Reduction of post-harvest losses in tomato using fungal bio-preservative for smallholder farmers

Abstract

Tomatoes (Solanum lycopersicum) are among the important fruits that are widely grown globally. However, they are susceptible to spoilage by fungi due to their high water content and soft endocarp. This spoilage leads to post-harvest losses (PHLs), which make tomato production unprofitable for majority of farmers in developing countries. These PHLs of tomatoes have been estimated to be 42% of annual global harvests. Accordingly, this study aimed to isolate, identify and characterise non-pathogenic, antagonistic fungal strains for the management of PHLs in tomato fruit. The tomato fruit (Cultivar Jasmine) were harvested from the research field of the Agricultural Research Council- Vegetable and Ornamental Plants (ARC- VOP) in Roodeplaat, South Africa. Fungal species were isolated and characterised from the collected tomatoes in order to evaluate their antagonistic properties against known fungal pathogens that cause spoilage in the tomatoes. The fungal colonies were morphologically identified with such identification being further confirmed by phylogenetic analysis of the internal transcribed spacer (ITS) regions 1 and 2, using ITS1 and ITS4 universal primers. The dual culture technique was used to test for the antagonistic ability (percentage growth inhibition (PGI)) of the isolates against eight pathogenic fungi (Rhizopus stolonifera ATCC 6227a, Rhizopus stolonifera ATCC 6227b, Geotrichum candidum ATCC 34614, Fusarium solani ATCC 36031, Fusarium oxysporum, Rhizoctonia solani, Alternaria solani and Alternaria alternata). Those fungal isolates that showed antagonistic properties against the pathogens were further tested for antifungal susceptibility, bile and acid tolerance, in order to test their abilities to serve as probiotics when they are consumed with the tomato fruit. The efficacy of these antagonists to reduce weight loss and spoilage percentage of tomatoes were evaluated in a 15 day storage trial under two storage conditions (8 °C refrigeration and uncontrolled room temperature). A total of 40 pure fungal isolates were identified and then clustered into 17 distinct operational taxonomic units (OTUs) based on 97% sequence similarity. The fungal isolates comprised 10 genera which were identified as Penicillium, Fusarium, Curvularia, Alternaria, Cladosporium, Lecythophora, Aureobasidium, Byssochlamys, Retroconis, and Epicoccum. Penicillium and Fusarium genera had the highest occurrence of 22.5% each as compared to Curvularia (15.0%), Alternaria (12.5%), Cladosporium (10.0%), Lecythophora (5.0%), Aureobasidium (5.0%), Byssochlamys (2.5%), Retroconis (2.5%), and Epicoccum (2.5%). Four fungal isolates (Byssochlamys spectabilis, Curvularia kusanoi, Epicoccum thailandicum and Retroconis fusiformis) showed high PGI against the growth of tomato fungal pathogens and were selected as antagonists. These antagonists also passed most of the standard criteria used for grading probiotics. Thereafter, during storage the C. kusanoi and E. thailandicum were the only antagonists that could reduce the weight loss and the spoilage percentage of tomatoes. Hence, from the findings, it was concluded that C. kusanoi and E. thailandicum showed potential as antagonists to preserve tomatoes during storage and they also possess beneficial (probiotic) properties. They are therefore, promising as bio-preservative agents that could be useful in extending the shelf life of tomatoes at storage and thereby preventing PHLs.