An Investigation of Alternative Antifungals against Phyllosticta citricarpa Kiely and Guignardia mangiferae

Abstract

Citrus fruits have been cultivated for many years and consumed mostly as fresh produce or juice and considered as one of the most important crops in the agriculture sector (Abirami et al., 2014: Paul, 2006). In South Africa, the citrus industry significantly contributes to the economy as the second largest earner of foreign exchange in terms of agricultural exports (Mabiletsa, 2003). However citrus production can be influenced negatively by pathogenic diseases resulting in significant economic decline such as Citrus Black Spot (CBS) caused by fungal pathogen Phyllosticta citricarpa (Timmer and Duncan, 1999; Davies and Albrigo, 1994). Recently infections due to P. citricarpa have resulted in lower citrus yield and huge economic loss impacting on the labour market due to job losses (Truter, 2010; Paul, 2006; Timmer and Duncan, 1999). (CBS) is a fungal disease of citrus leaves and fruit that causes superficial lesions on the rind of the fruit. However there are other strains associated with citrus, such as Guignradia magniferae (Meyer et al., 2001). This strain is non-pathogenic and does not cause citrus black spot symptoms. This endophytic fungus is able to live within a plant without causing apparent disease unlike P.citricarpa known to result in CBS infection. In South Africa including other countries such as China and Australia P. citricarpa is considered to be phytosanitory important because of its role in international trade (Truter, 2010; Baayen et al., 2002). If the fruits contain CBS lesions, they can result in the rejection of the whole imported batch. When lesions are spotted on the fruits, repackaging is recommended and fruits could be sold at a lower market value, resulting in huge economic losses. Synthetic fungicides with antimicrobial activity are used to extend the storage-life of citrus fruits (Halueendo, 2008). However, over the past two decades, there has been great public concern about safety and side effects of synthetic agents in food preservation besides health implications (Ayoola et al., 2008). These agents are known to remain on the plant or within its tissues following treatment resulting in potentially toxic and carcinogenic effects on human and food systems (Sellamuthu, 2013). This indicates that growers and suppliers of citrus fruits are faced with the challenge of providing consumers with products that are attractive, free from disease, defects, toxic residues, and with longer shelf or storage-life (Sellamuthu, 2013). Synthetic fungicides often used to control fungi are pesticides that inhibit fungal growth by targeting reproductive structures (ascospores and conidia) known to be important in the fungal life cycle. These structures play an important role in the growth and dispersal of fungi that habitually cause huge damages in agriculture resulting in critical losses of yield, quality and profit. Over the years farmers have used synthetic fungicides to control fungal growth; however synthetic fungicides use has increased concern due to their toxicity, polluting the environment and antimicrobial resistance. Moreover regular use of fungicides results in risk particularly if residues are retained in soil or transferred into water. This has a negative effect on soil organisms and carries a potential risk to long-term fertility of the soil. Furthermore, fungal resistance to currently available synthetic fungicides related to the overuse and abuse is more prevalent. In most cases resistance occurs due to modification of the target site (Halueendo, 2008). This is a concern because the products may become less effective or even inadequate for controlling fungal pathogens. This has influenced the search for rational approach to limit repeated use of synthetic fungicides. Kock et al., 2007 showed that anti-inflammatory compounds such as acetylsalicylic acid (ASA), benzoic acid (BA) and salicylic acid (SA) possess antifungal properties. Although alternative methods are being researched to control postharvest decay during storage, natural plant products such as essential oils (EO’s) and their hydrosol are gaining fame and the attention of researchers globally due to their biodegradable, eco-friendly, economical and safety properties. The EO’s reported in various studies have shown to exhibit antifungal properties by targeting structures responsible for the life cycle of fungal organism as well such as ascospores and conidia for both in vitro and in vivo in different fresh produce (Prakash et al., 2015). However there is limited information regarding the effect of essential oils against P. citricarpa. Therefore EO’s can be ideal candidates for use as alternative fungicides as well against citrus black spot. In this current study, EO’s and antifungal anti-mitochondrial compounds were tested for antifungal anti-mitochondrial properties. This is shown by a unique bioassay, with the mitochondrion-dependent sexual structure from P. citricarpa and G. mangiferae serving as indicators. It was revealed that the pathogenic fungi, P. citricarpa and G. mangiferae are also dependent on increased mitochondrion activity to effect spore-release and structure development. This was observed from the XTT assay (an indicator of metabolic activity) results where the activity of mitochondrion dehydrogenases were affected for both organism after the treatment with EO’s. The study also showed that some cheaper alternative methods such as using hydrosol can be used as antifungals against CBS, with the advantage over essential oils of being water soluble and consisting of EO’s traces (Nazzaro et al., 2013).