microorganisms which antagonize the Post-harvest disease causing pathogens. Antagonism between microorganisms is a ubiquitous phenomenon involving fungi and bacteria which naturally dwell in soil and surfaces of various plant organs.
It is assumed that biocontrol agents of plant diseases occurs naturally on aerial plant surfaces and may be one of the main reasons that crops are protected to some extent during their cultivation.
Basic approaches for the promotion and use of antagonistic microorganisms against Post-harvest diseases are the management of natural antagonists that already exist on fruit and vegetable surfaces or artificial introduction of antagonists against Post-harvest pathogens.
Naturally occurring epiphytic antagonist population on the surfaces of fruits and vegetables might manage the Post- harvest diseases. It has been demonstrated that there are natural antagonists in the phyllosphere and rhizosphere of plants that can suppress disease development.
Naturally occurring antagonists on the surfaces of apple and citrus fruit have been isolated and reapplied to the fruit as effective bio-control agents.
Biological control in the Post-harvest environment by artificially introduced antagonists may be an exceptionally productive area. One of the main reasons for the failure of biocontrol agents in the past has been our inability to control environmental conditions. Under storage conditions for harvested commodities, environmental conditions are often controlled and maintained.
Another reason, it is often difficult to direct biocontrol agents toward effective sites but in harvested commodities application is very easier. In testing biological control agents for Post-harvest diseases, the application from the laboratory to Post-harvest environment is not as complicated as that into the field.
Laboratory conditions can be made a more nearly approximate to Post-harvest storage conditions and scientist can anticipate a more rapid application of laboratory results for Post-harvest biological control of diseases than field applications.
Antagonists for the control of Post-harvest diseases will be applied to foodstuffs, should have special attention to their potential toxicity to mammals.
Wilson and Wisniewski (1989) described the desirable character for a potential antagonist which includes that it should be genetically stable, effective at low concentration, have simple nutrient requirements, capable of surviving in adverse environmental conditions, effective against a wide range of pathogens and on various fruits and vegetables, resistant to pesticides, preparable in a form that can be effectively stored and dispensed, non-productive of secondary metabolites that may be deleterious to humans and non-pathogenic to the host.
Management Process # 6. Chemical Control:
One of most commonly means of controlling plant diseases in field, greenhouses and storage is through the use of chemical pesticides that are toxic to the pathogen. Chemical substances continue to be essential for effective control of plant diseases.
It should be emphasized that the use of a fungicide or bactericide is not a substitute for appropriate storage conditions, since these compounds only seldom affect the physiological deterioration of the product.
Chemical compounds are more effective when the fruit or vegetable is held under appropriate refrigeration or a controlled atmospheric storage conditions which maintain the natural infection resistance of the produce. Chemical treatment may be the only means to prolong the Post-harvest life of fruits and vegetables.
They are an important tool used for controlling Post-harvest diseases may be fungicides and bactericides (kill to fungi and bacteria) or fungistats and bacteristats (inhibiting fungal and bacterial development).
Chemical treatments can be applied under various strategies and timings to eradicate or attenuate the established infections by preharvest applications to prevent infection in the field and Post-harvest applications to avoid infection through wounds.
A number of fungicides have been developed that are primarily or specifically applied for the management of Post-harvest diseases. Most of them are used as dilute solutions into which fruits and vegetables are dipped before storage or washed immediately after harvest.
Excellent fungicide should have effective performance, cost effective, low toxicity to humans and wildlife, safety to consumer and operator, low environmental impact, low residues in food and the ability to integrate with other disease control technologies.
(i) Pre-Harvest Chemical Treatments:
The efficient way to reduce initial infections in the field including quiescent infections is the application of broad spectrum protective fungicides to the developing fruit on the plant. Protective sprays inhibit spore germination and infection establishment in the lenticels or in floral parts of fruit.
Several investigations have indicated that field sprays may be effective in reducing wound decay or lenticels decay because of the sedimentation of the fungicide in the infection site and its safeguarding at appropriate levels.
A classic example of an effective preharvest treatment is the preventive spraying of citrus fruit with fixed copper compounds inhibits incipient infections of brown rot caused by Phytophthora citrophthora in the fruit peel.
In this case, the penetration and germination of the fungal zoospores into fruit occurs on the trees in presence of water and preventive sprays should be applied prior to rains for successful disease control.
Oranges stem end rot caused by Diplodia natalensis and Phomopsis citri remain quiescent on the tree in the button of the fruit are sprayed with benomyl before harvest to prevent the initiation of infection in fruits. Protective sprays in the plantation have been widely used to prevent anthracnose (Colletotrichum gloeosporioides) in various tropical and subtropical fruits.
