Citrus Postharvest Green Mold: Recent Advances in Fungal Pathogenicity and Fruit Resistance

Application of edible nanocomposites from chitosan/fenugreek seed mucilage/selenium nanoparticles for protecting lemon from green mold

Green (Penicillium digitatum) mold can severely endanger the citrus fruits production and quality. Targeting the protection of lemon fruits from green mold infestations with nanobiotechnology approach, the fenugreek seed mucilage (FM) was extracted and exploited for biosynthesis of selenium (SeNPs) nanoparticles; their nanocomposites (NCs) with chitosan (CT) was constructed and employed as antifungal materials and edible coating (ECs) to protect lemon fruits against green mold. The nanoparticles formation and conjugations were verified by infrared (FTIR) analysis and electron microscopy. The FM-synthesized SeNPs had particles average of 8.35 nm, were the NCs of them with CT had size mean of 49.33 nm and charged with +22.8 mV. The CT/FM/SeNPs composite exhibited superior antifungal actions toward P. digitatum isolates, up to 32.2 mm inhibition diameter and 12.5 mg/mL inhibitory concentration, which exceeded the actions of imazilil. The microscopic screening of exposed P. digitatum to NCs clarified their mycelial destructive action within 30 h. The coating of infected lemons with fabricated NCs led to complete elimination of green mold development after 10 days of coating, without any infestation remarks. The innovative fabrication of NCs from CT/FM/SeNPs is strongly suggested to protect citrus crops from green mold and preserve fruits quality.

Transcriptome Analysis of mfs2-Defective Penicillium digitatum Mutant to Reveal Importance of Pdmfs2 in Developing Fungal Prochloraz Resistance

Demethylation inhibitors (DMIs), including prochloraz, are popular fungicides to control citrus postharvest pathogens such as Penicillium digitatum (green mold). However, many P. digitatum strains have developed prochloraz resistance, which decreases drug efficacy. Specific major facilitator superfamily (MFS) transporter gene mfs2, encoding drug-efflux pump protein MFS2, has been identified in P. digitatum strain F6 (PdF6) to confer fungal strain prochloraz resistance. However, except for the drug-efflux pump function of MFS2, other mechanisms relating to the Pdmfs2 are not fully clear. The present study reported a transcriptome investigation on the mfs2-defective P. digitatum strain. Comparing to the wild-type strain, the mfs2-defective strain showed 717 differentially expressed genes (DEGs) without prochloraz induction, and 1221 DEGs with prochloraz induction. The obtained DEGs included multiple isoforms of MFS transporter-encoding genes, ATP-binding cassette (ABC) transporter-encoding genes, and multidrug and toxic compound extrusion (MATE) family protein-encoding genes. Many of these putative drug-efflux pump protein-encoding genes had significantly lower transcript abundances in the mfs2-defective P. digitatum strain at prochloraz induction, as compared to the wild-type strain, including twenty-two MFS transporter-encoding genes (MFS1 to MFS22), two ABC transporter-encoding genes (ABC1 and ABC2), and three MATE protein-encoding genes (MATE1 to MATE3). The prochloraz induction on special drug-efflux pump protein genes in the wild-type strain was not observed in the mfs2-defective strain, including MFS21, MFS22, ABC2, MATE1, MATE2, and MATE3. On the other hand, the up-regulation of other drug-efflux pump protein genes in the mfs2-defective strain cannot recover the fungal prochloraz resistance, including MFS23, MFS26, MFS27, MFS31, MFS33, and ABC3 to ABC8. The functional enrichment of DEGs based on Kyoto Encyclopedia of Genes and Genomes (KEGG), Clusters of Orthologous Groups (COG), and euKaryotic Orthologous Groups (KOG) database resources suggested some essential contributors to the mfs2-relating prochloraz resistance, including ribosome biosynthesis-related genes, oxidative phosphorylation genes, steroid biosynthesis-related genes, fatty acid and lipid metabolism-related genes, and carbon- and nitrogen-metabolism-related genes. The results indicated that the MFS2 transporter might be involved in the regulation of multiple drug-efflux pump protein gene expressions and multiple metabolism-related gene expressions, thus playing an important role in developing P. digitatum prochloraz resistance.

