The effect of garlic extracts on the control of postharvest pathogens and postharvest decay of apples

[摘要] ENGLISH ABSTRACT: Apples are an important export commodity for the South African market, andpostharvest losses that occur as a result of decay due to infection with pathogenic fungi suchas Botrytis cinerea Pers., Penicillium expansum (Link) Thom. and Neofabraea alba (E.J.Guthrie) are of major concern for all parties concerned with fruit production and distribution.Decay control of these fungi is primarily managed through the use of syntheticfungicides; however, pathogen development of resistance to these fungicides and recentworldwide concern over healthier living and a greener environment has called for thediscriminate use of synthetic chemicals. This has opened up an avenue for the developmentof safer and more environmentally friendly alternatives to control postharvest decays. Theuse of plant extracts and essential oils are favoured as natural sources of antimicrobials whilststill being safe for human consumption and having no negative impact on the environment.Allium sativum (garlic) is one such plant species that is well documented for its valuein improving human health and is readily available for consumption not just as a flavourcomponent of food but also to be taken as a daily herbal diet supplement. Given theantimicrobial effectiveness of garlic against human pathogens and ailments, its value as anantifungal agent against postharvest pathogens causing grey mould, blue mould and bull'seye rot of apples was investigated in vitro and in vivo within this study. Furthermore, anattempt was made to elucidate the chemical components of garlic extracts by gaschromatography-mass spectrometry (GC-MS).All experiments in this study were carried out with garlic extracts prepared from freshgarlic bulbs. For the in vitro experiments, two extract preparations of garlic, one containingethanol (Extract 1) and one where ethanol had been removed by evaporation (Extract 2), wastested for antifungal action within an amended media experimental design. Both extractpreparations were each subjected to two dilution series (0-80% garlic extract) with water andethanol as diluents. Both extract preparations were successful at retarding pathogen mycelialgrowth and spore germination; however, concentrations of Extract 2 (ethanol evaporated) anddiluted with distilled water provided markedly better inhibition of B. cinerea andP. expansum than the ethanolic dilutions of extract 2. Both extract preparations yieldedsimilar inhibitory results when tested against N. alba. Due to the results achieved in theamended media experiments, the use of a crude garlic extract without ethanol and diluted in water was considered to be the best option for further tests throughout the remainder of thestudy. In vitro volatile effects of crude garlic extracts at concentrations between 0 and 40%garlic extract were subsequently tested. Garlic volatiles were effective in inhibiting pathogenmycelial growth and spore germination of all three pathogens, at lower concentrationscompared to the amended media experiments. In vitro volatile exposure with garlic extractswas more effective at inhibiting N. alba than direct application of the extracts.Curative and protective application of garlic extracts and clove oil for increasedfungal inhibition through synergism was tested by direct and volatile exposure to thepathogens in vivo on three economically important apple cultivars; 'Granny Smith', 'GoldenDelicious', and 'Pink Lady'. Direct exposure of artificially wounded and inoculated fruit tothe garlic extract and clove oil revealed that garlic extracts applied curatively but notprotectively effectively controlled decay caused by B. cinerea and P. expansum on all applecultivars. Both curative and protective applications were ineffective in controlling N. alba. Invivo volatile exposure to the garlic extracts and clove oil did not inhibit decay on any of thecultivars and was not effective against any of the three pathogens investigated.A full chemical profile analysis was done by GC-MS analysis of garlic extractsamples. The compounds diallyl disulphide, allyl methyl trisulphide, allyl methyl disulphideand dimethyl trisulphide were detected in relatively high amounts. This result suggests thatthe abundance of sulphur and sulphur related compounds detected may be responsible for theantifungal action noted in the experimental studies.In conclusion, garlic was shown to have antifungal activity against B. cinerea,P. expansum and N. alba. The pathogens used in this study were not compared with eachother, but undoubtedly each pathogens reacts differently to exposure to the garlic extracts. Itwould therefore be advisable to investigate the effects of the extracts on each of thepathogens in a more in-depth study. More investigations into the application of the garlicextracts is required before it may be recommended for use; however, results for the use ofgarlic extracts against these postharvest pathogens and the postharvest decay they cause arepromising.

[发布机构] Stellenbosch University