Although black or white streaks are shocking when they appear on an otherwise healthy lawn, the incidence of slime mold is rarely harmful. Slime mold is actually caused by the small round fruiting bodies of a special type of fungi, called Myxomycetes, which are regularly present in the soil. It usually appears on warm humid days in late spring or early summer after extended periods of rain. ---"Slime Mold: Only a Cosmetic Problem," Gardening in the Panhandle (6/17/2013)