Use of yeasts from different environments for the control of Penicillium expansum on table grapes at storage temperature

Highlights

• Pathogenic and non-pathogenic Penicillium were found in table grapes in cold rooms.

• The most aggressive pathogenic isolates were identified as Penicillium expansum.

• Yeasts isolated from different sites were effective to control P. expansum.

• Non-Saccharomyces yeasts reduced the pathogenicity of P. expansum isolates assayed.

Abstract

A wide range of fresh fruits and vegetables is attacked by Penicillium species causing diseases during their postharvest handling. Many of these species are psychrotrophic and they are able to cause food spoilage at refrigeration temperature as happens with table grapes. After the harvest, grape bunches are stored inside boxes with SO₂ generator pads to reduce the contamination with fungal conidia. However, SO₂ residues are dangerous to people allergic to sulfites and they negatively affect the quality of fresh fruit. Biological control of phytopathogens with microbial antagonists naturally present on fruit surfaces could be helpful against postharvest diseases. The present study aimed to select native yeasts isolated from fermentation microenvironments and the surface of refrigerated grapes for their use in the biological control of P. expansum on table grapes stored in cold rooms. Non-pathogenic and pathogenic Penicillium species were isolated, and the four most aggressive pathogen isolates were identified as Penicillium expansum. Twenty yeast isolates identified as Aureobasidium pullulans, Cryptococcus magnus, Metschnikowia pulcherrima and Rhodotorula glutinis presented positive antagonistic activity against Penicillium expansum; they controlled the development of at least one of the fungi, significantly reducing the disease incidence. The results showed that three antagonistic yeasts (M. pulcherrima 22, 36 and 43) reduced the disease incidence and severity of all 4 P. expansum isolates. It was also found that the fruit surface is not the only source for isolation of biological control agents. Microenvironments with different stress conditions could be a promising source to isolate antagonistic microorganisms.

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.