Alternative methods for the control of postharvest citrus diseases

Authors

  • I. Talibi,

    Corresponding author
    1. Laboratoire de Biotechnologies et Valorisation des Ressources Naturelles, Faculté des Sciences, Université Ibn Zohr, Agadir, Morocco
    2. Domaine Elboura, Taroudant, Morocco
    • Correspondence

      Talibi Idriss, Laboratoire de Biotechnologies et Valorisation des Ressources Naturelles, Faculté des Sciences, Université Ibn Zohr, B.P 8106 Agadir, Morocco.

      E-mail: drisstalibi@hotmail.com

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  • H. Boubaker,

    1. Laboratoire de Biotechnologies et Valorisation des Ressources Naturelles, Faculté des Sciences, Université Ibn Zohr, Agadir, Morocco
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  • E.H. Boudyach,

    1. Laboratoire de Biotechnologies et Valorisation des Ressources Naturelles, Faculté des Sciences, Université Ibn Zohr, Agadir, Morocco
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  • A. Ait Ben Aoumar

    1. Laboratoire de Biotechnologies et Valorisation des Ressources Naturelles, Faculté des Sciences, Université Ibn Zohr, Agadir, Morocco
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Summary

The postharvest diseases of citrus fruit cause considerable losses during storage and transportation. These diseases are managed principally by the application of synthetic fungicides. However, the increasing concern for health hazards and environmental pollution due to chemical use has required the development of alternative strategies for the control of postharvest citrus diseases. Management of postharvest diseases using microbial antagonists, natural plant-derived products and Generally Recognized As Safe compounds has been demonstrated to be most suitable to replace the synthetic fungicides, which are either being banned or recommended for limited use. However, application of these alternatives by themselves may not always provide a commercially acceptable level of control of postharvest citrus diseases comparable to that obtained with synthetic fungicides. To provide more effective disease control, a multifaceted approach based on the combination of different postharvest treatments has been adopted. Actually, despite the distinctive features of these alternative methods, several reasons hinder the commercial use of such treatments. Consequently, research should emphasize the development of appropriate tools to effectively implement these alternative methods to commercial citrus production.

Introduction

Citrus is one of the most widely produced fruit globally. It is grown commercially in more than 137 countries around the world (Ismail and Zhang 2004). The contribution of the citrus industry to the world economy is enormous, and it provides jobs to millions of people around the world in harvesting, handling, transportation, storage and marketing operations. The importance of citrus fruit is attributed to its diversified use, which is widely consumed either as fresh fruit or as juice.

Due to their higher water content and nutrient composition, citrus fruit is very susceptible to infection by microbial pathogens during the period between harvest and consumption (Tripathi and Dubey 2003). Citrus fruits are usually quite acidic, in the pH range 2·2–4. For this reason, so the most of the decay in harvested fruits is caused by fungi. No bacterial postharvest disease of commercial importance is reported on citrus fruit. Contamination and infection by pathogenic fungi occur at different stages in the field and after harvest and usually follows mechanical injury of the fruit, which allows entry of these micro-organisms. Postharvest decays of fruit can also originate from latent infections occurring in the orchard such as black rot caused by Alternaria alternata pv. citri, brown rot caused by Phytophthora citrophthora and anthracnose caused by Colletotrichum gloeosporioides.

In developing and nondeveloped countries, high losses result from inadequate storage facilities and improper transport and handling (Ladaniya 2008). Citrus fruits are susceptible to a number of postharvest diseases that cause significant losses during the postharvest phase. Nevertheless, the most common and serious diseases that affect citrus fruit are green and blue moulds caused, respectively, by Penicillium digitatum Sacc. and Penicillium italicum Wehmer, followed in importance by sour rot caused by Geotrichum citri-aurantii Link ex Persn (Caccioni et al. 1998; Palou et al. 2002; Zheng et al. 2005). These pathogens are strict wound pathogens that can infect the fruit in the grove, in the packinghouse, or during subsequent handling and storage (Palou et al. 2008). The fungal inoculum is practically always present on the surface of fruit during the season and after harvest can build up to high levels unless appropriate packinghouse sanitization measures are adopted (Kanetis et al. 2007). Fruit infection by these fungi is enhanced during the fruit degreening operation and during wet and rainfall seasons (Liu et al. 2009b). Decay is also more prevalent as fruit increases in maturity, and at favourable temperatures and humidity (Baudoin and Eckert 1985).

Currently, synthetic fungicides are the primary means of controlling postharvest diseases of citrus fruit, especially imazalil (IZ), thiabendazole (TBZ), sodium ortho-phenyl phenate (SOPP), fludioxonil (FLU), pyrimethanil or different mixtures of these compounds (Ismail and Zhang 2004; Smilanick et al. 2005; Palou et al. 2008). However, the postharvest use of these fungicides is subject to registration and permission for use in various countries. Continuous use of these fungicides has resulted in the appearance of isolates of fungi with multiple fungicide resistances, which further complicate the management of the diseases (especially Penicillium rots) (Droby et al. 2002; Mercier and Smilanick 2005; Boubaker et al. 2009). In addition, these fungicides are not effective against all important pathogens. Indeed, sour rot is difficult to control with IMZ and TBZ (Suprapta et al. 1997; Mercier and Smilanick 2005). The synthetic fungicide guazatine is the only commercial product that can control sour rot (Brown 1988). However, this fungicide is no longer authorized in Morocco and several other countries. Furthermore, the use of fungicides is increasingly becoming restricted owing to stringent regulation, carcinogenicity, high and acute residual toxicity, long degradation period, environmental pollution and growing public concern about chemical residues in fruit (Tripathi and Dubey 2003; Palou et al. 2008).

Therefore, the challenge is to develop safer and eco-friendly alternative strategies of controlling citrus postharvest diseases, which pose less risk to human health and environment. Recently, several promising biological approaches have been proposed as potential alternatives to synthetic fungicides for the control of postharvest citrus disease. These biological control strategies include as follows: (i) use of antagonistic micro-organisms; (ii) application of naturally derived bioactive compounds; and (iii) induction of natural resistance. Among these biological approaches, the use of the microbial antagonists, either alone or as part of an integrated disease management policy, is quite promising and gaining popularity among consumers (Droby et al. 2002). Interestingly, most of the antagonistic micro-organisms are isolated from fruit surfaces as epiphytic microbial population. Continual laboratory experimentation followed by packinghouse experiments are needed to establish excellent biocontrol agents particularly against postharvest fungal pathogens.

