Fungus usually appears in black spots on leaves of plants or circles in your lawn. It can also appear as "rust" on Daylilies and other perennials. Fungus is best pretreated before it begins, but can also be treated once it begins. Living in the south with our climate, we tend to see it in the high humidity months. It's best if you rotate fungicides from time to time as well. We prefer Liquid Copper & Systemic Fungicides to prevent, treat and stop fungus.