This fungus penetrates the young fruits on the tree and establishes a quiescent infection. The developing fruits are sprayed to prevent spore germination and subsequent formation of appressoria and infective hyphae which are the quiescent stages of the fungus. Preharvest spray at every 7-14 days interval successfully prevents anthracnose on mangoes, avocados, papayas and bananas.
The use of protective spray in papaya also reduces the infectivity of Phytophthora, Alternaria, Botryodiplodia, Mycosphaerella and other fungi on fruit at the time of harvest.
Likewise, field sprays of mancozeb reduce the Post-harvest Rhizopus soft rot, probably by reducing field initiated fruit diseases caused by Colletotrichum and Phomopsis species because lesions caused by these fungi may serve as court of infection for Rhizopus stolonife, which requires wounds to penetrate the host.
Dealing with preharvest sprays, it is important to emphasize the need for careful selection of the fungicides and with the emergence of fungal resistant strains to chemicals.
Fungicidal sprays with the systemic benzimidazole compounds during the flowering period successfully controlled both preharvest and Post-harvest diseases but after the emergence of benzimidazole resistant B. cinerea strains, dicarboximide fungicides including iprodione and vinclozolin with different mode of action were adopted for preharvest appliance to soft fruits.
Furthermore, similar to the benzimidazole compounds the dicarboximides are also ineffective against Mucor and Rhizopus species and their use may result in increased incidence of Post-harvest diseases caused by these fungi. Thereafter, control strategies including the use of dicarboximides in combination with other fungicides or in rotation with an unrelated fungicide were developed.
Preharvest sprays may be a suitable means when considerable harvest injury is involved and handling practices make Post-harvest treatment difficult to apply soon after harvest.
Thus, orchard sprays may be the best means for reducing decay in peaches and other fruits that will be subjected to controlled ripening after harvest and in oranges that will be subjected to degreening, since both of these practices often increase decay by wound pathogens.
(ii) Post-Harvest Chemical Treatments:
Post-harvest treatments are generally more effective in protecting wound or injury infections. Open wounds, injury or cuts created during harvesting, handling and packaging are the key sites of infection for Post-harvest pathogens and the protection of wounds by chemicals considerably decrease the decay in storage.
Other potential sites of infection are the natural openings in the host surface such as lenticels and stomata, whose sensitivity to infection is increased by wounding or after washing the harvested commodity in water.
An efficient disinfection process should have its reach to the pathogenic microorganisms accumulated in all these sites. Application of the chemical should take place as soon as possible after harvest in order to avoid further development of the germinating spores in the host tissue.
The selection of the appropriate chemical depends on the sensitivity of the pathogen, tolerance of the host, ability of the chemical to penetrate the surface barriers of the infection site.
The first generation Post-harvest fungicides used for commercial control of diseases in fruits and vegetables are diphenyl (biphenyl), sodium o-phenylphenate, sec-butylamine and dicloran effective in preventing decay caused by wound pathogens such as species of Penicillium and Rhizopus but they have little effect on quiescent infections or other infections situated within the host tissue.
Diphenyl is a fungistat that has been used extensively by citrus exporters and played a significant role in distant marketing of citrus fruits. The chemical impregnated into paper wraps on each individual fruit or into paper sheets placed beneath and above the fruits within the container. It sublimes slowly into the atmosphere and protects the fruits during the entire period of shipping to distant markets.
The main function of biphenyl is the inhibition of sporulation of Penicillium spp. on decaying fruits and prevents contact infection of adjacent fruits through fungal spores from the surface of decayed fruits. Secbutylamine (aminobutane) developed as a Post-harvest fungicide for fruits have antifungal activity against wound infection by Penicillium digitatum and P. italicum on decaying fruit.
It can be applied to harvested fruit as a salt solution to protect oranges during the period of ethylene degreening or it can be added with wax formulations applied on lemons before storage.
The fungicide may also be volatilized and applied as a fumigation treatment which inhibits the development of Penicillium on the surface of the fruits. Dicloran is effective against several Post-harvest fungi predominantly watery soft rot caused by Rhizopus stolonifer in stone fruits and sweet potatoes.
The capability of dicloran to penetrate nearly a depth of 11 mm into peaches explains the inhibitory action of this fungicide treatment against established lesions of Rhizopus on peaches. However, this treatment is less effective against other decays of stone fruits such as the brown rot caused by Monilinia fructicola and the blue mould caused by Penicillium expansum.
