Scorza: Resistant stone fruits

Developing Plum Pox virus resistant stone fruits

Ralph Scorza
USDA-ARS Appalachian Fruit Research Station
2217 Wiltshire Road
Kearneysville, WV 25430
Ralph.scorza@ars.usda.gov

Breeding for resistance to Plum pox virus (PPV) requires an extensive knowledge of the virus including modes of transmission and virus strain and isolate characteristics.  Since resistance breeding requires significant effort and allocation of resources, critical evaluation of sources and levels of resistance are required. Reports of resistance must be critically reviewed to evaluate parameters such as the strain used for inoculation, the method of inoculation (aphid or grafting), the length of time resistance was evaluated, and the precise characterization of the response to infection in terms of severity of symptoms, and restriction of virus replication or movement in the host.  Considering these parameters there are few sources of high level resistance thus far identified in stone fruit species. Once an appropriate level of resistance is identified, in virtually all cases resistance must be combined with desirable agronomic, market and consumer traits.  Breeding technologies may include conventional hybridization and selection (HS), marker assisted HS, rapid cycle breeding (RCB) approaches, and genetic engineering (GE). Each method presents opportunities and challenges. Conventional HS is a long-term expensive endeavor yet it has a long track record of success.  Marker assisted HS can dramatically increase the efficiency, although not the long-term nature of HS, provided that genetic markers can be identified that are sufficiently accurate. The development of appropriate markers, especially for a multigenic source of resistance, can be expensive and challenging but with considerable pay-back. RCB-assisted HS can significantly reduce the generation time and speed cultivar development and can be combined with molecular markers for a rapid, efficient breeding approach. Currently this technology requires an initial genetic engineering step and is suitable only to those species that are amenable to GE (the final varieties would not be GE). Genetic engineering offers the possibility of using resistance genes from stone fruits, other plant species, and other organisms such as the virus itself to provide high levels of resistance. Multiple resistance genes can be readily combined to provide broad based stable resistance, and GE is a relatively rapid process. GE also offers a possibility that no other technology offers and that is the ability to insert resistance genes into established varieties. Yet the technology is limited to those species or varieties amenable to GE and these are relatively few. Also, GE crops are subject to intense and rather long-term, and costly regulatory review and may be rejected by some consumers and growers. Successful development of PPV resistant stone fruits will require a combination of approaches, collaboration between breeders, virologists, and molecular biologists, and the support of the stone fruit industry.