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This chapter focuses on sprout inhibition by irradiation of potato tubers, onion and garlic bulbs, and carrot roots. Factors affecting sprout inhibition and postirradiation changes in these subterranean organs were described.

Phytosanitary treatments such as irradiation disinfest host commodities of quarantine insect pests before they are exported to areas where the pests do not occur and are often the simplest approach to overcome regulatory trade barriers and gain market access. Irradiation is a versatile technology to disinfest fresh and durable agricultural commodities of quarantine pests. Irradiation is broadly effective against insects and mites, cost competitive with other disinfestation methods such as chemical fumigation, and fast. Irradiation generally does not significantly reduce commodity quality at the doses used to control insect pests and may extend shelf life. Additionally, irradiation can be applied to the commodity after packaging. Research methods for developing irradiation treatments for insect disinfestation and the history of developing regulations for phytosanitary irradiation are discussed.

Response of fungal cells to inactivation by ionizing radiation is governed by several factors of which the inherent resistance is the first. Factors affecting the sensitivity to irradiation of postharvest fruits and vegetables and their enzymatic activity have been reviewed. This chapter focuses on the suppressive effects of irradiation on decay development. The impact of irradiation alone or combined with other postharvest techniques on decay suppression and on the ripening process of fruits was discussed.

Several issues present barriers to the wider use of phytosanitary irradiation. Because irradiation is characterized as a food additive rather than a process, its use must be disclosed on a label. The labeling requirement may be a drawback for retailers who believe consumers are reluctant to buy irradiated food. The 1-kGy limit is archaic and reduces the efficiency of applying the treatment. The United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) has placed limits on the use of modified atmosphere packaging with irradiation, which disrupts commercial practices that ensure superior commodity quality. The limited number of countries that have approved phytosanitary uses of irradiation is hampering broader adoption because of closed markets.

The first chapter focuses on the purposes of applying ionizing radiation to fruits and vegetables. These include the extension of their useful shelf life by inactivating postharvest pathogens or/and by delaying ripening and senescence, and inhibiting sprouting of tubers, bulbs, and roots of subterranean vegetables. Most important purposes are the improvement of microbial safety associated with human pathogenic bacteria contaminating the fresh produce and providing relevant data on wholesomeness and safety of irradiated food. Chemical and nutritional changes occurring after irradiation should be provided.

Increased consumption of fresh-cut fruits and vegetables raised the concern about microbial safety because they serve as suitable growth media for human pathogenic bacteria associated with outbreaks of food-borne diseases. Recent studies evaluated the ability of irradiation combined with modified-atmosphere packaging (produced around fresh-cut produce) indicated that this combination was capable of eliminating or reducing the contaminating human bacteria and enhancing microbial quality with reduced irradiation doses. It was generally found that fresh and fresh-cut fruits and vegetables can tolerate low irradiation doses of up to 1 kGy, without causing significant changes in the quality of the produce. The fresh-cut fruits reviewed were mangoes, papayas, guavas, apples, and blueberries, and the fresh-cut vegetables reviewed were tomatoes, bell peppers, eggplants, cucumbers, melons, broccoli, celery, lettuce, watercress, spinach leaves, endive, green onions, carrots, potatoes, beets, and mushrooms.

This chapter discusses the radiation effects on mycotoxins in infected fruits and vegetables, including patulin, aflatoxins, ochratoxin, and Alternaria mycotoxins. The best way to reduce mycotoxin level is to eliminate the appearance of toxin-producing fungi at preharvest, during harvest, or mainly at postharvest stages.

This chapter reviews the postirradiation changes in fruits and vegetables, including microbiological changes, changes in quality parameters, nutritional changes (vitamin C content), and chemical changes, including antioxidant capacity, phenolic compounds, and enzymatic changes.

This chapter reviews the effects of radiation alone or combined with other postharvest treatments on quality parameters of fruits, including citrus fruits, avocados, mangoes, papayas, bananas, pineapples, litchis, persimmons, guavas, annona fruits, pomegranates, dates, figs, pome and stone fruits, strawberries, raspberries, blueberries, blackberries, and kiwifruits. Review on each fruit is accompanied by the beneficial effects of irradiation versus the adverse effects. To allow the use of radiation doses beneath those inducing peel damage, the combination of low doses with other postharvest techniques, acting synergistically or additively, were used. These included heating, modified or controlled atmosphere, modified-atmosphere packaging, various coatings and polymeric film wrapping, chemical application, heated chemicals, cold storage, and others.