Captan is another fungicide effective against Post-harvest diseases of various fruits and vegetables such as strawberries, peaches, cherries, pears, figs and potatoes, but it leaves visible residues of wettable powder on the surface of fruits. Sulfur dioxide (SO2) is applied as a Post-harvest fumigant to inhibit very superficial infections on the fruits and to prevent the contact spread or nesting of fungus during storage.
A common fumigation procedure consists of an initial application of 0.5 per cent SO2 (v/v) for 20 minute immediately after harvest followed by low concentration (0.1-0.2 per cent) fumigation for 30-60 minute on every 7-10 days during the storage period Coertze and Holz (1999).
The widely used systemic fungicides mainly benzimidazole compounds viz. thiabendazole, benomyl, carbendazim and thiophanate methyl introduced as Post-harvest fungicides in the late 1960s, have undoubtedly added a new dimensions to the control of Post-harvest diseases.
They have been used all over the world against a broad spectrum of pathogenic fungi to control decay of fruits caused by the two wound moulds Penicillium digitatum and P. italicum, stem end fungi Diplodia natalensis and Phomopsis citri, the brown rot caused by Monilinia fructicola in stone fruits, blue mould (Penicillium expansum), gray mould (Botrytis cinerea) and lenticel rot (Gloeosporium spp.) in apples, anthracnose (Colletotrichum gloeosporioides) in banana, papaya, mango and other tropical fruits and black rot (Ceratocystis paradoxa) in pineapple.
A group of systemic fungicide namely, imazalil introduced for Post-harvest disease management in the early 1970s, very efficient in controlling Penicillium digitatum and P. italicum in citrus fruits. It acts by inhibiting the biosynthesis of ergosterol an essential component in the membrane of fungal cells.
It was the first ergosterol biosynthesis inhibitor and with this property imazalil has become the most popular Post-harvest fungicide for controlling decay in fruits.
Similar to imazalil spectrum of activity prochloraz, etaconazole inhibit the synthesis of ergosterol. It eradicates initial infections by the two Penicillium species in citrus fruit including infections by benzimidazole resistant isolates.
They suppress the blue and green moulds and has a marked antisporulant activity of these fungi on diseased fruits, simultaneously protected them from infections through new wounds sustained after treatment. These fungicides also effective in controlling anthracnose and stem-end rot on papayas and virtually eliminated anthracnose from mango lots up to 72 per cent diseased fruit in the untreated control.
Fungicides were markedly efficacious against Fusarium rots and soft rots caused by Geotrichum and Rhizopus in melon fruits and provide only moderate protection against the stem end fungi Alternaria citri, Phomopsis citri and Diplodia natalensis.
An acylalanine fungicide metalaxyl (ridomil) acts as a strong inhibitor of the various developmental stages i.e. mycelial growth, sporangia formation, chlamydospores and oospores of Phytophthora spp. This systemic fungicide uniquely arrests incipient infections of Phytophthora in citrus fruits and prevents contact spread of brown rot during prolonged storage.
The antifungal effect on Phytophthora is much more pronounced than thiabendazole or imazalil but it has no influence on the development of other Post-harvest pathogens. However, the combination of metalaxyl with etaconazole in a water wax formulation broadens the scope of antifungal activity and controls Penicillium rots, Geotrichum sour rot and Phytophthora brown rot in citrus fruits.
A coordinated strategy of using the systemic fungicide metalaxyl in mixtures with protective multisite fungicides such as chlorothalonil has been found very effective for the management of Post-harvest diseases and slowing the development of resistant fungal strain.
A different selectively active fungicide Fosetyl al (Fosetyl aluminum) is found effective against incipient infections of Phytophthora. It is applied to harvested citrus fruits to protect against Phytophthora spp. and reduces the incidence of green mould (Penicillium digitatum) in storage.
Management Process # 7. Natural Chemical Compounds:
The use of synthetic fungicides has been the major commercial means of Post-harvest decay. However, the chemical residues remain on the fruit and vegetables or within its tissues following fungicidal treatment indicating that fungicide residues on food pose a great health risk to the consumer and that led to the search for safe alternatives to synthetic fungicides.
The fact that the effectiveness of synthetic fungicides has been reduced by frequent development of resistance by the pathogens further highlighted the need for new substances and methods for the management of storage diseases.
Naturally occurring plant products rapidly degrade on the host surface or metabolize quickly in the tissue, important sources of antifungal compounds with low toxicity to mammals and safe to environment, which may serve as substitutes for synthetically produced fungicides.
