A multifunctional broad spectrum antimicrobial composition is described. The composition can be incorporated into a wood preservative, or used as an additive to provide antimicrobial properties to water-based wood coatings. The composition is a concentrated water-miscible emulsion containing 3-iodo-2-propynyl N-butylcarbamate (IPBC), carbendazim (BCM) and propiconazole (PPCZ), and has antimicrobial activity against a wide variety of fungal organisms, including surface molds, blue stain fungi and wood rotting fungi. The composition can act as an in-can preservative and is useful for antimicrobial protection of wood and wood-based substrates.