Use of yeasts from different environments for the control of Penicillium expansum on table grapes at storage temperature
Introduction
Penicillium is one of the most common fungi in a diverse range of habitats and the species is well-known for its impact, both positive and negative, on humans (Visagie et al., 2014). Certain species produce penicillin, which revolutionized medical approaches to the treatment of bacterial infections (Houbraken and Samson, 2011). Other members are used in the food industry for the production of special cheeses (Errampalli, 2014). However, the main function of Penicillium is the decomposition of organic materials in natural habitats, and unfortunately, some species damage food crops (Pitt and Hocking, 2009). A wide range of fresh fruits and vegetables are attacked by several Penicillium species causing diseases during the postharvest handling (Quaglia et al., 2016; Sanzani et al., 2013). Many of these species are psychrotrophic and they are able to cause food spoilage at refrigeration temperature (Pitt and Hocking, 2009). Penicillium expansum, the major causal agent of blue mold in apples (Arrarte et al., 2017; Li et al., 2011; Vero et al., 2009), pears (Lutz et al., 2013; Meng et al., 2010), table grapes (Donoso and Latorre, 2006; Zoffoli and Latorre, 2011) and other deciduous fruits (De Paiva et al., 2017; Zhu et al., 2016); it is a destructive postharvest pathogen that colonizes these fruits and causes large economic losses during storage and shipment (Spadaro and Droby, 2015; Wisniewski et al., 2016).
Table grapes are highly perishable, non-climacteric and susceptible to severe postharvest changes such as loss of firmness, berry drop and decay (Meng et al., 2008). After harvest, table grapes are stored at 0 °C, between 40 and 100 days (according to the cultivar), in order to prolong their shelf-life (Zoffoli and Latorre, 2011). However, injuries on the berries promote germination of P. expansum conidia and the penetration of the germinal tube, colonizing the injured tissue and producing blue mold. Penicillium is known as a “wound pathogen”, since it requires fresh wounds or natural openings such as lenticels to infect the host (Errampalli, 2014).
The disease symptoms are characterized by a light brown skin discoloration followed by a soft and wet rot that can rapidly affect the entire berry (Franck et al., 2005). On the surface of rotting berries, blue-green mold colonies may appear. Decayed fruit has an earthy, musty odor (Zoffoli and Latorre, 2011). In addition to the spoilage, it is known that P. expansum is a major producer of patulin, a mycotoxin that affects the immune (Bourdiol et al., 1990), neurological (Devaraj et al., 1982) and gastrointestinal system (Abrunhosa et al., 2001) in humans (Ostry et al., 2017).
The major practice to treat table grapes during cold storage relies on the controlled emission of sulfur dioxide (SO2) by generator pads containing sodium metabisulfite (Sanzani et al., 2012). SO2 reduces surface contamination of grape bunches by fungal conidia and prevents the rapid spread of the mycelium after contact between diseased and healthy grapes (Zoffoli and Latorre, 2011). However, SO2 use has been restricted in some countries, since its residues are dangerous to people with sulfite allergies, and this synthetic compound is also harmful to fresh grapes, causing bleaching of the berries and browning of the rachis (Vasconcelos de Oliveira et al., 2014).
Many scientists are presently searching for alternatives to conventional control, focusing on microbial antagonists in order to prevent different diseases on diverse kind of crops (Grzegorczyk et al., 2017; Wallace et al., 2018).
Several microorganisms have been found to be good antagonists for the control of post-harvest diseases (Hua et al., 2014; Mahunu et al., 2016). Currently, microorganisms naturally present on fruit surfaces (such as bacteria and yeast) represent the majority of antagonists used against postharvest diseases (Droby and Wisniewski, 2018). Yeasts that naturally occur on fruits and vegetables have been specially targeted by many researchers as potential antagonists of postharvest diseases, because they exhibit a number of traits that enhance their potential for colonizing fruit surfaces (Sharma et al., 2009; Spadaro and Droby, 2016). However, yeasts isolated from sources such as the phyllosphere, roots, soil, and sea water have also proven to be good antagonists (Liu et al., 2013). For example, the psychrotrophic yeast Leucosporidium scotti, isolated from Antarctic soil, was found to be a good biocontrol agent against both blue and gray mold in apple (Vero et al., 2012). Nally et al. (2012) showed that yeasts isolated from fermenting must, exerted an inhibitory effect against Botrytis cinerea in table grapes.
There is little information on the control of Penicillium spp. using yeasts at low temperature (Lutz et al., 2013; Vero et al., 2012) or about the application of biocontrol yeasts isolated from fermenting musts (Nally et al., 2012; Pesce et al., 2018). In addition, there are no reports about biocontrol of P. expansum in table grapes at low temperature using oenological yeasts.
The present study aimed to select native yeasts isolated from fermentative microenvironments and the surface of refrigerated grapes for their possible use in the biological control of P. expansum in table grapes stored in cold rooms.
Section snippets
Source of microorganisms
Penicillium and yeast species were isolated from table grapes (Superior Seedless and Red Globe cultivars) with a soluble solids concentration (SSC) between 15.5 and 16% (Jayasena and Cameron, 2008; Maante et al., 2015). Grapes were stored inside boxes with SO2 generator pads, at 0 ± 1 °C for two months, in a packing house in 9 de Julio district (San Juan, Argentina).
Isolation of spoilage fungi
After two months, bunches that showed typical symptoms of blue mold were removed from the storage and used for isolation of fungi.
Isolation and phytopathogenicity of fungi
Sixteen Penicillium isolates were obtained from table grapes that showed blue mold characteristics. Five of them were isolated from Superior Seedless grapes (PSS) and the rest from Red Globe (PRG) cultivars (Table 1).
Pathogenicity in vivo assays showed that PRG5, PRG7, PRG8, PRG11 and PSS7 were considered non-pathogenic isolates, as they did not cause symptoms of blue mold at any of the concentrations assayed. On the other hand 11 fungal isolates caused different percentages of decay, but no at
Discussion
In this study, pathogenic and non-pathogenic fungi belonging to different species of Penicillium were isolated from grapes incubated at low temperature. The most aggressive pathogenic isolates belong to the species Penicillium expansum. Also, this species is one of the most frequently reported postharvest pathogen associated with table grapes (Franck et al., 2005; Zoffoli and Latorre, 2011). Even though the non-pathogenic isolates of Penicillium can be considered in this way under the
Conclusions
In the current study, four pathogenic isolates were identified as Penicillium expansum. Fruit surface is the main source for isolation of biological control agents. Also, microenvironments with different stress conditions are promising sources for pathogen antagonists. Therefore, using yeasts isolated from different sites can be effective as biological control treatments against P. expansum, reducing the disease incidence and severity. The most noticeable antagonistic activity against P.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Research Project CICITCA (Res. 21-18-CS). The authors would like to thank CONICET – Argentina for their financial support and the National University of San Juan.
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Equal contribution.