Control of Fungal Spores on Surfaces with UV-C Exposure Necessitates Complete Inactivation to Prevent Mycorrhizal Network Establishment

Mold infestations on surfaces present significant challenges to public health. Germicidal UV-C irradiation effectively inactivates spores suspended in water, yet information on surface spore mitigation is surprisingly absent. We show the effectiveness of 265–275 nm UV-C light to mitigate Aspergillus niger on nutrient-rich surfaces. UV-C mitigation of surface molds differs from inactivating spores suspended in water due to the unique characteristics of mycelial structures. Complete preinactivation of all viable cells during UV-C exposure is crucial to prevent mycelia formation; otherwise, even a single spore can gradually spread, covering surfaces by producing a progressive mycelial structure. A UV-C dose of 144 mJ/cm 2 from 265 nm LEDs achieved complete preinactivation at lower concentrations (100–1000 CFU/plate), while higher concentrations required increased UV-C doses. Intermittent duty cycling of light delivery (10 min ON then 50 min OFF) at 275 nm delivered from side-emitting optical fibers achieved comparable mitigation to continuous irradiation. Insufficient UV-C exposure induced more resistant mycelial structures that shielded live spores beneath. This study highlights complete preinactivation of viable molds, or sustained inhibition by UV-C light, is more effective than UV-C posttreatment. Mycelial alteration triggered by sublethal stress helps spores to persist in unfavorable environments, where microbial control is the goal.