Nurseries grow virus-free plants not just for the export market, but for the domestic market as well. Producing virus-free plants is important because viruses can stunt tree growth, reduce fruit yields, produce deformed, off color foliage, and increase susceptibility to other diseases. Virus-free trees grow faster, flower earlier and may be resistant to disease and environmental stresses. Twenty-eight nurseries, ranging in size from two acres to 1,400 acres, participate in Oregon’s virus certification program. Participating nurseries bear the cost of the program.
A virus certification program has been established in Oregon to include the fruit and ornamental plants belonging to the genera Malus, Prunus, Pyrus, Cydonia, and Chaenomeles. The program provides material discernibly free of economically harmful plant viruses and virus-like agents to commercial orchardists, nurseries, and homeowners. Plant materials produced under program guidelines meet standards required for export markets worldwide. The program is a cooperative effort between the Oregon Department of Agriculture and participating nurseries that have funded the program since its inception in 1976. Washington has a similar program and their nurseries participate in the program on a voluntary basis. Their program is financed by sales of nursery stock.
The virus-certified rootstock and cultivars are from registered stoolbeds obtained primarily from the Clean Plant Center of the Northwest program located at Prosser, Washington and approved sources which test for viruses and virus-like diseases. Budwood is ordered from these sources and budded onto virus-indexed seedling or clonal rootstocks to start their individual scion orchards for future propagation. These trees are planted on fumigated ground to establish a permanent scion orchard or mother block at the nursery. Each individual scion or budwood source tree is identified for tracking purposes to maintain a history of test results for diseases of economic importance. Similar records are maintained for the clonally propagated rootstocks, which are tested on a random basis.
To ensure that trees produced under the program remain free of virus diseases, registered scions and rootstocks are tested regularly, utilizing ELISA (enzyme-linked immunosorbent assay) protocols and molecular techniques, where it is applicable and appropriate. Many samples from scion trees and layering beds or seed trees can be tested each year due to the sensitivity and accuracy of these tests and the shorter turnaround time to obtain the results.
The intensive indexing of the scion orchards on a regular basis in the program has been instrumental in keeping the reintroduction of viruses and virus-like agents at a very low incidence level. If an infected tree is found in the scion orchard, it is removed so additional trees are not produced from that particular tree. At the present time there are very few insect vectors involved in spreading virus diseases in fruit and ornamental trees except for the recent outbreak of the aphid-transmitted plum pox virus. The primary method of reinfection of the Prunus species is by the spread of viruses on or in pollen and the associated movement with bees. During flower pollination, the virus particles are carried in the pollen tube; at fertilization, the virus infects the embryonic tissues. It is subsequently transported through the vascular system of the tree and it may take several years before symptoms are expressed. Although the efficiency of the process is low, the large amount of pollen involved in the system can provide an initial source of infection. Once infection occurs, the chance of spread to adjacent trees in the orchard increases.
Conversely, in the pome fruits this spread of virus disease by pollen is rare. Once cultivars are freed of harmful viruses, reinfection is slow. The main source of transmission would be by infected budwood, root-grafting, or soilborne virus vectors such as nematodes.
Budwood of the most common fruit cultivars are available from the Clean Plant Center of the Northwest program. There is a charge to obtain the material—current status can be found on their specific web page. There is additional information about submitting materials for virus testing and the fees charged for the various services that are offered. The varieties, which are grown for specific fruit growing regions, are changing constantly so information about local recommendations should be obtained from local county extension offices or grower representatives familiar with the area.
For more information, contact the Market Access & Certification Program Area – Plant Health Program (503-986-4620), Oregon Department of Agriculture, 635 Capitol St NE, Salem, OR 97301 or visit: http://www.oregon.gov/ODA/programs/PlantHealth/Pages/LabServices.aspx
In Washington, contact the Tree Fruit Certification Program (509-786-2226), Washington State Department of Agriculture, 24106 N. Bunn Rd., Prosser, WA 99350. For information about the Clean Plant Center of the Northwest program and the availability of virus-tested varieties, see their website: http://healthyplants.wsu.edu/.
The Clean Plant Center of the Northwest is part of the National Clean Plant Network (NCPN). The NCPN was created to protect US specialty crops, such as fruit trees, hops, and grapes, from the spread of economically harmful pests and diseases. The NCPN also ensures the global competitiveness of US specialty crop producers by creating high standards for our clean plant programs. The NCPN Fruit Trees commodity group has developed a state level model regulatory standard for the virus certification of fruit tree nursery stock. For more information about the NCPN and model regulatory standard, visit their website: http://ncpn-ft.org.