This chapter reviews the effects of radiation alone or combined with other postharvest techniques on shelf life and quality parameters of vegetables. The vegetables included are tomatoes, bell peppers, melons, lettuce, cilantro, cabbage, broccoli, carrots, potatoes, onions, garlic, and mushrooms. Each review is accompanied by beneficial effects of irradiation versus the adverse effects. The combined techniques included heat radiation, heat–chemical radiation, irradiation and modified-atmosphere packaging, irradiation with refrigeration, and wrapping materials.

Studies carried out for more than six decades clarified that ionizing radiation can serve as alternative physical means for shelf life extension of fresh produce, decay control, retardation of fruit ripening and senescence processes, maintenance of wholesomeness, enhancement of microbial safety, and sprout inhibition of subterranean vegetables. Irradiation has also been associated with maintenance or enhancement of antioxidant activity. Comparison of irradiation with other preservation techniques of fresh produce clarified its advantages. However, the acceptance of irradiation as a means for preservation of fresh produce and microbial safety has generally met with public opposition because of economic and logistic factors or psychological problems. Attention has been given to the need of sensitive and reliable analytical techniques to identify irradiated food.

In 2006, US Department of Agriculture Animal and Plant Health Inspection Service published a landmark rule providing generic radiation quarantine treatments. The rule approved radiation doses of 150 Gy for any tephritid fruit fly and 400 Gy for all other insects except the pupa and adult stages of Lepidoptera. The generic radiation treatments apply to all fresh horticultural commodities. Therefore, if a pest risk assessment demonstrates that no pupae or adult Lepidoptera are associated with a commodity, export approval can be forthcoming with no further research. Generic treatments are the culmination of decades of research but not an end point. Future research on quarantine and phytosanitary uses of radiation should focus on (1) development of specific doses for quarantine Lepidoptera not covered by the generic treatments, (2) reduction of dose levels for specific pests and commodities to shorten treatment time and minimize any deleterious effects of radiation treatment on commodity quality, (3) development of generic doses below 400 Gy for important groups of quarantine arthropods other than fruit flies, and (4) development of information on commodity tolerance and development of value-added irradiated fresh products using generic radiation treatments. Generic treatments will facilitate safe trade between countries that have approved phytosanitary uses of radiation for fresh agricultural commodities.

Publisher Summary Aflatoxins are found in fruits of tropical and subtropical regions, where typical environmental conditions support growth of aflatoxigenic aspergilli, and mycotoxin production. Aspergillus species are widespread in nature. They are regarded as soil fungi, frequently colonizing plant debris and decaying agricultural crops, and are among the commonest airborne fungi. They occur with high frequency as saprophytes on a wide range of substrates, including foods and feeds, and several species are among the typical pathogens of harvested fruits and vegetables. During their life cycle, several Aspergillus species are capable of producing a wide range of mycotoxins harmful to humans and animals that consume them. The major mycotoxins associated with Aspergillus species in fruits and vegetables are aflatoxins produced mainly by aflatoxigenic strains of A. flavus and A. parasiticus in figs and dates and ochratoxin A (OTA) produced by A. carbonarius and other ochratoxigenic aspergilli in grapes, dried vine fruits and wines. Some of the mycotoxins produced by Aspergillus species during their developmental stages in fruits and vegetables are among the most important carcinogenic mycotoxins; they include aflatoxins, ochratoxin A (OTA) and sterigmatocystin.

Publisher Summary This chapter provides an overview of Penicillium expansum, the cause of the destructive blue mold rot of pome and stone fruits. It is a major concern for human health, because of the production of patulin during pathogenesis. Human exposure is primarily via consumption of infected apple juices and other apple-based products. Patulin occurrence has been reported in juices of other fruits, such as pears, grapes, cherries, apricots and oranges. The factors affecting P. expansum growth and patulin formation include fungal strain, host fruit cultivar, and storage conditions, especially the storage atmosphere. The chapter discusses the worldwide occurrence of patulin in fruit products. It also explains patulin contamination in infant apple products, and comparisons between its levels in—organic fruits versus conventional ones, industrial versus hand-made products, and cloudy versus clear apple juices. Penicillium mycotoxins other than patulin, including ochratoxin A, citrinin, penicillic acid, cyclopiazonic acid, and penitrem, as well as the co-occurrence of different mycotoxins, are also discussed.

