The South African stone fruit industry has a turn-over of R2.7 billion (Hortgro, 2018). Roughly 286.8 thousand tons of peaches, nectarines and plums were produced in 2016/17. Peaches and nectarines are primarily processed or destined for the local market (only 7.9% is exported); whereas plums are predominantly exported (75%). The main export markets include the Middle East for peaches, the UK for nectarines and Europe for plums (Hortgro, 2018). Stone fruits have a relative to high perishability and are susceptible to bruising, as well as a wide variety of postharvest pathogens (Crisosto and Mitchell, 2011; Kader, 2011). Fruit can become infected during packing, storage, distribution, at retail or at home. Therefore they require well-organised supply chain management systems to ensure that they retain their high quality and arrive blemish-free at the destination markets, particularly when export-ed. Extended supply chains can promote ripening and increase susceptibility to postharvest pathogens (Prusky, 1996; Prusky et al., 2016). The nature of extended supply chains and multi-fruit handling systems (packing, distribution, storage and re-pack) also increase the risk of cross-infection between pathogens of different fruit types (i.e. stone, pome and citrus fruits). By better understanding and applying the knowledge of host-pathogen interactions of the main postharvest pathogens of stone fruits, decay and market end-losses can be reduced. Important postharvest pathogens of stone fruits include Monilinia, Rhizopus, Mucor, Penicillium, Alternaria, Botrytis, Cladosporium, Colletotrichum and Stigmina (Snowdon 2010; Pitt and Hocking, 2009). Monilinia species are the most problematic post-harvest pathogens of peach, nectarine and apricot, but Penicillium expansum is also recognised as an important pathogen in terms of losses and mycotoxin production (Ceponis and Friedman 1957; Wells et al. 1994; Pitt and Hocking 2009; Snowdon 2010). Recent findings have illustrated other Penicillium species as a potential risk to nectarine and plum (Louw and Korsten 2016). Other reported Penicillium pathogens of stone fruits are Penicillium crustosum on peach (Restuccia et al. 2006), Penicillium chrysogenum on blackplum (non-commercial plum-like fruit) (Eseigbe and Bankole 1996) and Penicillium digitatum on nectarine (Navarro et al. 2011). Further investigation is needed to determine the importance of these Penicillium species in the stone fruit industry. Recent studies completed at the University of Pretoria have confirmed the presence of these Penicillium species in fresh produce supply chains (local and export) (Scholtz and Korsten, 2016). The presence of P. digitatum in stone fruit environments was already reported by Ma et al. (2003) more than 15 years ago. No project has since attempted to make a link between the presence of these pathogens and losses of stone fruits. A similar need persists for the pome fruit industry (Louw and Korsten, 2014). The nature of fresh produce chains (time and money), lack of industry transparency and suitable expertise therein hinder accurate identification of pathogens and quantification of losses (decay). Furthermore, export chain studies require a lot of funding and stakeholder support to make it a success. Based on this, a lab based study was done at the University of Pretoria to determine the pathogenic profile of some of these Penicillium species on various nectarine and plum cultivars.
Results and discussion
New findings confirmed P. digitatum and P. crustosum patho-genic on nectarine and plum. The most destructive species was P. digitatum, causing larger lesions than P. expansum in a shorter period of time (Fig. 1). The most important postharvest pathogen of citrus, Penicillium digitatum, has been confirmed a pathogen of pome fruits (Louw and Korsten, 2014) and is now also highly aggressive on stone fruits. This pathogen caused the largest lesions on most of the cultivars evaluated. Only with Sunburst and Honey Star was P. expansum the most aggressive. The third largest lesions were caused by P. crustosum and the smallest by P. solitum. Similar to P. expansum, P. crustosum can also produce penitrem A and patulin (Frisvad and Samson 2004; Frisvad et al. 2004; Pitt and Hocking 2009), which are harmful mycotoxins. This is a particular concern in juicing and processing facilities. Decay associated with P. solitum can be regarded as negligible (not a concern to industry). Disease incidence (amount of rotten fruit), on the other hand was consistently high for P. expansum (89.47-100%) and P. crustosum (96.67-100%), but fluctuated significantly for P. digitatum (7.5-100%) and P. solitum (21.88-100%).