Metabolomics approach to understand molecular mechanisms involved in fungal pathogen-citrus pathosystems

Citrus is a crucial crop with a significant economic impact globally. However, postharvest decay caused by fungal pathogens poses a considerable threat, leading to substantial financial losses. Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii and Phyllosticta citricarpa are the main fungal pathogens, causing green mold, blue mold, sour rot and citrus black spot diseases, respectively. The use of chemical fungicides as a control strategy in citrus raises concerns about food and environmental safety. Therefore, understanding the molecular basis of host-pathogen interactions is essential to find safer alternatives. This review highlights the potential of the metabolomics approach in the search for bioactive compounds involved in the pathogen-citrus interaction, and how the integration of metabolomics and genomics contributes to the understanding of secondary metabolites associated with fungal virulence and the fungal infection mechanisms. Our goal is to provide a pipeline combining metabolomics and genomics that can effectively guide researchers to perform studies aiming to contribute to the understanding of the fundamental chemical and biochemical aspects of pathogen-host interactions, in order to effectively develop new alternatives for fungal diseases in citrus cultivation. We intend to inspire the scientific community to question unexplored biological systems, and to employ diverse analytical approaches and metabolomics techniques to address outstanding questions about the non-studied pathosystems from a chemical biology perspective.

Discovery and Transcriptional Profiling of Penicillium digitatum Genes That Could Promote Fungal Virulence during Citrus Fruit Infection

Green mold caused by Penicillium digitatum (Pers.:Fr.) Sacc is the most prevalent postharvest rot concerning citrus fruits. Using the subtractive suppression hybridization (SSH) technique, different P. digitatum genes have been identified that could be involved in virulence during citrus infection in the early stages, a crucial moment that determines whether the infection progresses or not. To this end, a comparison of two P. digitatum strains with high and low virulence has been carried out. We conducted a study on the gene expression profile of the most relevant genes. The results indicate the importance of transcription and regulation processes as well as enzymes involved in the degradation of the plant cell wall. The most represented expressed sequence tag (EST) was identified as PDIP_11000, associated with the FluG domain, which is putatively involved in the activation of conidiation. It is also worth noting that PDIP_02280 encodes a pectin methyl esterase, a cell wall remodeling protein with a high expression level in the most virulent fungal strains, which is notably induced during citrus infection. Furthermore, within the group with the greatest representation and showing significant induction in the early stages of infection, regulatory proteins (PDIP_68700, PDIP_76160) and a chaperone (PDIP_38040) stand out. To a lesser extent, but not less relevant, it is worth distinguishing different regulatory proteins and transcription factors, such as PDIP_00580, PDIP_49640 and PDIP_78930.

Antifungal molecular details of MNQ-derived novel carbon dots against Penicillium digitatum

It is urgent to develop high-efficiency and low-toxicity natural antifungal agents on green mold caused by Penicillium digitatum. The effect of 2-methoxy-1, 4-naphthoquinone (MNQ) inhibition of P. digitatum was not very satisfactory. MNQ-derived carbon dots (MNQ-CDs) synthesized through a solvothermal route were used as antifungal agents against P. digitatum. The antifungal activity of prepared MNQ-CDswas enhanced compared to MNQ, and the minimum inhibitory concentration was 2.8 μg/mL. A total of 441 genes and 122 metabolites have undergone significant changes. The omics data revealed that MNQ-CDs primarily modified the metabolism of aromatic amino acids and synthesis of the cell membrane in P. digitatum, thereby inhibiting its propagation. Furthermore, compared with MNQ, MNQ-CDs had a better control effect on the green mold of citrus fruits, and could more significantly inhibit the propagation of P. digitatum. This study provides a new idea for the design of new and efficient antifungal materials.

