In crop improvement programs for disease resistance, a number of strategies are used to acquire the desired trait. One source of plants with improved disease resistance can be material that is recovered from plant tissue culture. The term plant tissue culture encompasses several different types of culture. Some specific types of culture include embryo culture, meristem culture, tissue or callus culture, and protoplast culture. Incorporation of an antiviral compound into tissue culture medium can eliminate viruses from plant tissue. Embryo rescue is used to ensure the survival of immature embryos when wide crosses are made between distantly related plants. During the tissue culture process, genetic variability can occur. Somaclonal variation can be a source of disease resistance in crops where genes for resistance are not available or are difficult to introgress in a breeding program. The main advantage of protoplast fusion is that it provides a possible method of hybridization between two parents where crossing barriers may exist.