Citrus blue and green molds caused by Penicillium digitatum, P. italicum, and P. polonicum, are the major postharvest diseases of citrus fruit. In the present study, Actinomycin X-2 (Act-X-2), a naturally occurring antibiotic produced by Streptomyces species, was found to show excellent antifungal effect against these three pathogens with a minimum inhibitory concentration (MIC) value of 62.5 mu g/mL for them all, which was better than the positive control thiophanate-methyl. Act-X-2 significantly reduced the percentage of spore germination, and highly inhibited the mycelial growth of P. italicum, P. digitatum, and P. polonicum with EC50 values being 34.34, 13.76, and 37.48 mu g/mL, respectively. In addition, Act-X-2 greatly decreased the intracellular protein content while increasing the reactive oxygen species (ROS) level and superoxide anion (O-2(-)) content in the mycelia of pathogens. In vivo test indicated that Act-X-2 strongly inhibited the infection of navel oranges by these three Penicillium species, with an inhibition percentage of >50% for them all at the concentration of 10 MIC. Transcriptome analysis suggested that Act-X-2 might highly influence the ribosomal functions of P. polonicum, which was supported as well by the molecular docking analysis of Act-X-2 with some key functional proteins and RNAs of the ribosome. Furthermore, Act-X-2 significantly reduced the decay percentage and improved the firmness, color, and sugar-acid ratio of navel oranges spray-inoculated with P. polonicum during the postharvest storage at 4 degrees C for 60 d.