A marine lipopeptides-producing Bacillus amyloliquefaciens HY2-1 with a broad-spectrum antifungal and antibacterial activity and its fermentation kinetics study

The antagonistic Bacillus amyloliquefaciens HY2-1 was a marine microbiology that was isolated previously from the seabed silt of Beibu Gulf in China by dual culture with Penicillium digitatum. As a continuous study, the present work focused on evaluating the antimicrobial activity, identifying the produced active components, and revealing the fermentation characteristics of B. amyloliquefaciens HY2-1, respectively. It was found that B. amyloliquefaciens HY2-1 exhibited a broad-spectrum antimicrobial activity against the tested seven phytopathogenic fungi and five pathogenic bacteria by producing Bacillus lipopeptides such as fengycin A (C₁₄ to C₁₉ homologues) and surfactin (C₁₄ and C₁₅ homologues). Morphological observation of P. digitatum under light microscope, scanning electron microscopy, transmission electron microscopy, and fluorescence microscope inferred that B. amyloliquefaciens exerted the antagonistic activity by damaging the fungal cell membrane, thus inhibiting the mycelium growth and sporification of phytopathogenic fungi. As a marine microbiology, our results showed that B. amyloliquefaciens could survive and metabolize even at the culture condition with 110 g/L of NaCl concentration, and the produced antimicrobial compounds exhibited excellent thermostability and acid-alkali tolerance. The dynamic models were further constructed to theoretically analyze the fermentation process of B. amyloliquefaciens HY2-1, suggesting that the synthesis of antimicrobial compounds was coupled with both cell growth and cell biomass. In conclusion, the marine lipopeptides-producing B. amyloliquefaciens HY2-1 showed a promising prospect to be explored as a biocontrol agent for plant disease control of crops and postharvest preservation of fruits and vegetables, especially due to its outstanding stress resistance and the broad-spectrum and effective antagonist on various phytopathogenic fungi.

Efficient control of the fungal pathogens Colletotrichum gloeosporioides and Penicillium digitatum infecting citrus fruits by native soilborne Bacillus velezensis strains

Destruction of citrus fruits by fungal pathogens during preharvest and postharvest stages can result in severe losses for the citrus industry. Antagonistic microorganisms used as biological agents to control citrus pathogens are considered alternatives to synthetic fungicides. In this study, we aimed to identify fungal pathogens causing dominant diseases on citrus fruits in a specialized citrus cultivation region of Vietnam and inspect soilborne Bacillus isolates with antifungal activity against these pathogens. Two fungal pathogens were characterized as Colletotrichum gloeosporioides and Penicillium digitatum based on morphological characteristics and ribosomal DNA internal transcribed spacer sequence analyses. Reinfection assays of orange fruits confirmed that C. gloeosporioides causes stem-end rot, and P. digitatum triggers green mold disease. By the heterologous expression of the green fluorescent protein (GFP) in C. gloeosporioides using Agrobacterium tumefaciens-mediated transformation, we could observe the fungal infection process of the citrus fruit stem-end rot caused by C. gloeosporioides for the first time. Furthermore, we isolated and selected two soilborne Bacillus strains with strong antagonistic activity for preventing the decay of citrus fruits by these pathogens. Molecular analyses of 16 S rRNA and gyrB genes showed that both isolates belong to B. velezensis. Antifungal activity assays indicated that bacterial culture suspensions could strongly inhibit C. gloeosporioides and P. digitatum, and shield orange fruits from the invasion of the pathogens. Our work provides a highly effective Bacillus-based preservative solution for combating the fungal pathogens C. gloeosporioides and P. digitatum to protect citrus fruits at the postharvest stages.

Advances in Fungal Elicitor-Triggered Plant Immunity

There is an array of pathogenic fungi in the natural environment of plants, which produce some molecules including pathogen-associated molecular patterns (PAMPs) and effectors during infection. These molecules, which can be recognized by plant specific receptors to activate plant immunity, including PTI (PAMP-triggered immunity) and ETI (effector-triggered immunity), are called elicitors. Undoubtedly, identification of novel fungal elicitors and their plant receptors and comprehensive understanding about fungal elicitor-triggered plant immunity will be of great significance to effectively control plant diseases. Great progress has occurred in fungal elicitor-triggered plant immunity, especially in the signaling pathways of PTI and ETI, in recent years. Here, recent advances in fungal elicitor-triggered plant immunity are summarized and their important contribution to the enlightenment of plant disease control is also discussed.

