An important component of seed production is ensuring the desired genetic identity of the subsequent generation that will be used for propagation. A key practice in maintaining genetic purity is called roguing, where fields are carefully examined and off-type plants removed. Off-type plants occur as random mutations, as a result of segregation in open-pollinated crops, or due to inadvertent mixture with seeds of other crops or cultivars. Seed crops must be carefully rogued prior to flowering to remove all off-type plants whose pollen may contaminate the remainder of the crop. Guidelines for when to rogue and what to look for are specific to each crop and cultivar and can be obtained from a seed certifying agency or the breeder who developed the cultivar. Roguing is also essential for removing male-fertile plants from female rows in hybrid seed production. Seedborne diseases, such as lettuce mosaic virus, may also be removed by careful rouging. The quality of the stock seed used for establishment determines the quality of the seed produced. Much more stringent criteria must be used for propagating the early generations than later ones. An advantage of the root-to-seed or bulb-to-seed production systems in biennial crops is that there is an opportunity for quality evaluation and roguing at the time that the roots or bulbs are dug and transplanted to the seed production field. In a seed-to-seed production scheme, where the plants are established in the final production field by seed and are not transplanted, there is no opportunity to rogue for root or bulb quality. In this case, the quality of the stock seed must be extremely high.