Thi Thuy Tien Nguyen¹*, Hien Trang Nguyen¹, Thanh Long Le ¹, and Thy Dan Huyen Nguyen¹

¹Department of Engineering and Food Technology, University of Agriculture and Forestry, Hue University, Hue, Thua Thien Hue, 530000, Vietnam.

Abstract

Forty-eight Streptomyces strains were screened for antifungal activities against P. digitatum P2 and P. italicum I6 isolated from oranges infected with green mold and blue mold diseases, respectively, using the dual culture method. Among 48 screened strains of Streptomyces, S. murinus NARZ showed the strongest activity by completely inhibiting the growth of both pathogens. The antifungal activities of S. murinus NARZ were evaluated based on the percentage inhibition of radial growth (PIRG, %) using culture filtrate (CF) and ethyl acetate crude extract. The EC₅₀ values of the 7-day S. murinus CF were 13.25% and 33.72% against P. digitatum P2 and P. italicum I6, respectively. The EC₅₀ and MIC₉₀ values of the crude extract were 45.03 µg/mL and 193.15 µg/mL for P. digitatum P2, and 68.25 µg/mL and 295.60 µg/mL for P. italicum I6, respectively. Notably, out of the six International Streptomyces Project Media (ISP2, ISP3, ISP4, ISP4G, ISP4Y, and ISP8), ISP3 medium was optimal for S. murinus NARZ cultivation to produce antifungal metabolites, with 50% CF yielding 100% inhibition against both Penicillium pathogens. Furthermore, the CF had notable thermal resistance; the PIRGs decreased from 100% to 74.22% for P. digitatum P2 and to 64.59% for P. italicum P6 after treatment at 121°C for 15 min. The tests showed that oranges sprayed with 100% CF of S. murinus NARZ remained symptom-free of blue and green mold diseases after a seven-day incubation following artificial inoculation with Penicillium species. This study highlights the potential of Streptomyces strains, particularly S. murinus NARZ, as effective biocontrol agents against postharvest fungal diseases in oranges.

Keywords: Antifungal activities, Biocontrol, Postharvest diseases, Penicillium, Streptomyces murinus