Survival of plant pathogens such as fungi, bacteria, nematodes, and, less commonly, viruses in association with soil particles or undecomposed plant matter in soil can provide an important source of inoculum for disease outbreaks. Sterilization, when strictly defined, refers to the nonselective inactivation of all organisms in a given substrate. Soil pasteurization is an example of partial sterilization which uses lower than maximum temperatures to destroy pathogens while preserving many beneficial microorganisms. The earliest attempts to eliminate soil-borne plant pests through heat were probably the indirect and unplanned result of primitive types of slash-and-burn farming in which litter is burned prior to cultivation. Roasting soil in flame-heated pans to control Thielaviopsis root rot and weeds was widely practiced in tobacco seed beds in the southern US starting in the late nineteenth and continuing through the early twentieth centuries. Hot water is often used effectively to rid plant propagative material such as seeds, bulbs, and nursery stock of pathogens.