Genome-wide identification of the Penicillium digitatum bZIP gene family and the roles of one key member, PdatfA

Penicillium digitatum is the most common cause of postharvest decay in citrus fruits around the world. Previous studies revealed that the bZIP gene family plays crucial roles in development, stress adaptation, and pathogenicity in fungi. However, little is known about the bZIP genes in P. digitatum. In this study, we systematically identified the bZIP family in 23 Penicillium species and analyzed their evolutionary relationships. We found that gene loss and gene duplication shaped the evolution of the Penicillium bZIP family. P. digitatum experienced 3 bZIP gene loss events, but with no gene duplication. We subsequently characterized the biological functions of one important member, PdatfA in P. digitatum by constructing the deletion mutant. Results showed that ΔPdatfA exhibited a moderate growth defect, reduced pigmentation, and slightly increased resistance to fungicides iprodione and fludioxonil. However, ΔPdatfA displayed similar rot symptoms to that of the wild type. The ΔPdatfA mycelia were not affected in response to oxidative stress while its conidia showed enhanced resistance due to the upregulation of catalases. Our results provide new insights into the evolution and functions of the bZIP gene family in Penicillium.

The plasma membrane H+-ATPase is critical for cell growth and pathogenicity in Penicillium digitatum

The plasma membrane H⁺-ATPase (PMA1) is a major cytosolic pH regulator and a potential candidate for antifungal drug discovery due to its fungal specificity and criticality. In this study, the function of Penicillum digitatum PMA1 was characterized through RNA interference (RNAi) and overexpression technology. The results showed that silencing the PMA1 gene reduces cell growth and pathogenicity, and increases susceptibility of P. digitatum to proton pump inhibitors (PPIs). Under scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examination, cell morphology was significantly altered in the PMA1- silenced mutant (si57). When compared with wild type (WT) and the overexpressed mutant (oe9), the cell walls of the si57 mutant were thicker and their cell membrane damage manifested particularly at sites of polarized growth. Consistent with the morphological change on the cell wall, chitin and glucan content of the cell wall of si57 were significantly lower and accompanied with increased activities of chitinase and glucanase. The lower ergosterol content in the si57 mutant then increased cell membrane permeability, ultimately leading to leakage of cytoplasmic contents such as ions, reduced sugars and soluble proteins. Furthermore, significantly decreased activity of cell wall degrading enzymes of si57 during citrus fruit infections indicates a reduced pathogenicity in this mutant. We conclude that PMA1 in P. digitatum plays an important role in maintaining pathogenesis and PMA1 could be a candidate novel fungicidal drug discovery for citrus green mold. KEY POINTS: Silencing PMA1 gene decreased the growth and pathogenicity of P. digitatum. Silencing PMA1 gene damaged cell wall and cell membrane integrity of P. digitatum. PMA1 appears to be a suitable fungicidal target against citrus green mold.

Endophytic Fungi Associated with Coffee Leaves in China Exhibited In Vitro Antagonism against Fungal and Bacterial Pathogens

Coffee endophytes have been studied for almost 74 years, and several studies have demonstrated coffee-endophytic fungi with antibacterial and antifungal potential for human and plant pathogens. In this study, we isolated and identified a total of 235 strains of endophytic fungi from coffee leaf tissues collected in four coffee plantations in Pu’er city, Yunnan province, China. Molecular identification was carried out using maximum likelihood phylogenetic analysis of nuclear ribosomal internal transcribed spacer (ITS1-5.8S rDNA-ITS2) sequences, while the colonization rate and the isolation frequency were also calculated. Two pathogenic fungi (Alternaria alternata and Penicillium digitatum) and two pathogenic bacteria (Pseudomonas syringae and Salmonella enterica subsp. enterica) were used for screening the antagonistic activities of 61 strains of coffee-endophytic fungi by a dual-culture test assay while maximum likelihood phylogenetic analysis confirmed their natural classification. This is the first study of coffee-leaf-endophytic fungal diversity in China, and the results revealed that coffee-endophytic fungi from this study belong to the Ascomycota, distributed among two classes, 10 orders, and 17 families. Concurrently, endophytic fungi isolates distributed in Arthrinium, Biscogniauxia, Daldinia, Diaporthe, and Nigrospora showed strong antagonistic activities against the pathogens. For the pathogens Alternaria alternata and Pseudomonas syringae, Nigrospora XCE-7 showed the best inhibitory effects with inhibition rates of 71.76% and 61.11%, respectively. For the pathogen Penicillium digitatum, Daldinia ME-9 showed the best inhibitory effect with a 74.67% inhibition rate, while Biscogniauxia PTE-7 and Daldinia T5E-1-3 showed the best inhibitory effect with a rate of 60.42% against the pathogen Salmonella enterica subsp. enterica. Overall, our study shows the diversity of coffee endophytes in four coffee-growing areas in Pu’er city, Yunnan province, China, and their potential use as biological control agents against two fungal and two bacterial pathogens.

