In the plant world, rust is the common name for several fungal diseases that affect turfgrasses like zoysia and St. Augustine, ornamental plants like gladiolas and daylilies, and edible plants like corn, beans, and squash. The first sign of a rust infection is typically small white, yellow or orange spots that appear on the leaves, typically during the cooler months. As the disease advances, the leaves may develop lesions that turn yellow, brown, or even black, and may eventually fall off. --- "Rust", Gardening in a Minute.