Calla lilies grow from bulbs. Those planted in the ground flower in summer, after which the bulbs may be dug up and replanted the following year. Container plants may be maintained indoors year round. Calla lilies wilt for two reasons—one more serious than the other.
Calla lilies are susceptible to tomato spotted wilt virus, which causes leaf wilt. Lilies also wilt when they need to rest; wilting may mean they require a period of dormancy.
If a calla lily's leaves turn yellow and wilt or curl, the plant needs dormancy. White or brown spots and streaks on the leaves indicate the presence of tomato spotted wilt virus.
Colorado State University Extension recommends discarding calla lilies infected with tomato spotted wilt virus. If your plant needs a rest period, reduce watering for six weeks to allow it to rest, allowing the soil to dry. When watering resumes, the calla lily should again grow normally.