Antifungal application of biosynthesized selenium nanoparticles with pomegranate peels and nanochitosan as edible coatings for citrus green mold protection

Background

Citrus production and trading are seriously affected by fungal decays worldwide; the green mold infection by Penicillium digitatum could be the most disastrous. The substitutions of chemical and synthetic fungicides with effectual natural alternatives are global demands; plant extract from pomegranates peels (PPE), biosynthesized selenium nanoparticles with PPE (PPE/SeNPs) and chitosan nanoparticles (NCT) were suggested as efficacious fungicidal agents/nanocomposites to control P. digitatum strains.

Method

PPE from Punica granatum was extracted and employed directly for synthesizing SeNPs, whereas NCT was produced using ionic gelation method of chitosan extracted from white prawn (Fenneropenaeus indicus) shells. The physiochemical, biochemical and structural characterization of generated molecules were conducted using infra-red spectroscopy, particles’ size (Ps) and charge assessment and electron microscopes imaging. Antifungal potentialities were investigated in vitro and in infected fruits with P. digitatum by applying NCT nanocomposites-based edible coating.

Results

The synthesis of PPE-synthesized SeNPs and NCT was successfully achieved, the molecular bonding in synthesized agents/composites were proved with infrared spectroscopy to have both biochemical and physical interactions. The nanoparticles had 82.72, 9.41 and 85.17 nm mean diameters for NCT, PPE/SeNPs and NCT/PPE/SeNPs nanocomposites, respectively. The nanoparticles had homogenous spherical shapes and good distribution attributes. The entire agents/nanocomposites exhibited potent fungicidal potentialities toward P. digitatum isolates; NCT/PPE/SeNPs nanocomposite was the most forceful and significantly exceeded the fungicidal action of standard fungicide. The direct treatment of fungal mycelia with NCT/PPE/SeNPs nanocomposite led to remarkable lysis and deformations of P. digitatum hyphae within 12 h of treatment. The coating of infected orange with NCT-based edible coatings reduced the green mold infection signs by 91.7, 95.4 and 100%, for NCT, NCT/PPE and NCT/PPE/SeNPs based coating solutions, respectively.

Conclusions

NCT, PPE-synthesized SeNPs, and their innovative nanocomposites NCT/PPE/SeNPs are convincingly recommended for formulating effectual antifungal and edible coatings to eliminate postharvest fungal pathogen, both with protection from their invasion or with destructing their existing infections.

Graphical Abstract

Deciphering Molecular Determinants Underlying Penicillium digitatum's Response to Biological and Chemical Antifungal Agents by Tandem Mass Tag (TMT)-Based High-Resolution LC-MS/MS

Penicillium digitatum is a widespread pathogen responsible for the postharvest decay of citrus, one of the most economically important crops worldwide. Currently, chemical fungicides are still the main strategy to control the green mould disease caused by the fungus. However, the increasing selection and proliferation of fungicide-resistant strains require more efforts to explore new alternatives acting via new or unexplored mechanisms for postharvest disease management. To date, several non-chemical compounds have been investigated for the control of fungal pathogens. In this scenario, understanding the molecular determinants underlying P. digitatum's response to biological and chemical antifungals may help in the development of safer and more effective non-chemical control methods. In this work, a proteomic approach based on isobaric labelling and a nanoLC tandem mass spectrometry approach was used to investigate molecular changes associated with P. digitatum's response to treatments with α-sarcin and beetin 27 (BE27), two proteins endowed with antifungal activity. The outcomes of treatments with these biological agents were then compared with those triggered by the commonly used chemical fungicide thiabendazole (TBZ). Our results showed that differentially expressed proteins mainly include cell wall-degrading enzymes, proteins involved in stress response, antioxidant and detoxification mechanisms and metabolic processes such as thiamine biosynthesis. Interestingly, specific modulations in response to protein toxins treatments were observed for a subset of proteins. Deciphering the inhibitory mechanisms of biofungicides and chemical compounds, together with understanding their effects on the fungal physiology, will provide a new direction for improving the efficacy of novel antifungal formulations and developing new control strategies.