Publisher Summary Alternaria toxigenicity in fruits varies not only with the fungal species or strain, but also with the host fruit species or cultivar and with the conditions under which the toxin is synthesized. Alternaria species are among the most common postharvest pathogens of fruits and vegetables. During pathogenesis several species are capable of producing several mycotoxins that elicit adverse effects in humans and animals. These mycotoxins include tenuazonic acid, alternariol, alternariol methyl ether, altenuen, and altertoxins. Although A. alternata is regarded as the major mycotoxin-producing species, other species such as A. citri, A. solani, A. longipes, and the A. tenuissima, A. arborescens, and A. infectoria species-groups also produce the characteristic Alternaria mycotoxins. Alternaria mycotoxins transferred from a rotten part of the fruit into the surrounding sound tissue result in the presence of mycotoxins in fruit tissue, even in the absence of fungal mycelium. Alternaria species naturally contaminate the aerial parts of plants, and are easily isolated from decaying matter.

This chapter focuses on the biological control that refers to the use of naturally found microorganisms that antagonize the postharvest pathogens. Antagonism between microorganisms is a ubiquitous phenomenon involving fungi and bacteria, which naturally inhabit the soil and the surfaces of various plant organs. It is assumed that biocontrol 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. One of the approaches to the isolation of antagonistic microorganisms for controlling postharvest diseases is through the promotion and management of natural epiphytic antagonists, which are already present on fruit and vegetable surfaces. Preharvest pesticide application and various postharvest treatments, such as fungicide and wax sprays, washes and dips, can greatly affect the resident microflora, both qualitatively and quantitatively.

Fruits become more susceptible to invasion by postharvest pathogens as they ripen. Treatments and conditions that lead to delayed ripening and senescence can, therefore, indirectly suppress postharvest disease development. These include low-temperature storage, low-O 2 and high-CO 2 atmospheres, ethylene removal from the atmosphere, growth regulators, calcium application; these, as well as other treatments or strategies, may contribute to maintaining the natural resistance typical of the young fruit or vegetable. Low temperatures that affect host and pathogen simultaneously prevent moisture loss from the host tissues and consequent shriveling; they retard metabolic activity and delay physiological changes that lead to ripening and senescence. Since fruits and vegetables become generally more susceptible to pathogens as they mature and approach senescence, the retardation in the physiological activity of the host is accompanied by a delay in decay development after harvest.

This chapter deals with the synthetic fungicides, which are the primary means used for controlling postharvest diseases. The resistance developed by postharvest pathogens to some fungicides, and the withdrawal of a number of key fungicides in response to health concerns over pesticide contamination, have stimulated the search for alternative technologies for postharvest disease control. Among the various possible alternative means of control, much attention has been drawn to the wide range of natural substances with antimicrobial activity. The chapter also examines inducible resistance in harvested tissues, which has recently joined the general concept that resistance in plants can be enhanced by modulating their natural defense mechanisms. Activation of defense responses in harvested crops has been demonstrated in various host–pathogen interactions via application of physical, chemical, or biological elicitors.

This chapter concentrates on the secretion of extracellular enzymes and toxins, which are the two major attack means with which the pathogen may be equipped, and the ability of the pathogen to detoxify host defense compounds. It also defines the cutinase enzyme, which hydrolyzes the primary alcohol ester linkages of the polymer. Cutin is found in many plant pathogenic fungi, including Botrytis cinerea . However, the importance of this enzyme for penetration of the cuticle seems to be a matter of debate. Many lines of evidence suggest that cuticular penetration of Colletotrichum gloeosporioides into the host is associated with the induction of extracellular cutinase production, and that insertion of the cutinase gene into this pathogen facilitates infection of an intact host. The chapter also explains the pectic substances, which constitute an important component of the primary cell wall of plants, and form the major component in the middle lamella, which are responsible for the tight link between the cells and the integrity of the plant tissue.

This chapter discusses the means for maintaining host resistance and other physical means: heating, ionizing radiation, and ultraviolet illumination. It defines the heat treatment that may be applied to the commodity by means of hot water dips and sprays, hot vapor or dry air, or infrared or microwave radiation. Practical systems have used mainly hot water or vapor. While hot water was originally used for fungal control and was extended to removal of insects from fresh commodities, vapor heating—developed for insect control—may also serve to reduce fungal decay. Thus, the vapor heat treatment used for disinfestation of Carabao mangoes in the Philippines also significantly reduced the incidence of anthracnose and stem-end rot in fruit, although the onset of decay was not delayed by the treatment.