The second approach for disease control is the use of natural plant-derived compounds. Indeed, these compounds have gained popularity and scientific interest for their antibacterial and antifungal activities (Tripathi and Dubey 2003; Du Plooy et al. 2009; Liu et al. 2009b; Gatto et al. 2011; Talibi et al. 2011a,b; Talibi et al. 2012a,b). The use of natural plant products is an interesting alternative or a complementary control method because of their antifungal activity, nonphytotoxicity, systemicity and biodegradability (Tripathi and Dubey 2003; Ameziane et al. 2007; Gatto et al. 2011).

However, commercially speaking, the application of these biological control methods may not always provide acceptable levels of control of postharvest citrus diseases. It is possible to increase the efficiency of these methods by combining them with other postharvest treatments. Indeed, different disease management strategies have been integrated to provide more effective disease control in comparison with a single approach. Low-toxicity chemicals, particularly common food additives and Generally Regarded As Safe (GRAS) compounds, have been evaluated for their efficacy for the control of citrus pathogens. Strictly speaking, GRAS compounds do not fall into the category of organic ingredients, but they are much less harmful than many other inorganic chemicals.

The purpose of this study is to provide an updated overview of the published data on alternative control methods to conventional chemical fungicides for the control of postharvest citrus pathogens (Janisiewicz and Korsten 2002; Spadaro and Gullino 2004; Palou et al. 2008; Droby et al. 2009; Sharma et al. 2009; Nunes 2012; Liu et al. 2013).

Integrated strategies to control postharvest citrus diseases

Antagonistic micro-organisms as biocontrol agents

It has been demonstrated that natural microbial antagonists exist on fruit surfaces that can suppress disease development (Wilson and Wisniewski 1989). The use of antagonistic micro-organisms for controlling the postharvest diseases of citrus fruit is based on two approaches: (i) the use of natural epiphytic antagonists that already exist on fruit surfaces, and (ii) the artificial introduction of selective microbial antagonists that control postharvest diseases. The importance of naturally occurring microbial antagonists is revealed when washed fruits develop more rot than unwashed ones (Wilson and Wisniewski 1989). Chalutz and Wilson (1990) found that washed, dried and stored citrus fruit get infected much more rapidly than unwashed fruit. This suggests that the resident epiphytic microflora on citrus fruit is capable of controlling citrus diseases. Moreover, according to their origin, these naturally occurring antagonists are more apt to gain public acceptance. Several antagonistic micro-organisms, found to be effective in controlling citrus diseases in the postharvest phase, were isolated from the surface of citrus fruit (Wilson and Chalutz 1989; Borras and Aguilar 1990; Chalutz and Wilson 1990; Droby et al. 1998; Taqarort et al. 2008) (Table 1). Currently, available microbial antagonists were developed for the control of decay originating mainly from active infection of fruit wounds and not from quiescent infections. Furthermore, until now, biological control is clearly not going to completely replace the use of fungicides for the control of the postharvest citrus diseases, but it will increasingly be a part of integrated control programmes for major fungal diseases.

Table 1. Microbial antagonists used for the successful control of citrus postharvest diseases and their mode of action on citrus fruits
Microbial antagonistPathogenMode of actionReferences
  1. ND, not determined.

Bacteria
Pseudomonas syringae Penicillium digitatum Antibiosis, competition for nutrientBull et al. (1998)
P. digitatum Competition for nutrient and spaceSmilanick and Denis-Arrue (1992)
Penicillium sppAntibiosisWilson and Chalutz (1989)
Pseudomonas glathei P. digitatum Competition for nutrient and space, induction of resistanceHuang et al. (1995)
Pseudomonas corrugata P. digitatum Antibiosis and competition for nutrients and spaceSmilanick and Denis-Arrue (1992)
Pseudomonas fluorescens P. digitatum Antibiosis and competition for nutrients and spaceSmilanick and Denis-Arrue (1992)
Enterobacter cloacae P. digitatum Competition for nutrient and spaceWilson and Chalutz (1989)
Bacillus subtilis P. digitatum AntibiosisSingh and Deverall (1984)
P. digitatum AntibiosisLeelasuphakul et al. (2008)
P. digitatum AntibiosisYánez-Mendizábal et al. (2011)
Geotrichum citri-aurantii
Bacillus amyloliquefaciens Penicillium crustosum Production of volatile compoundsArrebola et al. (2010)
Bacillus thuringiensis Guignardia citricarpa Antibiosis, induction of resistanceLucon et al. (2010)
Serratia plymuthica Penicillium sppAntibiosis and competition for nutrientsMeziane et al. (2006)
Pantoea agglomerans Penicillium sppNDCañamás et al. (2008)
P. digitatum Triggers H2O2 productionTorres et al. (2011)
Triggers enzymatic activities
Fungi
Muscodor albus G. citri-aurantii Production of volatile compoundsMercier and Smilanick (2005)
P. digitatum
Aureobasidium pullulans P. digitatum AntibiosisLiu, et al. (2007)
Verticillium lecanii P. digitatum Induction of resistanceBenhamou (2004)
Yeasts
Pichia guilliermondii Penicillium italicum Competition for nutrient and space, directly parasitizing the pathogenArras et al. (1998)
P. digitatum Induction of resistanceRodov et al. (1994)
Pichia anomala P. digitatum Competition for nutrient and spaceTaqarort et al. (2008)
Pichia pastoris G. citri-aurantii AntibiosisRen et al. (2011)
Pichia membranaefaciens Penicillium sppInduction of resistanceLuo et al. (2012)
Candida saitoana P. italicum Competition for nutrient and space, direct parasitismEl-Ghaouth et al. (2000)
Candida famata P. digitatum Induction of resistanceArras (1996)
Candida guilliermondii P. digitatum Induction of resistanceArras (1996)
Candida oleophila P. digitatum Induction of resistanceDroby et al. (2002)
Candida sake P. digitatum NDDroby et al. (1999)
Rhodotorula glutinis P. digitatum Competition for nutrient and spaceZheng et al. (2005)
Rhodosporidium paludigenum G. citri-aurantii Competition for nutrient and spaceLiu et al. (2010)
Debaryomyces hansenii P. digitatum Competition for nutrient and spaceTaqarort et al. (2008)
Cryptococcus laurentii P. italicum Competition for nutrient and spaceLiu et al. (2010)
G. citri-aurantii
A. pullulans Penicillium sppCompetition for nutrient and spaceWilson and Chalutz (1989)
Kloeckera apiculata P. italicum Competition for nutrient and spaceLong et al. (2006)
Wickerhamomyces anomalus P. digitatum AntibiosisPlatania et al. (2012)