Identification of Volatile Organic Compounds in Extremophilic Bacteria and Their Effective Use in Biocontrol of Postharvest Fungal Phytopathogens

Phytopathogenic fungal growth in postharvest fruits and vegetables is responsible for 20-25% of production losses. Volatile organic compounds (VOCs) have been gaining importance in the food industry as a safe and ecofriendly alternative to pesticides for combating these phytopathogenic fungi. In this study, we analysed the ability of some VOCs produced by strains of the genera Bacillus, Peribacillus, Pseudomonas, Psychrobacillus and Staphylococcus to inhibit the growth of Alternaria alternata, Botrytis cinerea, Fusarium oxysporum, Fusarium solani, Monilinia fructicola, Monilinia laxa and Sclerotinia sclerotiorum, in vitro and in vivo. We analysed bacterial VOCs by using GC/MS and 87 volatile compounds were identified, in particular acetoin, acetic acid, 2,3-butanediol, isopentanol, dimethyl disulphide and isopentyl isobutanoate. In vitro growth inhibition assays and in vivo experiments using cherry fruits showed that the best producers of VOCs, Bacillus atrophaeus L193, Bacillus velezensis XT1 and Psychrobacillus vulpis Z8, exhibited the highest antifungal activity against B. cinerea, M. fructicola and M. laxa, which highlights the potential of these strains to control postharvest diseases. Transmission electron microscopy micrographs of bacterial VOC-treated fungi clearly showed antifungal activity which led to an intense degeneration of cellular components of mycelium and cell death.

Unveiling the Role Displayed by Penicillium digitatum PdMut3 Transcription Factor in Pathogen-Fruit Interaction

Zn2Cys6 transcription factors are unique to fungi and are involved in different regulatory functions. In this study, we have identified the Penicillium digitatumPdMut3 gene, which encodes a putative Zn (II) 2Cys6 DNA-binding protein. Elimination of PdMut3 in Pd1 strain caused increased virulence during citrus infection. The transcription of the PdMut3 gene showed a higher expression rate during fungal growth and less transcription during fruit infection. Furthermore, the deletion of the gene in the wild-type isolate of P. digitatum did not produce any modification of the sensitivity to different fungicides, indicating that the gene is not associated with resistance to fungicides. In contrast, PdMut3 null mutants showed a reduction in growth in minimal media, which was associated with severe alterations in conidiophore development and morphological alterations of the hyphae. Mutants showed greater sensitivity to compounds that interfere with the cell wall and an invasive growth block. Thus, PdMut3 might have an indirect role in fungi virulence through metabolism and peroxisomes development.

Potential of microbial endophytes to enhance the resistance to postharvest diseases of fruit and vegetables

Food loss of fruit and vegetables caused by postharvest diseases is a major issue worldwide. The method used to prevent and control postharvest diseases is usually to use chemical fungicides, but long-term and large-scale use will make the pathogens resistant and potentially have a negative impact on human health and the ecological environment. Therefore, finding a safe and effective biological control method instead of chemical control is a hot research topic in recent years. Endophytes, colonizing plants asymptomatically, can promote the growth of the hosts and enhance their resistance. The use of endophytes as biological control agents for postharvest diseases of fruit and vegetables has attracted increasing attention in the last 20 years. Compared with chemical control, endophytes have the advantages of being more environmentally friendly, sustainable, and safer. However, there are relatively few relevant studies, so herein we summarize the available literature. This review focuses mainly on the recent progress of using endophytes to enhance the resistance of postharvest fruit and vegetables to diseases, with the emphasis on the possible mechanisms and the potential applications. Furthermore, this article suggests future areas for study using antagonistic endophytes to prevent and control fruit and vegetable postharvest diseases: (i) screening more potential broad-spectrum anti-pathogen endophytes and their metabolic active substances by the method of macrogenomics; (ii) elucidating the underlining molecular mechanism among endophytes, harvested vegetables and fruit, pathogens, and microbial communities; (iii) needing more application research to overcome the difficulties of commercialization practice. © 2020 Society of Chemical Industry.

