Avirulent Isolates of <i>Penicillium chrysogenum</i> to Control the Blue Mold of Apple Caused by <i>P. expansum</i>

Blue mold is an economically significant postharvest disease of pome fruit that is primarily caused by <i>Penicillium expansum</i>. To manage this disease and sustain product quality, novel decay intervention strategies are needed that also maintain long-term efficacy. Biocontrol organisms and natural products are promising tools for managing postharvest diseases. Here, two <i>Penicillium chrysogenum</i> isolates, 404 and 413, were investigated as potential biocontrol agents against <i>P. expansum</i> in apple. Notably, 404 and 413 were non-pathogenic in apple, yet they grew vigorously in vitro when compared to the highly aggressive <i>P. expansum</i> R19 and Pe21 isolates. Whole-genome sequencing and species-specific barcoding identified both strains as <i>P. chrysogenum</i>. Each <i>P. chrysogenum</i> strain was inoculated in apple with the subsequent co-inoculation of R19 or Pe21 simultaneously, 3, or 7 days after prior inoculation with 404 or 413. The co-inoculation of these isolates showed reduced decay incidence and severity, with the most significant reduction from the longer establishment of <i>P. chrysogenum</i>. In vitro growth showed no antagonism between species, further suggesting competitive niche colonization as the mode of action for decay reduction. Both <i>P. chrysogenum</i> isolates had incomplete patulin gene clusters but tolerated patulin treatment. Finally, hygromycin resistance was observed for both <i>P. chrysogenum</i> isolates, yet they are not multiresistant to apple postharvest fungicides. Overall, we demonstrate the translative potential of <i>P. chrysogenum</i> to serve as an effective biocontrol agent against blue mold decay in apples, pending practical optimization and formulation.