The effect of biocontrol agents and plant extracts on postharvest quality of ‘kent’ mangoes.

Abstract

Mango is a tropical fruit grown in all parts of the world, hence is economically important. This fruit is beneficial as it has medicinal properties helpful to human health. Mango fruit is climacteric with a high respiration rate, leading to rapid ripening, and limiting the shelf life. This also limits the import and export, whilst the fruit is under duress, there are also fungal pathogens that infect this fruit both pre and postharvest. Anthracnose is one of the diseases affecting mangoes which is caused by the fungal pathogen Colletotrichum gloeosporioides (Penz.) Penz. & Sacc. The susceptibility of mango fruit to fungal diseases affects the total yield and profits of the producers. Synthetic fungicides have been used to control the infection caused by fungal pathogens. However, excessive use of fungicides has a negative impact on human health and is hazardous to the environment. Notably, fungal pathogens develop resistance to fungicides over time. The chemicals such as prochloraz have also been used as a treatment in cold storage to prolong the shelf life of mangoes, and the risk of consumers ingesting these chemicals is reported. Contrary to fungicides, this study evaluated the efficacy of biocontrol agents and plant extracts against C. gloeosporioides and in prolonging the shelf life of mangoes. During the in vitro studies, 150 yeast and 150 Bacillus strains were isolated from different plant leaf materials and screened against C. gloeosporioides. A dual assay was conducted, and potato dextrose agar (PDA) was inoculated with the fungal pathogen and biocontrol agents and then stored at 28˚C for 7 days. The best two Bacillus isolates (SL and Sl) and 1 yeast isolate (Ba) that inhibited the pathogen by more than 65% and were selected from a dual culture assay in secondary screening and identified using BLAST. These isolates were further used as potential biocontrol agents against the pathogen in vivo on ‘Kent’ mango fruits. The BLAST results identified isolate Ba as Meyerozyma guilliermondii, isolates SL and Ss as Burkholderia contaminans. These isolates were overall the best treatments with mycelial growth inhibition of 67.91%, 70.00%, and 74.04%, respectively. M. guilliermondii and two B. contaminans inhibited anthracnose disease incidence on ‘Kent’ mango fruit by 50% compared to the control treatment (0%) after 7 days at 25˚C. The scanning electron microscopy (SEM) images showed breakage and shrinkage of C. gloeosporioides mycelia in vitro, and there was little damage to mycelia and no spore germination on mango fruit. The different plant extracts, Aloe vera, Tetradenia riparia, Pelargonium sidoides, and Moringa oleifera were prepared and screened against C. gloeosporioides. PDA was amended with different concentrations of the plant extracts (1%, 1.5%, 2%, 2.5%, and 3%) then inoculated with the pathogen, and stored at 28˚C

for 7 days. P. sidoides was selected as the best-performing plant extract at 2.5% and 3% concentrations with 100% mycelial growth inhibition in vitro. Anthracnose disease incidence was significantly reduced by P. sidoides at 3% and 2.5% concentrations (≤50% and ≤75%, respectively) compared to the control fruit with 100% infection. The SEM interaction of 2.5% and 3% of P. sidoides with C. gloeosporioides in vitro; and mango fruit treated with 2.5% and 3% of P. sidoides and the pathogen had a limited number of spores and thin mycelia in comparison to the control. Furthermore, this study evaluated the individual and combined effect of B. contaminans, P. sidoides, and carboxymethyl cellulose (CMC) edible coating on mango fruits at 10˚C for 21 days, followed by 7 days at 25°C. Untreated fruit had the highest mass loss (18.53%) compared to all treatments (CMC, BCA, BCA+PE and PE) which had 15.3%, 14.64%, 14.18% and 14.04%, respectively. Mango fruit coated with PE had DPPH scavenging activity at 51.11 μM TE/g DM, whereas CMC treated fruit had 29.66 μM TE/g DM, even lower than untreated fruit which had 36.52 μM TE/g DM. Phenolic content was recorded at 116.4 μg GEA/g DM on PE treated fruit compared to 95.1 μg GEA/g DM for the control fruit. The concentration of AA was 51.95 mg/g on PE treated mango compared to 24.61 mg/g on the control fruit. This study showed that B. contaminans, P. sidoides, and CMC edible coating can be exploited as postharvest treatments to extend the shelf life of mango fruit.