Which of the following is NOT a common fungicide ingredient?

Formaldehyde is not a common fungicide ingredient used in horticulture. Unlike the other options, which are recognized fungicides effective in controlling various fungal diseases in plants, formaldehyde is primarily used as a disinfectant and preservative due to its ability to kill bacteria and fungi, but it is not typically employed as a fungicide in agricultural practices.

Chlorothalonil, mancozeb, and propiconazole are all widely utilized in the horticulture industry to manage fungal pathogens. Chlorothalonil is a broad-spectrum fungicide effective against many foliar diseases. Mancozeb is a multi-site fungicide commonly used to protect crops from a range of fungal diseases. Propiconazole is a systemic fungicide that helps in the prevention and treatment of fungal infections by being absorbed by the plants.