Fruits and vegetables have number of constitutive and inducible compounds that are antimicrobial have not been fully explored as biological control agents. Such compounds could be used in or on the plant where they are produced or extracted and applied to other harvested crops.
(i) Essential Oils and Plant Extracts:
They are sources of antifungal activity against a wide range of fungi. A rapid assay to determine antifungal activity in both plant extracts and essential oils has recently been described by Wilson (1997). The major plants showing the highest persistent antifungal activity were garlic and pepper against Botrytis cinerea, which served as a test fungus.
They found that essential oils from the leaves of Melanleuca leucadendron, Ocimum callum, and Citrus medica were able to protect several stored commodities from biodegradation by Aspergillus flavus and A. versicolor.
The next best inhibitors were essential oils of clove buds (Eugenia caryophyllata) and cinnamon leaf (Cinnamomum zeylanicum). The most frequently occurring constituents in essential oils showing high antifungal activity were Z-limonene, cineole, a-pinene, P-pinene, P- myrcene and camphor against harvested produce.
(ii) Volatile Compounds:
A large class of compounds glucosinolates produced by plants of the Cruciferae are other natural substances with potential antimicrobial activity.
Glucosinolates on enzymatic hydrolysis produce isothiocyanates which have antifungal activity has been tested on several Post-harvest pathogens in vitro and in vivo on artificially inoculated pears with encouraging results. Wilson (1987) found that numbers of volatiles produced by peach on ripening are highly fungicidal.
The most effective volatiles in inhibiting spore germination were benzaldehyde, benzyl alcohol, y- caprolactone and y-valerolactone. Benzaldehyde totally inhibited spore germination of Botrytis cinerea at 25 ml/L and germination of M. fruetieola at 125 ml/L.
The benzaldehyde, methyl salicylate, and ethyl benzoate compound have been recorded as growth suppressors completely inhibited the growth of Monilinia fruetieola, B. cinerea and Rhizopus rot.
Acetaldehyde is a natural volatile compound produced by various plant organs and accumulates in fruits during ripening, has shown fungicidal properties against various Post-harvest pathogens. In sublethal concentration it is capable of inhibiting both spore germination and mycelial growth of common storage fungi.
Fumigation of fruits with acetaldehyde inhibits Penicillium expansum development, while fumigation of strawberries with acetaldehyde considerably reduces the incidence of Post-harvest decay caused by Rhizopus stolonifer and Botrytis cinerea.
Gel derived from Aloevera plants has been found to have antifungal activity against four common Post-harvest pathogens: Penicillium digitatum, P. expansum, B. cinerea and A. alternata. The natural gel suppressed both germination and mycelial growth, with P, digitatum and A. alternata being the most sensitive species.
The antifungal potential of the gel in decay suppression was exhibited on P. digitatum-inoculated grapefruit, and was expressed in a delay in lesion development as well as a significant reduction in infection incidence at shelf life conditions.
Plant volatiles have been implicated in resistance mechanisms to Post-harvest diseases. Sitton and Patterson (1992) suggested that resistance of fruit against rots in high CO2 storage was due to the production of high levels of acetaldehyde and ethyl acetate by the fruits in storage atmosphere.
Prasad and Stadelbacher (1973) used acetaldehyde vapour as a fumigant for the management of Botrytis cinerea and Rhizopus stolonifer rots of strawberries and raspberries and to control the green peach aphid on lettuce.
(iii) Chitosan:
It is an animal derived polymer formed by de-acetilation of chitin. It can serve as a coating for the fruit or vegetable and for regulating gas moisture and exchange around the product.
When applied as a coating, chitosan delayed the ripening of tomatoes and reduced decay incidence indirectly by modifying the internal atmosphere. A marked reduction of decay and the formation of structural defence barriers have been recorded in chitosan-treated bell peppers, cucumbers, strawberries, litchi and carrots.
These included the induction of callose synthesis, thickening of host cell walls, formation of papillae and plugging of some intercellular spaces with fibrillar material, probably impregnated with antifungal phenolic-like compounds.
However, chitosan also exhibits a direct fungicidal activity and can affect Post-harvest pathogens by suppressing their growth, inhibition of spore germination, germ-tube elongation and radial growth of B. cinerea and K. stolonifer, inducing cellular alterations and damage in culture.
In parallel, chitosan may also induce the formation of antifungal hydrolases, such as chitinase and P-1,3-gluconase, which may lead to the reduction in the chitin content of fungal cell walls and the stimulation of various structural defence barriers in fruits such as bell peppers and tomatoes.
