Host-Pathogen-Fungal Interactions Provide Translative Potential for a Biologically-Based Approach to Mitigate Blue Mold Decay of Stored Apple Fruit

Blue mold is an economically significant postharvest disease of pome fruit that is primarily caused by Penicillium expansum. 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 Penicillium spp. isolates, 404 and 413, were non-pathogenic in apple, yet grew rigorously in vitro when compared to the highly aggressive P. expansum R19 and Pe21 isolates. Whole genome sequencing and species-specific barcoding identified both strains as P. chrysogenum. Each P. chrysogenum strain was inoculated in apple with subsequent co-inoculation of R19 or Pe21 simultaneously, 3, or 7 days after prior inoculation with 404 or 413. Co-inoculation of these isolates showed reduced decay incidence and severity, with most significant reduction from longer establishment of P. chrysogenum. In vitro growth showed no antagonism between species, further suggesting nutrient acquisition or niche colonization as the mode of action for decay reduction. Both P. chrysogenum have incomplete patulin gene clusters, yet tolerate patulin treatment. Finally, hygromycin resistance was observed for both P. chrysogenum isolates, yet they are not multi-resistant to apple postharvest fungicides. Overall, we demonstrate the translative potential of P. chrysogenum to serve as an effective biocontrol agent against blue mold decay in apples, pending practical optimization and formulation.