Yeasts as biocontrol agents

Treatment of citrus fruit with antagonistic yeasts is one of the best alternatives to control postharvest diseases (Janisiewicz and Korsten 2002). This importance is attributed to several positive characteristics that make yeasts effective microbial agents for the control of postharvest diseases of fruit. First, yeasts can colonize the surface of fruit for long period even under dry conditions (Janisiewicz 1987). Second, yeasts produce extracellular polysaccharides, which enhance their survivability and restrict the growth of pathogen propagules. Third, yeasts can use nutrients rapidly and proliferate at a high rate (Sharma et al. 2009). Effective control of citrus fruit decay was observed with yeasts such as Candida guilliermondii (syn.:Pichia guilliermondii), Pichia anomala, Candida saitoana, Candida famata, Candida oleophila, Candida sake, Aureobasidium pullulans and Kloeckera apiculata (Wilson and Chalutz 1989; Chalutz and Wilson 1990; Lima et al. 1997; El-Ghaouth et al. 2000; Droby et al. 2002; Taqarort et al. 2008) (Table 1). Actually, two products based on antagonistic yeasts are commercially available to manage postharvest diseases: Biosave (Pseudomonas syringae Van Hall) and ‘Shemer’ (Metschnikowia fructicola Kurtzman & Droby) (Droby et al. 2009). Two early yeast-based products AspireTM (Ecogen, Philadelphia, PA) and YieldPlus (AnchorYeast, Cape Town, South Africa) are no longer available (Droby et al. 2009). A few others are still in different stages of commercial improvement and expected to be released in the market in the future. However, application of these biocontrol products alone did not provide commercially acceptable control of fruit diseases. The biocontrol ability of these antagonists could be enhanced by manipulation of the environment, using mixtures of beneficial organisms, physiological and genetic enhancement of the biocontrol mechanisms and integration of biocontrol with other methods such as low doses of fungicides and controlled atmosphere storage (Spotts et al. 2002).

Bacteria as biocontrol agents

Use of bacteria for postharvest disease control of citrus has focused on their application as biofungicides. Plant-associated bacteria are ubiquitous in most plant species and can be isolated from surface plant tissues, soils, roots and the rhizosphere of various plants. Moreover, endophytic bacteria are of interest as biocontrol agents because of their rhizosphere competence and their ability to colonize internal tissues of plants and thus provide an internal defence against pathogens (Liu et al. 2009a). Antagonistic bacteria are well known for their production of substances with antifungal and antibacterial properties (Smilanick and Denis-Arrue 1992; Leelasuphakul et al. 2008; Lucon et al. 2010; Yánez-Mendizábal et al. 2011). Significant advances in the control of postharvest citrus diseases have been achieved by the use of bacterial antagonists such as Ps. syringae, Ps. fluorescens, Burkolderia (Pseudomonas) cepacia, Bacillus subtilis, B. thuringiensis, Pantoea agglomerans, Enterobacter cloacae and Serratia plymuthica (Singh and Deverall 1984; Smilanick and Denis-Arrue 1992; Bull et al. 1998; Meziane et al. 2006; Cañamás et al. 2008; Lucon et al. 2010) (Table 1). However, only one species of bacteria has been mass-produced and commercialized under the trade name Bio-Save 100 and 110 based on a strain of Ps. syringae, to control postharvest citrus pathogens.

Fungi as biocontrol agents

Biological control of postharvest citrus diseases by fungal antagonists is less developed compared with yeasts and bacteria. However, antagonistic fungi such as Muscodor albus and Homoptera parasite have shown a reduction in postharvest citrus fruit decay (Borras and Aguilar 1990; Benhamou 2004; Mercier and Smilanick 2005) (Table 1). Antagonistic fungi exhibit a broad spectrum in terms of disease control and volatile antimicrobial compounds (Mercier and Smilanick 2005; Verma et al. 2007). The fungus M. albus, a biofumigant that produces certain low-molecular-weight volatiles, has been used to fumigate whole rooms of lemons to control pathogens during storage. It is reported to be effective on green mould and sour rot (Mercier and Smilanick 2005). This fungus produces 28 organic volatile compounds that show some inhibitory effect against pathogenic fungi and bacteria (Strobel et al. 2001). This antagonism is due to the richness of its antimicrobial metabolites and to physiological conformation (Verma et al. 2007). The potential of fungal antagonists can be improved by continual improvement in isolation, formulation and application methods, particularly in the postharvest environment.

Mode of action of antagonistic micro-organisms

Although considerable amount of research have been reviewed on the use of the microbial antagonists, few attempts have been made to study microbial interactions in fruit surface and wounds. This is due to difficulty in studying the complex interaction occurring between the pathogen, antagonist, host and other micro-organisms present on the fruit surface. Understanding the mechanism by which the biocontrol of fruit diseases occurs is critical to the eventual improvement and wider use of biocontrol methods. Several mechanisms, operating alone or in concert, are involved in antagonistic interactions in the fructoplane. Nutrient and space competition, antibiosis and parasitism are the major mechanisms involved. Additional mechanisms such as induced resistance, interference with pathogen-related enzymes and undoubtedly a number of still unknown mechanisms may complete the microbial arsenal (Sharma et al. 2009; Liu et al. 2013). Often, more than one mechanism is implicated, but in no case has a sole mechanism been found responsible for biological control (Janisiewicz and Korsten 2002; Liu et al. 2013). A good understanding of the relationships between pathogens, antagonistic micro-organisms, fruit and the environment is essential for the successful implementation of the biological control in the postharvest phase.

Antibiosis. Antibiotic production is one of the major modes of action of antagonists. This mechanism is found more in bacteria than in yeasts and in filamentous fungi. Bull et al. (1998) demonstrated that syringomycin produced by Ps. syringae controlled green mould on lemons and inhibited the growth of G. citri-aurantii, P. digitatum and P. italicum. Similarly, Pseudomonas cepacia was also found to be effective in controlling green mould in lemon by producing antibiotics (Smilanick and Denis-Arrue 1992). Moreover, B. subtilis and its antibiotics are considered to be potent biological control agents to suppress growth of P. digitatum in the postharvest protection of citrus (Leelasuphakul et al. 2008). However, the production of these antibiotics was not generally detected on the fruit; raising a doubt on the role of the antibiosis in postharvest diseases control which explains the fact that antibiosis may not comprise the entire mode of action of antagonists on citrus (Bull et al. 1998). Although antibiotic-producing bacteria have a potential to be used as biocontrol agents of postharvest diseases, importance is being given to the development of nonantibiotic-producing microbial antagonists for the control of postharvest diseases of fruits (El-Ghaouth et al. 2004). The possibility of rapid development of pathogen resistance towards antibiotic substances may be an obstacle in the practical use of antibiotic-producing micro-organisms for decay control.

