Resistance breeding programs The first formal resistance breeding programs for apple scab began in the early 20th century with the development of the PRI Apple Breeding Program by Purdue University, Rutgers University, and the University of Illinois. Since its inception in 1945, the PRI Apple Breeding Program has used controlled crosses between cultivated apples and wild Malus species to develop 1500 resistant cultivars, 16 of which (including 'Prima,' 'Jonafree,' and 'Goldrush') have been named released into market. Modern genetic work has found that a total of fifteen genes may confer resistance to apple scab. Many of these genes have been isolated from wild Malus spp. populations in East Asia, where a high level of species diversity still remains. Of these resistance genes, the Vf (Rvi6) gene is the most well-studied and is currently being used by researchers seeking to develop resistant cultivars using transgenic technology. While the development of transgenic resistant cultivars may reduce management costs in orchards, limited market acceptance pose a barrier to early adoption by commercial growers. Moreover, researchers have observed a breakdown of resistance genes by Venturia populations, posing another significant barrier to the success of this technology. See also Venturia inaequalis References Apple tree diseases Fungal tree pathogens and diseases