(iv) Latex:
It present in some fruits is another natural fungicide or a source of fungicides, which is regarded as both safe and effective against various diseases of banana, papaya and other fruits.
Papaya latex is a complex mixture of sugars with several enzymes, notably proteases, glucosidases, chitinases and lipases. The water-soluble fraction of papaya latex can completely digest the conidia of many fungi, including important Post-harvest pathogens.
A small cystein-rich protein isolated from the latex of the rubber tree (Hevea brasiliensis) showed strong antifungal activity in vitro against several fungi such as B. cinerea and species of Fusarium and Trichoderma.
Hevein, which is a chitin-binding protein, might interfere with fungal growth by binding or cross-linking newly synthesized chitin chains. Therefore, the broad range of natural fungicidal plant volatiles is available and numerous opportunities exist to explore their usefulness in controlling Post-harvest diseases.
Management Process # 8. Other Safe Compound:
Civic concerns about pesticide residues on food crops which have made registration of novel fungicides very complicated have led to attention in these compounds and investigations of their potential as safe alternatives for Post-harvest disease control of fruits and vegetables.
Safer compound are widely used in the food industry in packinghouses. They are common food additives used against spoilage control of harvested produce.
These compounds have a broad spectrum of activity against food borne fungi, bacteria and yeasts and are usually recognized as safe compounds for many applications. Such treatments are inexpensive, pose a minimal risk of injury on fruit and can be a useful tool in the management of fungicide-resistant isolates.
(i) Hydrogen Peroxide:
It is a compound that degrades into O2 and H2O leaving no harmful residue. Vapour phase hydrogen peroxide treatment was found significantly to reduce the number of germinable Botrytis cinerea spores on grapes. It also resulted in reduced decay in uninoculated grapes after 12 days of storage at 10°C, without affecting grape colour or soluble solids content.
A disinfectant Sanosil-25 containing 48 per cent hydrogen peroxide and silver salts as stabilizing agents, inhibits spore germination and mycelial growth of Alternaria alternata, Fusarium solani and B. cinerea, and markedly decreases decay in melons when incorporated into a wax treatment, without causing any phytotoxic effects.
Dipping commercially harvested eggplants and red peppers in 0.5% Sanosil-25 reduced decay development by A. alternata and B. cinerea after storage and shelf life, to a commercially accepted level.
(ii) Organic Acids:
Acetic acid and other organic acids such as propionic acid are commonly used by food manufacturers as antimicrobial preservatives in a variety of food products.
The possibility of using vaporized acetic acid against Post-harvest decay has been studied in various fruits. Sholberg and Gaunce (1995) found acetic acid applied as a vapour at low concentrations in air extremely effective in reducing or preventing decay in various cultivars of tomatoes, oranges, apples, grapes pears and stone fruits.
(iii) Salts:
Bicarbonate and carbonate salts are common food additives widely used in the food industry for pH control, taste and texture modifications and spoilage control have a broad spectrum of activity against food borne bacteria and yeasts. Immersion of citrus fruits in sodium bicarbonate solutions reduce the incidence of Post-harvest green mould caused by P. digitatum in lemon and orange fruits.
A comparison among the inhibitory and fungistatic effects of various carbonate and bicarbonate salts on P. digitatum spore germination showed that the effective dose (ED50) concentrations of sodium carbonate, potassium carbonate, sodium bicarbonate, ammonium bicarbonate and potassium bicarbonate were 5.0, 6.2, 14.1,16.4 and 33.4 mM, respectively.
A direct inhibitory effect of sodium bicarbonate on in vitro mycelium growth was similarly recorded for K. stolonifer, A. alternata and Fusarium spp., the major pathogens of stored melons. Coating harvested melons with wax containing 2 per cent sodium bicarbonate resulted in a marked reduction in decay incidence after storage, while the fresh appearance of the fruit was maintained.
(iv) Chlorine:
It is an effective and economical biocide that has long been accepted as a potent disinfectant for sanitation purposes and is recognized as safe in many countries.Chlorinated water with calcium hypochlorite or sodium hypochlorite is routinely used in dump tanks as standard procedure to wash and treat tomatoes and certain vegetables in commercial packinghouses.
The chlorinated water prevents accumulation of pathogens in water as well as reduces the pathogen population on surface of harvested produce. Chlorine acts on fungal propagules by direct contact and can inactivate spores which are suspended in water or located on the surface of fruits or vegetables. It does not act on pathogens under the fruit skin or after infection has occurred.