Siderophores have also been reported to be produced by microbial antagonists. Pulcherrimin, a siderophore produced by the yeast Metschnikowia pulcherrima, has shown ability to reduce the growth of various postharvest fungal pathogens (Saravanakumar et al. 2008). Similarly, Rhodotorula glutinis controlled grey mould of apple by producing the siderophore rhodotorulic acid (Sansone et al. 2005). These studies established that competition for iron plays an important role in the biocontrol of pathogens by antagonistic micro-organisms.

Competition for nutrients and space. The nutrient sources in the peel of citrus fruit are frequently not sufficient for all micro-organisms, which makes the competition between pathogen and nonpathogens for nutrient resources or sites an important issue in biocontrol. Many investigations on different biocontrol systems have concluded that the successful competition of microbial antagonists with fruit-infecting pathogens for nutrients and space may be a possible mechanism of biocontrol (Wilson and Wisniewski 1989; Arras et al. 1998; Li et al. 2008; Liu et al. 2012). The nonpathogenic micro-organisms (especially yeasts) protect the surface of citrus fruits by rapid colonization of wounds and thereby exhausting the limited available substrates so that none are available for pathogen to growth. Several yeast species, P. guilliermondii, C. saitoana, R. glutinis, Rhodosporidium paludigenum, K. apiculata, Hanseniaspora guilliermondii and Metschnikowia andauensis, were reported to compete with citrus postharvest pathogens for nutrients and space (Arras et al. 1998; El-Ghaouth et al. 2000; Zheng et al. 2005; Long et al. 2006; Taqarort et al. 2008; Liu et al. 2010) (Table 1). The competition for nutrients and space is favoured by the attachment capability of antagonistic yeasts to pathogen hyphae. The attachment may enhance nutrient competition as well as interfere with the ability of the pathogen to initiate infection (El-Ghaouth et al. 2002). Currently, there are only fragmented data regarding the antagonist–pathogen interaction in terms of competitions for limiting nutrient essential for pathogenesis.

Direct parasitism. In direct parasitism, the pathogen is directly attacked by a specific microbial antagonist that kills it or its propagules. Parasitism depends on close contact and recognition between antagonist and pathogen, on the secretion of lytic enzymes and on the active growth of the parasite into the host (El-Ghaouth et al. 2002; Spadaro and Gullino 2004). It is often referred to as hyperparasitism or mycoparasitism when interactions involve a fungus. In the literature, the role of direct parasitism of the microbial antagonists in controlling postharvest pathogens of citrus fruit is less documented. Arras et al. (1998) showed that P. guilliermondii cells had the ability to attach to the hyphae of P. italicum. Also, the antagonistic yeast C. saitoana was found to attack P. italicum by direct parasitism (El-Ghaouth et al. 2000). The firm attachment of microbial antagonists to fungal pathogens, in conjunction with the enhanced activity of cell-wall degrading enzymes allowing invasion by mycoparasites, may have an important role in the biocontrol activity of antagonists.

Induced resistance. Several microbial antagonists elicit a wide range of defence responses termed induced resistance in citrus fruit (Borras and Aguilar 1990; Rodov et al. 1994; Droby et al. 2002; Benhamou 2004; Lucon et al. 2010). Droby et al. (2002) reported that the application of C. oleophila to surface wounds of grapefruit elicited systemic resistance against P. digitatum. They also demonstrated that the induction of pathogen resistance required viable yeast cells. Rodov et al. (1994) reported that P. guilliermondii induced resistance to green mould by eliciting the production of phytoalexins (e.g. scoparone and scopoletin). Similarly, Arras (1996) showed that scoparone accumulation could be 19 times higher when the antagonist C. famata was inoculated 24 h prior to P. digitatum, and only four times higher if inoculated 24 h after the pathogen. The accumulation of the phytoalexin scoparone was correlated with increased antifungal activity in the flavedo and resulted in enhanced resistance of the fruit to infection by P. digitatum. Recent studies have shown that microbial antagonists trigger a variety of cellular responses, such as activation of reactive oxygen species (ROS) and secretion of lytic enzymes such as b-1,3 glucanases and chitinases (Castoria et al. 2005; Chan et al. 2007; Friel et al. 2007; Macarisin et al. 2007; Xu and Tian 2008).

Although a causal connection between the accumulation of the host defence responses and bioprotection by microbial antagonists has not yet been clearly established, the occurrence of high levels of host antifungal compounds in protected tissue suggests their implication in disease resistance (El-Ghaouth et al. 2004).

Application methods of microbial antagonists

Infection of citrus fruit by pathogens can occur in the field prior to harvest thus it may be advantageous to apply microbial antagonists at this point, as well as in the postharvest phase. An important consideration for the application of antagonists at preharvest is their ability to colonize the surface of fruit both in the field and during storage and to persist on the fruit surface to maintain efficient control of decay (Ippolito and Nigro 2000; Cañamás et al. 2008). However, according to Sharma et al.(2009), this approach has still many limitations and does not provide commercially acceptable control of fruit diseases.

Unlike preharvest application, postharvest application of microbial antagonists is the most used and practical method for controlling postharvest diseases of citrus fruit (Sharma et al. 2009). In this case, microbial antagonists are applied either as postharvest spray, dip or drench applications (El-Ghaouth et al. 2001; Mercier and Smilanick 2005; Cañamás et al. 2008; Usall et al. 2008; Arrebola et al. 2010). Postharvest application of Ps. syringae, Ps. cepacia, B. subtilis, Trichoderma viride and Debaryomyces hansenii resulted in control of P. digitatum in citrus fruit (Singh and Deverall 1984; Wilson and Chalutz 1989; Borras and Aguilar 1990; Smilanick and Denis-Arrue 1992; Bull et al. 1998).

