White fungus on bird of paradise can be caused by powdery mildew, a fungal disease that thrives in humid conditions. Powdery mildew can cause white or grayish patches on the leaves, stems, and flowers of the plant. To treat powdery mildew, it is important to remove any infected parts of the plant and improve air circulation around the plant. Fungicides can also be used to treat powdery mildew.