Submitted to: Acta Horticulture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 20, 2000
Publication Date: May 1, 2001
Interpretive Summary: Germplasm of pome and stone fruits is imported into the United States from many countries. Approximately 150 items per year are imported by scientists for use in breeding programs to improve upon these crops, in such traits as insect and disease resistance, yields and flavors, and improved shelf life. There are many destructive diseases of these crops in other countries that do not occur in the US. This necessitates that each importation be held in quarantine for close observation and testing before it is released and distributed throughout the US. The focus of these tests are for viruses, viroids, and phytoplasmas using techniques such as grafting to sensitive indicator hosts, and laboratory methods to detect virus-specific proteins or nucleic acids. Items that have completed quarantine testing are distributed to scientists, commercial firms, and are also added to the national germplasm collections. The emphasis of this report is on recent changes and improvements in the procedures USDA uses to expedite imported stone and pome fruit germplasm through the quarantine process for the immediate and long term benefit of breeders and commercial growers. Information in this paper will also benefit other fruit tree virologists and regulatory officials in the US and worldwide.
Approximately 150 accessions of temperate fruit tree germplasm are imported annually into the USA through the US Department of Agriculture's National Plant Germplasm Quarantine Center in Beltsville, MD. These accessions are destined for USDA and university researchers, commercial interests, national repositories, and private citizens. Pome fruit species are tested for latent viruses using graft bioassays, while bioassays and ELISA are used to test stone fruit germplasm for viruses. Particular effort is made to prevent the introduction of plum pox virus into the USA. A recent advance is the incorporation of PCR techniques with nested, broad-spectrum primers to test for phytoplasmas such as apple proliferation, pear decline, and peach X diseases. The procedure is applied to extracts from both dormant budwood and leaf midribs. Tissue or dot blot hybridizations with cRNA probes are used to test for several viroids, including peach latent mosaic, apple scar skin, and pear blister canker viroids. The incorporation of molecular procedures has resulted in changes in USDA regulations, so that accessions received as dormant budwood may be eligible for provisional release after about one year in quarantine. During this period, the accessions are tested for all described quarantine pathogens at least once, and twice for some, such as plum pox virus and phytoplasmas. Efforts continue to incorporate procedures that improve the reliability of quarantine testing and reduce the time accessions are held in quarantine. Notably, experiments are in progress to introduce PCR testing for cherry green ring mottle virus and the apple latent viruses.