Biological control of citrus diseases with natural plant products

With consumer trends for natural alternatives to chemical-based fungicides and changes in legislation, the use of natural products such as plant extracts may provide a solution for both industry and consumers. Recently, attention has been paid towards the exploitation of higher plant products as novel botanical fungicides in citrus diseases management. More than 1340 plant species are known to be potential sources of antimicrobial compounds, and about 10 000 secondary plant metabolites have been chemically defined for their role as antimicrobials (Cowan 1999; Tripathi and Dubey 2003). Plant extracts have the advantage of being biodegradable, not phytotoxic, are GRAS to mammals. Therefore, higher plants can be exploited for the discovery of new natural fungicides, which can replace synthetic ones. Some phytochemicals of plant origin have been formulated as botanical pesticides and used successfully in integrated pest management programmes (Tripathi et al. 2004).

Use of essential oils

Essential oils are natural, volatile, complex compounds known for their antimicrobial, antioxidant and medicinal properties (Bakkali et al. 2008). The development of resistant strains of fungi against essential oils may be less likely as it is for many synthetic fungicides because several active components are often present in the final product and synergistic interactions may exist between the different components of the oils (Tripathi and Dubey 2003; Tripathi et al. 2004). Furthermore, most of the components of essential oils seem to have no specific cellular targets (Carson et al. 2002). The volatility, ephemeral nature and biodegradability of essential oil compounds may be especially advantageous for treatment of postharvest citrus diseases because only low levels of residues can be expected.

Plaza et al. (2004a) reported that thyme and cinnamon essential oils significantly reduced the incidence of green and blue moulds of citrus. Also, thyme oil was reported to control most postharvest citrus rots, such as green mould, blue mould and sour rot (Arras and Usai 2001; Liu et al. 2009b). Many studies (Table 2) have documented the antifungal effects of plant essential oils against citrus fruit pathogens (Chebli et al. 2003; Tripathi et al. 2004; Alilou et al. 2008; Du Plooy et al. 2009; Solaimani et al. 2009; Badawy et al. 2011). The antifungal activity of the essential oils suggests that they may be considered as a potential alternative to the synthetic fungicides for the control of postharvest citrus pathogens. However, despite their potent antifungal activity, commercial implementation of treatments with essential oils is strongly restricted in citrus because of problems related to potential phytotoxicity, intense sensory attributes or technological application as fumigants or in aqueous solutions (Palou et al. 2008).

Table 2. Plant extracts used for the control of citrus postharvest diseases
Plant speciesPlant extractPathogenReferences
Cymbopogon sp.Aqueous extract Penicillium digitatum Abd-El-Khair and Hafez (2006)
Lantana sp.Aqueous extract P. digitatum Abd-El-Khair and Hafez (2006)
Sanguisorba minor Methanol extract P. digitatum Gatto et al. (2011)
Penicillium italicum
Borago officinalis Methanol extract P. digitatum Gatto et al. (2011)
P. italicum
Sonchus oleraceus Methanol extract P. digitatum Gatto et al. (2011)
P. italicum
Thymus spEssential oil Geotrichum citri-aurantii Liu et al. (2009b)
Thymus capitatus Essential oil P. digitatum Arras and Usai (2001)
Thymus vulgaris Essential oil P. digitatum Fatemi et al. (2011)
P. italicum
Alternaria alternata pv. citri
Zataria multiflora Essential oil P. digitatum Solaimani et al. (2009)
P. italicum
Chrysanthemum Essential oil G. citri-aurantii Chebli et al. (2003)
P. digitatum
Cistus villosus Aqueous extract G. citri-aurantii Talibi et al. (2012a)
Methanol extractTalibi et al. (2012b)
Halimium antiatlanticum Aqueous extract G. citri-aurantii Talibi et al. (2012a)
Methanol extractTalibi et al. (2012b)
Halimium umbellatum Aqueous extract G. citri-aurantii Talibi et al. (2012a)
P. italicum Askarne et al. (2012)
Methanol extract G. citri-aurantii Talibi et al. (2012b)
Mentha spicata Essential oil P. digitatum Du Plooy et al. (2009)
Lippia scaberrima Essential oil P. digitatum Du Plooy et al. (2009)
Allium sativum Aqueous and ethanol extracts P. digitatum Obagwu and Korsten (2003)
Mentha arvensis Essential oil P. italicum Tripathi et al. (2004)
Zingiber officinale Essential oil P. italicum Tripathi et al. (2004)
Ocimum canum Essential oil P. italicum Tripathi et al. (2004)
Acacia nilotica Aqueous extract P. italicum Tripathi et al. (2002)
Punica granatum Methanol extract P. digitatum Tayel et al. (2009)
Withania somnifera Methanol extract P. digitatum Mekbib et al. (2007)
Acacia seyal Methanol extract P. digitatum Mekbib et al. (2007)
Bubonium imbricatum Essential oil P. digitatum Alilou et al. (2008)
Citrus sp.Essential oil P. digitatum Badawy et al. (2011)
Ageratum conyzoides Essential oil P. italicum Dixit et al. (1995)
Simmondsia chinensis Oil emulsion P. italicum Ahmed et al. (2007)
Cinnamomumzeylanicum Essential oilPenicillium sppKouassi et al. (2012)
Parastrephia lepidophylla Aqueous extract P. digitatum Sayago et al. (2012)

Use of crude plant extracts

The preservative nature of some plant extracts has been known for centuries, and there has been renewed interest in the antimicrobial properties of extracts from aromatic plants. In recent years, several studies have been focused on screening of plant extracts for new antifungal compounds that can be used to control postharvest citrus diseases. Aqueous or organic solvent extracts of plants from different origins are sources of antifungal activity against citrus postharvest pathogens under different experimental conditions (Obagwu and Korsten 2003; Abd-El-Khair and Hafez 2006; Ameziane et al. 2007; Mekbib et al. 2007; Gatto et al. 2011; Talibi et al. 2011a; Askarne et al. 2012; Talibi et al. 2012a,b) (Table 2).

Besides the aqueous extracts, organic solvent extracts of several plants have been tested against citrus pathogens. Treatment of mandarin fruit with methanol extracts of Cistus villosus, Halimium umbellatum and Ceratonia siliqua successfully controlled the citrus sour rot (Talibi et al. 2012a,b; Askarne et al. 2013). Also of interest, methanol extracts from Sanguisorba minor showed good control of green mould (Gatto et al. 2011). Ameziane et al. (2007) showed that methanol extract of C. villosus is more active against P. digitatum and G. citri-aurantii than chloroform extracts. Thus, the nature of extraction influences the antifungal activity on the plants tested. This difference in biological activity is due to the polarity of each solvent, that is, the nature of the molecules extracted with each solvent. Methanol is a polar solvent, which can extract several compounds with antimicrobial activities such as alkaloids, triterpene glycosides, tannins, sesquiterpene lactones and phenolic compounds.The potential use of crude plant extracts to control postharvest citrus diseases requires a detailed examination of their biological activity and dispersion in fruit tissues and the development of a formulation which inhibits growth of pathogens without producing phytotoxic effects on fruit.