This chapter discusses pathogens and their origin. During the prolonged storage of fruits and vegetables, a series of physiological processes occurs, which leads to the senescencing of the vegetal tissues and, in parallel, to their increased susceptibility to weak pathogens that attack senescencing vegetal tissues. Fungi and bacteria responsible for in-storage decay often originate in the field or the orchard. When penetration into the host takes place in the field, the pathogen, which is then in its early or quiescent stages of infection, will get to the storeroom within the host tissue without eliciting any symptoms of decay. The chapter also deals with spore germination, which is a preliminary stage to fungal penetration into the host. The right environmental temperature, available water or moisture and, sometimes, the presence of nutrients transferred from the host into the water, are the most important environmental factors that aid spore germination.

The postharvest infection of fruits and other plant organs may induce a number of alterations in their physiological and biochemical processes, or in the host tissue constituents, as a result of host–pathogen interactions. Process changes may include acceleration of ethylene evolution, stimulation of the respiratory enzymes, enhanced pectolytic activity, altered protein synthesis or polyamine-synthesis enzyme activity, and tissue changes may include increased cell-wall soluble pectin, and changed organic acid and sugar contents. The chapter also explains the ethylene synthesis, which is stimulated by mechanical wounding, which occurs while introducing the inoculum into the host. However, comparison between rates of ethylene production elicited by wounds of certain dimensions and those elicited by fungal lesions of similar size indicates that ethylene production in fungus-infected fruits is considerably greater than that in wounded ones.

Chemical substances intended for controlling postharvest diseases may be fungicides and bactericides, or fungistats and bacteristats. To serve its purpose, the chemical should come into contact with the pathogen; the minimal effective concentration is pathogen dependent. Chemical treatments can be applied under various strategies and with various timings: preharvest application to prevent infection in the field, sanitation procedure to reduce the level of inoculum in the environment to which injured fruits or vegetable are liable to be exposed, and postharvest application to prevent infection through wounds and to eradicate or attenuate established infections to prevent their development and spread during storage. In order to choose the appropriate strategy for decay control, one has to understand the mode of infection of the pathogen, its time of infection, and the environmental factors that may affect disease development.

This chapter highlights pathogen development within the host, which is correlated in many cases with the activity of cell wall-degrading enzymes. These enzymes are also responsible for cell death and the liberation of nutrients, which then become available to the pathogen. Liberation of nutrients results in a 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. The chapter also defines the Polyphenols and tannins as inhibitors of polygalacturonase (PG) activity of Sclerotinia fructicola (Monilinia fructicola) in apples and other pathogen/host combinations, although they have no effect on the pathogen itself. Decay development in many fruits and vegetables may be the result of the delicate balance between active production of enzymes and their inhibition by polyphenols, mainly in their oxidized condition, as a result of polyphenol-oxidase activity.

This chapter discusses the characteristic pathogens of various fruits and vegetables. Each fruit and vegetable has its own group of characteristic pathogens to which it is susceptible, and for which it serves as a suitable host. Harvested fruits and vegetables are naturally attacked by a relatively small group of pathogens: approximately forty species. Moreover, each fruit or vegetable has its own typical pathogens out of this particular group. Fruits of tropical and subtropical origin, such as mango, papaya, avocado, or citrus fruits are typically attacked by the fungus, Colletotrichum gloeosporioides, which causes anthracnose. This pathogen penetrates into the fruit while still in the orchard, but the anthracnose symptoms in the rind break out only as the fruit ripens after harvest. In a similar manner, Gloeosporium musae attacks the unripe banana fruits in the orchard, to initiate a quiescent infection, which becomes active only during storage.

This chapter reviews preharvest factors that affect disease development, along with harvesting, and handling. The first preharvest factor which may affect postharvest quality is the cultivar, since different cultivars may vary greatly in their susceptibility to diseases. In fact, one of the aims of plant breeding and genetic engineering is to incorporate resistance genes in new varieties of crop plants. Differences in cultivar characteristics can markedly affect the keeping quality of the fresh produce. Another preharvest factor is the health of the planting material. Various pathogens may contaminate the planting material and cause disease in the field or in storage. Many pathogens persist in the soil or survive on plant debris in the field, from which winds and rain may be directly responsible for their dispersal to potential hosts. Other pathogens, such as Phytophthora spp., which infect potato tubers or citrus fruits, are actually dependent on rainwater for germination of their spores and initiation of infection.