In vivo application of garlic extracts in combination with clove oil to prevent postharvest decay caused by Botrytis cinerea, Penicillium expansum and Neofabraea alba on apples

Highlights

• Garlic extracts and clove oil are effective in vivo antifungals when used against the pathogens Botrytis cinerea and Penicillium expansum.

• Garlic extract effectively reduced lesion diameter when applied curatively on all apple cultivars.

• Curative application of extracts was better than protective application of extracts onto the three apple cultivars.

• Garlic extracts and clove oil was not effective for the control of Neofabraea alba on apples.

Abstract

Curative and protective applications of garlic extracts and clove oil directly or through volatile exposure were tested in vivo for potential to inhibit decay caused by postharvest pathogens Botrytis cinerea, Penicillium expansum and Neofabraea alba on three apple cultivars, ‘Granny Smith’, ‘Golden Delicious’ and ‘Pink Lady’. Curative application of the extracts by direct exposure proved to be more effective than a protective application for decay management of B. cinerea and P. expansum on all three cultivars. The efficacy of garlic extracts and clove oil, individually, as well as a combination treatment, did not differ significantly (P < 0.05) from each other; however, all treatments significantly (P ≤ 0.05) reduced decay when compared to the control treatments. Direct exposure of fruit that were artificially inoculated with N. alba, to the extracts, did not result in any inhibition compared to the control treatments. Exposure of inoculated fruit to the volatiles of the extracts did not inhibit postharvest decay on any of the apple cultivars, and in some cases, resulted in increased lesion diameters. This study demonstrated that the curative application of garlic extracts has the potential to reduce postharvest decay caused by B. cinerea and P. expansum, when applied directly to the fruit.

Introduction

Postharvest losses of fruit are significant and can reach levels of up to 50% depending on the commodity (Janisiewicz and Korsten, 2002, Singh et al., 2011). Apples are prone to postharvest losses due to infection and subsequent decay caused by postharvest pathogens such as Botrytis cinerea, Penicillium expansum and Neofabraea alba. Government regulations regarding minimum residue levels (MRLs), coupled with public concern about the possible harmful effects of fungicides (Singh et al., 2012), as well as the concern over pathogens acquiring resistance to commonly used fungicides, have promoted the search for new and novel alternatives to agrochemicals (Combrink et al., 2011). Plant extracts and essential oils are a rich source of natural compounds that have antimicrobial potential (Okigbo et al., 2009, Shinde et al., 2011) and have thus come to the forefront in the search towards safer agrochemicals.

Extracts and essential oils derived from thyme, cinnamon, peppermint, sweet basil, ginger, chilly, clove and garlic (Burt, 2004, Ziedan and Farrag, 2008, Okigbo et al., 2009, Combrink et al., 2011, Singh et al., 2012) are among some of the plant extracts that have been investigated for their antimicrobial effects against plant pathogens. Investigations into garlic (Allium sativum L.), has highlighted a wide degree of antimicrobial applications (Ankri and Mirelman, 1999) for this plant. The antimicrobial action of garlic against plant pathogens, have been reported by Nashwa and Abo-Elyousr (2012) for the ability to reduce early blight disease on tomato. Okigbo et al. (2009) reported that garlic caused effective inhibition of mycelial growth of root rot pathogens affecting cassava, and controlled decays caused by species of Penicillium on citrus fruit (Obagwu and Korsten, 2003).

The use of plant extracts and essential oils targets a niche market such as organic farming; however, any available products are registered primarily for pre-harvest use (Dayan et al., 2009), leaving ample potential for use of such products for postharvest application. Furthermore, the majority of the work in the postharvest sector has focused on in vitro suppression of pathogens and little work has been done on the in vivo effects of plant extracts and essential oils on fruit and vegetables, postharvest (Combrink et al., 2011, Singh et al., 2011).

A previous study which addressed the effect of garlic extracts on postharvest pathogens in amended media (Daniel, 2014) showed that garlic extract preparations were successful at retarding pathogen mycelial growth and spore germination of B. cinerea, P. expansum and N. alba. In vitro volatile effects of garlic extracts at concentrations between 0 and 40% garlic extract were subsequently tested. Garlic extract volatiles were effective in inhibiting pathogen mycelial growth and spore germination of all three pathogens, at lower concentrations compared to the amended media experiments.

In keeping with this, the present study aimed to investigate the ability of garlic extracts and clove oil to inhibit decay caused by the postharvest pathogens B. cinerea, P. expansum and N. alba on apples.