Use of natural products extracted from plants

Higher plants contain a wide spectrum of secondary substances such as phenols, flavonoids, quinones, tannins, essential oils, alkaloids, saponins and sterols (Tripathi et al. 2004). Among the numerous natural plant products with potential antimicrobial activity are as follows: acetaldehyde, benzaldehyde, benzyl alcohol, ethanol, methyl salicylate, ethyl benzoate, ethyl formate, hexanal, (E)-2-hexenal, lipoxygenases, jasmonates, allicin, glucosinolates and isothiocyanates, etc. (Utama et al. 2002; Tripathi and Dubey 2003; Palou et al. 2008). Utama et al. (2002) demonstrated the efficacy of acetaldehyde, benzaldehyde, cinnamaldehyde, ethanol, benzyl alcohol, nerolidol and 2-nonanone as volatile fungitoxicants for the protection of citrus fruit against P. digitatum. Citral was reported to inhibit mycelial growth and spore germination of P. digitatum (Klieber et al. 2002). Also, citral has been highlighted as an active compound in citrus fruit against decay caused by P. digitatum (Fisher and Phillips 2008). Production of citral in the flavedo of citrus fruit has been described as a preformed defence mechanism against infection by P. digitatum (Rodov et al. 1995). Jasmonates (jasmonic acid and methyl jasmonate) have been found to be effective in postharvest control of P. digitatum either after natural or artificial inoculation of grapefruit (Droby et al. 1999). Exposure of fruit to jasmonates also effectively reduced chilling injury incidence after cold storage (Droby et al. 1999). As they are naturally occurring compounds and are given in low doses, jasmonates may provide a more environment-friendly means of reducing the use of synthetic chemicals. A naturally occurring compound isolated from the flavedo tissue of grapefruit (Citrus paradisi) identified as 7-geranoxy coumarin exhibited antifungal activity against P. italicum and P. digitatum during in vitro and in vivo tests (Agnioni et al. 1998) (Table 3).

Table 3. Natural compounds tested against citrus postharvest pathogens
CompoundCausal agentReferences
Benzaldehyde Penicillium digitatum Wilson and Wisniewski (1989)
Cinnamaldehyde P. digitatum Wilson and Wisniewski (1989)
Acetaldehyde vapour P. digitatum Prasad and Stadelbacher (1973)
Heptanol Geotrichum citri-aurantii Suprapta et al. (1997)
Decanol G. citri-aurantii Suprapta et al. (1997)
Geraniol G. citri-aurantii Suprapta et al. (1997)
Citronellol G. citri-aurantii Suprapta et al. (1997)
Citral G. citri-aurantii

Suprapta et al. (1997)

Klieber et al. (2002)

Zhou et al. (2014)

P. digitatum Fisher and Phillips (2008)
Thymol P. digitatum Jafarpour and Fatemi (2012)
Menthol P. digitatum Jafarpour and Fatemi (2012)
7-geranoxy coumarin P. digitatum Agnioni et al. (1998)
Penicillium italicum
Jasmonic acid P. digitatum Droby et al. (1999)
Methyl jasmonate P. digitatum Droby et al. (1999)
Nerolidol P. digitatum Droby et al. (1999)
2-nonanone P. digitatum Droby et al. (1999)
Kaempferol P. italicum Tripathi et al. (2002)

Mode of action of plant extracts

As the exploitation of natural plant products to protect the postharvest decay of citrus fruit is in its infancy, there is little information about their mechanism of action. Nevertheless, some data address their modes of action. Considering the large number of bioactive chemicals in plant extracts, it is most likely that their antimicrobial activity is not attributable to one specific mechanism but to diverse modes of action (Carson et al. 2002). Droby et al. (1999) reported that jasmonates suppress the development of P. digitatum in grapefruit. This control might be due to the induction of host resistance responses (Droby et al. 1999). Also, Tripathi and Dubey (2003) reported that jasmonates play an important role as signal molecules in plant defence responses against pathogen attack. The same authors showed that essential oils play a role in plant defence mechanisms against phytopathogenic micro-organisms, and the synergism between their different components reduces the development of resistant races of fungi. Also methanol extracts of Withania somnifera and Acacia seyal controlled green mould by a stimulatory effect on host defence mechanisms (Lanciotti et al. 2004; Mekbib et al. 2007). These defence mechanisms resulted in: (i) synthesis of cell wall that could serve as a physical and biological barrier to invading pathogens or (ii) an increase in the total soluble phenolic compound concentration of orange peels (Mekbib et al. 2007). Phenolic compounds are known to alter membrane functionality of pathogens (Lanciotti et al. 2004). However, more investigations on the mode of action of such plant products are required before their recommendation for the control of citrus postharvest diseases.

Application methods of plant extracts and criteria for selecting a good product

In in vivo trials, the efficacy of postharvest treatments of plant extracts on citrus fruits depended on their method of application. The incorporation of essential oils into fruit coatings primarily applied to retain moisture is gaining popularity (Du Plooy et al. 2009). Essential oil of Simmondsia chinensis (jojoba oil) was applied by Ahmed et al. (2007) as a coating for ‘Valencia’ oranges. They effectively maintained fruit quality for up to 60 days (Ahmed et al. 2007). Also Du Plooy et al. (2009) showed that the advantage of using coatings amended with essential oils, rather than vapour, is that there is closer contact between the essential oils and fruit surfaces, allowing exposure of each fruit to similar concentrations of inhibitor over a longer period. Another method of application of botanicals in controlling citrus postharvest diseases is the immersion of citrus fruits in plant solutions. Essential oil of Shiraz thyme showed antifungal activity against P. digitatum only when dip applied (Solaimani et al. 2009). The same authors reported that the dipping method was significantly better than spray method on control of green mould. Keeping in view the merits of the botanicals as postharvest fungitoxicants and to ensure proper application of plant extracts, the products which are found efficacious during in vivo application must meet the following conditions: (i) the product should be effective even after treatments of short duration; (ii) the treatment should not have an effect on quality parameters such as acidity, flavour and aroma and (iii) the lowest suitable dose of the treatments for practical application should be utilized (Tripathi and Dubey 2003).

