The present invention relates to compositions comprising a garlic extract and chitosan or a salt thereof. These compositions present an enhanced antifungal activity, or activity for controlling powdery mildew disease, in various types of plants, vegetables, fruits or flowers, particularly when compared to garlic extract alone or to chitosan or a salt thereof alone. These compositions also present an extended shelf life when compared to garlic extract alone. Methods for preparing such compositions and methods for using them are also disclosed.