. . . . . . . . is a phenolic factor present in onion which has anti-fungal properties.

Catechol is a phenolic factor present in onion that has anti-fungal properties.
The pungency in onion is due to sulphur bearing compound in the volatile oil allyl propyl disulphide.
The red colour of the outer skin of onion bulbs is due to protocatechoic and catechol a phenolic factor present in red onions which also has antifungal properties.