Control of citrus postharvest diseases by food additives and GRAS compounds

Antifungal compounds that leave low or nondetectable residues in the citrus fruit are actively sought in research programmes. Organic and inorganic salts are widely used in the food industry; they are common food additives for leavening, pH control, taste and texture modifications (Smilanick et al. 1999). These compounds have a broad spectrum of activity against bacteria and fungi and are GRAS compounds for many applications, by European and North America regulations. In addition to their consistent antimicrobial activity, they are inexpensive, readily available, with favourable safety profile for humans and the environment and suitable for commercial postharvest handling practices (El-Mougy et al. 2008; Deliopoulos et al. 2010) (Table 4).

Table 4. Salts and food additives used for the control of citrus postharvest diseases
Name of saltCausal agentReferences
Acetic acid Penicillium digitatum Sholberg (1998)
Formic acid P. digitatum Sholberg (1998)
Propionic acid P. digitatum Sholberg (1998)
Sorbic acid Geotrichum citri-aurantii Kitagawa and Kawada (1984)
Benzoic acid G. citri-aurantii El-Mougy et al. (2008)
Penicillium italicum
P. digitatum
Boric acid G. citri-aurantii Talibi et al. (2011b)
Potassium sorbate P. digitatum Smilanick et al. (2008), D'Aquino et al. (2013)
G. citri-aurantii El-Mougy et al. (2008)
P. italicum El-Mougy et al. (2008)
Sodium carbonate P. italicum  
P. digitatum Plaza et al. (2004c)
Sodium bicarbonate P. digitatum Smilanick et al. (1999)
G. citri-aurantii Smilanick et al. (2008)
P. italicum Palou et al. (2001)
Sodium benzoate G. citri-aurantii El-Mougy et al. (2008)
P. italicum
P. digitatum
P. digitatum Hall (1988)
Sodium salicylate G. citri-aurantii Talibi et al. (2011b)
Sodium propionate P. digitatum Hall (1988)
Sodium ethylparaben P. italicum Moscoso-Ramírez et al. (2013)
Calcium chloride P. digitatum Droby et al. (1997)
Calcium polysulfide P. digitatum Smilanick and Sorenson (2001)
G. citri-aurantii

Use of GRAS compounds and food additives

Among food preservatives, potassium sorbate has been evaluated for the control of citrus green and blue moulds and sour rot (Kitagawa and Kawada 1984; Hall 1988; Palou et al. 2002; El-Mougy et al. 2008; Smilanick et al. 2008; Youssef et al. 2012a,b). This compound, classified as a minimal risk active ingredient and exempt from residue tolerances, is more appropriate for application as an aqueous solution. However, it is not a popular control agent because its low efficacy, and it was reported to delay, rather than stop, green mould infections (Smilanick et al. 2008). Sodium benzoate and benzoic acid are known for their bactericidal and bacteriostatic properties and have the advantage of being nontoxic and tasteless (El-Mougy et al. 2008). Their effect on postharvest citrus diseases was reported against sour rot, green and blue moulds of stored citrus fruit (Hall 1988; El-Mougy et al. 2008). A comparison among the inhibitory effects of various food additives and low-toxicity chemicals against P. digitatum and P. italicum showed that potassium sorbate and sodium benzoate were the most effective on oranges and lemons (Palou et al. 2002). To extend the shelf life of fresh fruit, many other salt compounds are actually used in many countries, aiming at the destruction of the pathogens or inhibition of their growth. Certain compounds, such as sodium bicarbonate or sodium carbonate, have been highly successful in controlling green mould (Smilanick et al. 1999). Smilanick et al. (2008) reported that sodium bicarbonate reduced the incidence of citrus sour rot. Youssef et al. (2014) demonstrated that both sodium carbonate and bicarbonate exert a direct antifungal effect on P. digitatum and induce citrus fruit defence mechanisms to postharvest decay. Moreover, these treatments pose a minimal risk of phytotoxicity to the fruit and can be a useful tool in the management of fungicide-resistant isolates, which have become particularly problematic (Smilanick et al. 1999). A significant reduction in incidence of P. digitatum and P. italicum was noted in the case of oranges treated with ammonium molybdate and sodium molybdate (Palou et al. 2002). Besides these salts, other compounds such as orthophosphoric acid, sodium propionate, calcium polysulfide, calcium chloride, EDTA, sodium salicylate and boric acid have been evaluated for the control of citrus green or blue moulds or sour rot and reduced the incidence and severity of these diseases (Hall 1988; Droby et al. 1997; El-Mougy et al. 2008; Talibi et al. 2011b) (Table 4).

Mode of action of salt compounds

Although many researchers have focused on the control of postharvest diseases of citrus fruit by the application of salt compounds, the mechanisms by which salts inhibit micro-organisms are not well understood. The various modes of action of salt compounds are membrane disruption, inhibition of essential metabolic functions, stresses on pH homeostasis through the accumulation of anions within the cell and the activation of defence mechanisms in fruit (Smilanick et al. 2005; Youssef et al. 2014). Bicarbonates are effective growth inhibitors of various phytopathogenic fungi in vitro. Most citrus fungal pathogens grow better in acidic to neutral conditions than in alkaline conditions. The principal mode of action of the bicarbonate ion is through its buffering capacity sustaining an alkaline environment. When this happens, pathogens, such as P. digitatum which require an acidic environment, expend more energy on fungal acid production than hyphal extension and therefore growth may be inhibited (Pelser and Eckert 1977). Inhibition of G. citri-aurantii, P. digitatum and P. italicum by sorbic acid and its salts may be caused by alteration of cell membrane function and cell transport function, inhibition of enzymes and protein synthesis, and uncoupling of oxidative phosphorylation in mitochondria (El-Mougy et al. 2008). The pH of the bicarbonate and carbonate solutions is important for the control of postharvest citrus diseases, because it directly affects the germination of conidia (Pelser and Eckert 1977) and influences the virulence of pathogens through their colonization of host tissue (Smilanick et al. 2005). However, we and other workers showed that pH alone cannot explain the inhibitory effect of these compounds (Palmer et al. 1997; Smilanick et al. 1999; Talibi et al. 2011b; Youssef et al. 2014).