Decay caused by P. digitatum depended on specific conditions. Disease incidence was higher and lesions larger on fruit that were older or riper (Louw and Krosten, 2016). This was particularly the case with nectarines. Disease incidence for P. digitatum on NE
3-48-49 was 20 % one day after the harvest of fruit. The diameter of lesions was 77.28±4.73 mm after seven days incubation. With the second experiment (two days postharvest), disease incidence was 80% and the lesion diameter comparable (72.18±12.08 mm). In the case of Bright Pearl, no lesions were caused on one-day postharvest fruit, whereas five-day postharvest fruit yielded lesions of 26.60.18±11.07 mm in diameter, at 40% disease incidence. New disease symptoms were observed on nectarine and plum cultivars (Fig. 2) (Louw and Korsten, 2014). Symptoms were easier to detect on light coloured cultivars. Penicillium digitatum caused green mould while P. expansum, P. crustosum and P. soli-tum caused blue mould on nectarines and plums. Decay started as softening and discolouring (browning) of infected and colonised tissue. Lesions were sunken; and white mycelial grew on the brown infected tissue [after three days ambient (room conditions) storage], followed by spore forma-tion (after three to five days storage). It was only after sporulation that symptoms became distinguishable (able to link the pathogen to the disease). Penicillium crustosum was the fastest to sporulate, and P. solitum the slowest. Although P. digitatum was slow to produce mycelia in comparison to the large lesions, sporula-tion followed quickly thereafter. With P. digitatum, rotten fruit later had a wrinkled appearance that became apparent when it was overlaid with the lime green spores. Overall, it was observed that more spores were produced on the nectarine than on the plum cultivars. The disease incidence and lesions diameter of Penicillium species from different environmental backgrounds (pear and citrus export chains) were compared. The different environmental isolates didn’t deliver significantly different results. This showed that the environmental background of the isolates had no impact on the severity of decay. It also illustrated the potential of cross-contamination and -infection of citrus pathogens on stone fruits and vice versa. This was further confirmed when scanning electron microscopy was used to view the germination, infection, colonisation and sporulation (production of new spores) of P. digitatum, P. expansum and P. crustosum on nectarines, pears and lemons (Fig. 3). All the Penicillium species were able to germinate and grow on all of the hosts within 48h ambient storage. Sporulation was observed for P. expansum on lemons and P. crustosum on nectarines, and P. digitatum on nectarines and lemons within the 48h. Sporulation on the other fruits would most probably have occurred if they were stored for longer periods of time. This is important to note when different fruit types are handled, transported and stored in close proximity to one another. It is a key factor in effective management of a product and retaining quality by ensuring that inoculum build-up does not occur in the supply chain that may later result in cross-contamination, if other fruit types enter the contaminated space. Temperature, fruit ripeness and inoculum loads are important factors to consider when dealing with cross-infection of pathogens on new hosts. At the end, or close to the end of fresh produce supply chains (i.e. repack facilities), fruit tends to be riper; the cold chain could be broken; inoculum levels could be high; and different fruit from different countries is handled in the same facility. This environment is ideal for cross-contamination and -infection of fruit resulting in decay.
Fig. 1: Lesions (mm) caused by different Penicillium species on nectarine and plum cultivars (20 fruit per cultivar) after seven days incubation (five days incubation for Nectar-gold) at room conditions.
(ADAPTED FROM LOUW AND KORSTEN, 2016)
Fig. 2 Symptoms caused by Penicillium species on nectarine and plum cultivars after seven days incubation at ambient conditions.
(LOUW AND KORSTEN, 2016)
Fig. 3 Scanning electron micrographs of Penicillium species on nectarines, pears and lemons.
A- P. crustosum sporulating on nectarines (48h).
B- P. digitatum sporulating on nectarines (48h).
C- P. expansum porulating on lemons (48h).
D- P. expansum producing metula (sporulation structures) on nectarines but no conidia (48h).
(ADAPTED FROM LOUW AND KORSTEN, 2016)
This study illustrated the pathogenic ability of P. digitatum, P. crustosum and P. solitum on nectarines and plums. Penicillium expansum is a well-known pathogen of stone fruits. Lesions caused by P. solitum were small and thus considered unimportant to industry. Little is known of P. crustosum and P. digitatum on stone fruits, even though they seem to pose a potential threat to industry. Apart from decay, P. crustosum is also a concern based on its ability to synthesise mycotoxins (i.e. patulin and penitrem A). The importance of proper management in the fresh produce chain was highlighted. The ripeness of fruit, susceptibility of the cultivar, inoculum load and diversity, and environmental conditions (cold storage) are all important factors to consider. Penicillium species are postharvest pathogens that tend to become more of a concern later in fresh produce chains when the above mentioned factors are more in favour of the pathogen. Penicillium digitatum was observed opportunistic on stone fruit but able to cause rapid decay when conditions were conducive. This has also been observed on pome fruits [Louw and Korsten (2014) and Vilanova et al. (2014)]. Effective management of the cold chain, ensuring good facility hygiene and careful handling of fruit to prevent cross-contamination, should remain the most effective weapon against decay and losses. It’s when citrus, pome and stone fruits are handled in close proximity that Penicillium digitatum could become a major concern. In SA, the export seasons of these fruit types overlap and should therefore be afforded special consideration. Higher inoculum levels of P. digitatum can also be present in fresh produce chains where citrus fruit is handled. When it comes to citrus (primary postharvest pathogen of citrus), Penicillium digitatum can tolerate much harsher conditions and, therefore has a larger footprint when it is involved. Also, stone fruit is not targeted for specific prevention or treatment against P. digitatum, since it has never been consid-ered a pathogen of the fruit. There is, therefore a need to investigate the importance of P. digitatum on stone fruits in fresh produce chains.
This work is based on research supported in part by the National Research Foundation (NRF) of South Africa [UID: 78566 (NRF RISP grant for the ABI3500), UID: 97884 (student support)]. The grant holders acknowledge that opinions, findings and conclusions or recommendations expressed in any publication generated by the NRF-supported research are that of the authors, and the NRF accepts no liability what-soever in this regard. We also thank the commercial producer who participated in this study.
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