Penicillium italicum: An Underexplored Postharvest Pathogen

In the agricultural sector, citrus is one of the most important fruit genus in the world. In this scenario, Brazil is the largest producer of oranges; 34% of the global production, and exporter of concentrated orange juice; 76% of the juice consumed in the planet, summing up US$ 6.5 billion to Brazilian GDP. However, the orange production has been considerable decreasing due to unfavorable weather conditions in recent years and the increasing number of pathogen infections. One of the main citrus post-harvest phytopathogen is Penicillium italicum, responsible for the blue mold disease, which is currently controlled by pesticides, such as Imazalil, Pyrimethanil, Fludioxonil, and Tiabendazole, which are toxic chemicals harmful to the environment and also to human health. In addition, P. italicum has developed considerable resistance to these chemicals as a result of widespread applications. To address this growing problem, the search for new control methods of citrus post-harvest phytopathogens is being extensively explored, resulting in promising new approaches such as biocontrol methods as “killer” yeasts, application of essential oils, and antimicrobial volatile substances. The alternative methodologies to control P. italicum are reviewed here, as well as the fungal virulence factors and infection strategies. Therefore, this review will focus on a general overview of recent research carried out regarding the phytopathological interaction of P. italicum and its citrus host.

Biological control of postharvest fungal decays in citrus: a review

Citrus (Citrus spp.) species produce a variety of fruits that are popular worldwide. Citrus fruits, however, are susceptible to postharvest decays caused by various pathogenic fungi, including Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii, Aspergillus niger, and Aspergillus flavus. Decays resulting from infections by these pathogens cause a significant reduction in citrus quality and marketable yield. Biological control of postharvest decay utilizing antagonistic bacteria and fungi has been explored as a promising alternative to synthetic fungicides. In the present article, the isolation of antagonists utilized to manage postharvest decays in citrus is reviewed, and the mechanism of action including recent molecular and genomic studies is discussed as well. Several recently-postulated mechanisms of action, such as biofilm formation and an oxidative burst of reactive oxygen species have been highlighted. Improvements in biocontrol efficacy of antagonists through the use of a combination of microbial antagonists and additives are also reviewed. Biological control utilizing bacterial and yeast antagonists is a critical component of an integrated management approach for the sustainable development of the citrus industry. Further research will be needed, however, to explore and utilize beneficial microbial consortia and novel approaches like CRISPR/Cas technology for management of postharvest decays.

Protection of Citrus Fruits from Postharvest Infection with Penicillium digitatum and Degradation of Patulin by Biocontrol Yeast Clavispora lusitaniae 146

Fungal rots are one of the main causes of large economic losses and deterioration in the quality and nutrient composition of fruits during the postharvest stage. The yeast Clavispora lusitaniae 146 has previously been shown to efficiently protect lemons from green mold caused by Penicillium digitatum. In this work, the effect of yeast concentration and exposure time on biocontrol efficiency was assessed; the protection of various citrus fruits against P. digitatum by C. lusitaniae 146 was evaluated; the ability of strain 146 to degrade mycotoxin patulin was tested; and the effect of the treatment on the sensory properties of fruits was determined. An efficient protection of lemons was achieved after minimum exposure to a relatively low yeast cell concentration. Apart from lemons, the yeast prevented green mold in grapefruits, mandarins, oranges, and tangerines, implying that it can be used as a broad-range biocontrol agent in citrus. The ability to degrade patulin indicated that strain 146 may be suitable for the control of further Penicillium species. Yeast treatment did not alter the sensory perception of the aroma of fruits. These results corroborate the potential of C. lusitaniae 146 for the control of postharvest diseases of citrus fruits and indicate its suitability for industrial-scale fruit processing.