The effectiveness of calcium against P. digitatum on grapefruit could be due to its direct effects on host tissue by making cell walls more resistant to pathogen penetration. Most of the calcium that penetrates into the host tissue seems to accumulate in the middle lamella region of the cell wall. These cations form bonds between adjacent pectic acids or between pectic acids and other polysaccharides, forming cross-bridges which make the cell walls less accessible to the action of pectolytic enzymes of the pathogen (Droby et al. 1997). Ammonium molybdate affects metabolic processes in several organisms (Bodart et al. 1999). The basis of its biological activity was reported to be its ability to inhibit acid phosphatase which interferes with phosphorylation and dephosphorylation (Mukhopadhyay et al. 1988), one of the most important processes of cell regulation (Hunter 1995).

Use of combined strategy to control postharvest citrus diseases

Different disease management strategies applied both at pre- and postharvest stages have been integrated to provide more effective disease control than possible with a single approach. Several lines of evidence suggest that the combination of microbial antagonists with other alternative control methods can be a promising approach to overcome some of the drawbacks in biocontrol activity (Huang et al. 1995; Droby et al. 1998; El-Ghaouth et al. 2000; Arras et al. 2002; Janisiewicz and Korsten 2002; Porat et al. 2002; Plaza et al. 2004b,c; Zhang et al. 2004) (Table 5). The combination of microbial antagonists with heat (Porat et al. 2002), GRAS compounds (Usall et al. 2008) and UV-C (Stevens et al. 1997) produced a synergic effect and was superior to all the treatments alone in controlling green and blue moulds. Such combined treatments can be easily implemented on a commercial scale in many citrus packinghouses because they are compatible with existing facilities and postharvest handling practices. In general, five objectives may be pursued by the integration of two or more treatments: additive or synergistic effects to increase the effectiveness or the persistence of individual treatments; complementary effects to combine preventive and curative activities; to delay the development of fungicide-resistant isolates, to control fungicide-resistant isolates already present within packinghouses; and to facilitate a reduction in fungicide rates in order to minimize fruit residues and chemical costs (Palou et al. 2008; Smilanick et al. 2008). For example, combination of biocontrol agents with salts or food additives, low levels of conventional fungicides or physical control treatments is possible to improve the action of biological control agents.

Table 5. Combination of biocontrol antagonists with other control methods
AntagonistCombined withDisease controlledReferences
Combination with physical control methods
Candida oleophila Hot waterGreen mouldPorat et al. (2002)
Ultraviolet light-CD'hallewin et al. (2004)
Pseudomonas glathei Heat treatmentGreen mouldHuang et al. (1995)
Bacillus subtilis Hot waterGreen mouldObagwu and Korsten (2003)
Pantoea agglomerans Heat treatmentGreen mouldPlaza et al. (2004c)
Debaryomyces hansenii Ultraviolet light-CGreen mouldStevens et al. (1997)
Combination with low levels of conventional fungicides
C. oleophila ThiabendazoleStem-end rotBrown and Chambers (1996)
Penicillium rotsDroby et al. (1998)
Pichia guilliermondii ImazalilStem-end rotArras et al. (2002)
ThiabendazolePenicillium rots
Kloeckera apiculata CarbendazimBlue mouldLong et al. (2006)
Combination with food additives and other salts
C. oleophila Sodium bicarbonateGreen mouldPorat et al. (2002)
Bacillus subtilis Sodium bicarbonateGreen mouldObagwu and Korsten (2003)
Pseudomonas syringae Sodium bicarbonateGreen mouldSmilanick et al. (1999)
Sodium carbonate
Pantoea agglomerans Sodium carbonateGreen mouldUsall et al. (2008)
Sodium bicarbonateTeixidó et al. (2001)
Cryptococcus laurentii Sodium bicarbonateGreen mouldZhang et al. (2004)
Pichia guilliermondii Calcium chlorideGreen mouldDroby et al. (1997)
Candida saitoana GlycolchitosanGreen mouldEl-Ghaouth et al. (2000)
2-Deoxy-D-glucoseEl-Ghaouth et al. (2001)
Kluyveromyces marxianus Sodium bicarbonateGreen mouldGeng et al. (2011)

Combination of microbial antagonists with other control methods

As previously mentioned, the main shortcoming of the use of microbial antagonists has been inconsistency in their performance, especially when used as a stand-alone product to replace synthetic fungicides. Furthermore, as infection of citrus fruit occurs either prior to harvest or during harvesting and processing, microbial antagonists are expected to display both a protective and curative activity comparable to that observed with synthetic fungicides. However, the biocontrol agents often fail to control previously established infections (Ippolito and Nigro 2000; Zheng et al. 2005). The combination of biological control with other control methods is one of the most promising means of establishing effective integrated disease management strategies. Several approaches have been evaluated to enhance the biocontrol properties of antagonists (Table 5).

Combination of salts and food additives with physical control treatments

Salts and food additives are more effective when combined with curing and hot water treatments. Sorbate potassium was reported to control postharvest sour rot when it was applied as a hot water solution (Kitagawa and Kawada 1984; Smilanick et al. 2008). Plaza et al. (2004c) reported that dipping fruits in a sodium carbonate solution following a curing treatment satisfactorily reduced incidence of green and blue moulds during subsequent long-term storage. Besides heat treatments, salts are applied with wax coatings, a critical operation in citrus fruit packinghouses. Youssef et al. (2012a,b) reported that wax mixed with sodium bicarbonate, potassium carbonate and potassium sorbate significantly reduced the incidence of postharvest green and blue moulds.

Conclusion

From this review, it appears that some significant progress has been made towards the biological and integrated control of postharvest diseases of citrus fruit. Some biofungicides are already on the market in a few countries and will probably become more widely available as they are registered in more areas. Other microbial antagonists should reach the market soon. Postharvest conditions provide an ideal niche for microbial antagonists as they are less subject to sudden weather changes and are often equipped with a sophisticated climate control systems. However, so far, only a few products with high biocontrol potential have been made available on a commercial scale. With intensive research being carried out in various laboratories, the possibility of identifying potent microbes and developing suitable biocontrol products for commercial marketing appears to be bright. On the other hand, it is unrealistic to assume that microbial antagonists have the same fungicidal activity as fungicides. Improved postharvest usage strategies of microbial antagonists include integration with other low-risk treatments to optimize performance while allowing identification of methods that reduce the use of conventional synthetic fungicides for the control of postharvest diseases of citrus fruit. In the development of these new strategies, emphasis should be placed on minimizing human health risks and environmental toxicity. Research should provide appropriate tools (microbial antagonists, natural substances, GRAS compounds, etc.…) to tailor a complete postharvest citrus diseases management strategy.

Conflict of interest

No conflict of interest declared.

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