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| Inhibitory Effects of Five Antifungal Substances on Development of Postharvest Pathogen Rhizopus oryzae |
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Abstract Rhizopus oryzae is a major postharvest pathogen of tropical fruits and vegetables, and quite harmful to storage and preservation wihch result in huge economic losses. It can cause Rhizopus soft rot in fruit, destroy fruit quality and flavor, and also produce the ergot alkaloid agroclavine which is toxic to humans and animals. In the present study, we isolated the main pathogen from naturally infected strawberries (Fragaria ananassa L. cv. Zhangji), and identified it by morphological observation and internal transcribed spacer region of ribosomal DNA (rDNA-ITS) analysis. After that, five exogenous antifungal substances including sodium bicarbonate (SB), boric acid (BA), cinnamon oil (Co), sodium nitroprusside (SNP) and phosphite (Phi) were selected for known antifungal activities and commercially availability. And then, inhibitory effects of five substances on pathogen development and pathogenicity were evaluated via assessment of physiological properties, biochemical indexes and inoculation experiments on tomato (Solanum lycopersicum cv. Ailsa Cragi) fruits. Results indicated that the fungus isolated from strawberry was demonstrated as the main pathogen through an artificial infection test and it was highly pathogenic. The infected position of fruit appeared obvious browning, softening and hydration, and the symptoms were similar to those of soft rot caused by Rhizopus stolonifer. Meanwhile, after amplification using ITS universal primers, an approximately 600 bp fragment of ITS rRNA gene sequence was obtained and perfectly matched with that of the R. oryzae strains. In addition, antifungal activities of five substances were positive and dose-dependent. When the concentration of SB, BA, SNP, Co and Phi reached 0.2%, 0.2%, 0.5%, 0.02% and 5 mmol/L, respectively, spore germination rates of R. oryzae were all reduced to below 10% after 6 h of incubation, versus the germination rate was over 90% in the control at the same time. Therefore, these concentrations were considered to be the minimum inhibitory concentrations (MICs) of five substances. Under the MICs, despite the difference of antifungal efficacies, all of five exogenous antifungal could significantly delay germ tube elongation and mycelium extension of R. oryzae, lower fungal biomass accumulation, disturb carbohydrate absorption of pathogen, and effectively control soft rot of tomato caused by R. oryzae. Among five substances, BA, Co and Phi showed more inhibitory efficacies than those of SB and SNP in in vitro experiments; SB, BA and Phi worked better than Co and SNP in in vivo experiments. Our results explored the possibility of employing those antifungal substances as part of general program for management of postharvest diseases caused by fungal pathogens, and will provide some theoretical basis for integrated control of R.oryzae.
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Received: 16 June 2014
Published: 06 January 2015
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