Furocoumarins and coumarins photoinactivate Colletotrichum acutatum and Aspergillus nidulans fungi under solar radiation

The increasing tolerance to currently-used fungicides is a major problem both in clinical and agricultural areas leading to an urgent need for the development of novel antifungal strategies. This study investigated the in vitro antimicrobial photo treatment (APT) of conidia of the plant-pathogenic fungus Colletotrichum acutatum and the ascomycete Aspergillus nidulans with the furocoumarins 8-methoxypsoralen (8-MOP) and isopimpinellin, and a mixture of two coumarins (7-methoxy coumarin and citropten). Subcellular localization of the photosensitizer 8-MOP was also determined in C. acutatum conidia. Additionally, the effects of APT on the leaves of the plant host Citrus sinensis were determined. APT with 8-MOP (50 μM) led to a reduction of approximately 4 logs in the survival of the conidia of both species, and the mixture of the two coumarins (12.5 mg L−1) resulted in a reduction of approximately 4 logs for A. nidulans and 3 logs for C. acutatum. Isopimpinellin (50 μM) displayed a reduction of 4 logs for A. nidulans but less than 2 logs for C. acutatum. Washing the conidia to remove unbound photosensitizers before light exposure reduced the photodynamic inactivation of C. acutatum both with 8-MOP and the mixture of the two coumarins. The reduction was smaller for A. nidulans. 8-MOP spread throughout the cytoplasm and accumulated in structures such as lipid bodies of C. acutatum conidia. No damage to orange tree leaves was observed after APT with any of the photosensitizers.