Clean Plant


The Process: Progression of grape material from importation to vineyards

By law, all imported grapevines must be quarantined by a USDA-approved facility to prevent the introduction of foreign pests and diseases into the U.S. Scientists at Cornell University and the University of California at Davis hold permits to import grapevines. Foundation Plant Services (FPS) at UC Davis is the largest facility and processes 50 to 100 foreign and domestic grapevine selections a year.  What follows is an ideal case of a foreign grape introduction being processed through the FPS “pipeline” from importation to State certified material.  However, each case is unique and there are many technical challenges that may delay the process.


Dormant grape cuttings on their way to get packaged for shipment.


The USDA National Plant Germplasm Quarantine Center is the first stop for imported grapevines coming into the US.

Source and Entry into the US

Imports come to FPS through cultivar exchanges, breeders or an individual. We use the term selection to refer to grapevines in our collections rather than cultivar or clone because it signifies that the grape is from a single vine source. Since cuttings are harvested when the vine is dormant, most selections arrive at FPS between January and March.

Prior to receiving the material at FPS, imported cuttings are visually inspected for pests and obvious pathogens at the USDA National Plant Germplasm Quarantine Center in Beltsville, MD. If they pass inspection, the canes are forwarded to FPS where they are inspected again by a county agricultural inspector.  Canes are dipped in a fungicide, a pesticide and finally hot water (hot water dipping eliminates the phytoplasma Flavescence doree).


Testing Begins: The Woody Index

Next, the single source vine canes are divided into two bundles - one is used for the woody index and one is used for propagation. For the woody index, dormant buds from the test canes (also known the candidate vine) are chipbud grafted onto virus-sensitive indicator plants. The indicators will show disease symptoms if a pathogen was transmitted from the buds to the indicator.


Chipbudding into the indicator plant.


Indicator plants are planted in a field and observed several times over two growing seasons. Here the trunk is being inspected for pitting symptoms.

Virus Elimination Therapy

The bundle of canes for propagation is used to make plants for lab testing and tissue culture treatment.  By December, these plants are often large enough to begin virus elimination therapy. Virus elimination is done using tissue culture techniques (watch a video of microshoot tip excision). Most selections are treated preemptively for virus elimination even before the results of virus testing are known in an effort to shorten the possible release time. Also, to qualify for planting in the Russell Ranch foundation vineyard selections must go through tissue culture for virus elimination even if virus tests are negative.  This is to eliminate viruses that may not be known yet and for which there is no test.


Herbaceous Host Index

Spring of the year after importation is the optimal time for another test - the herbaceous host index. Leaves and stems of the candidate vine are ground and inoculated to a panel of herbaceous indicator plants in the greenhouse. The indicator plants will show symptoms within a few weeks if the leaves of the candidate vine contained mechanically transmissible viruses, especially nepoviruses. 


The herbaceous host index detects viruses that are mechanically transmissible. Four species are inoculated.


Microshoot tip culture requires an average of 7 months for a tip to grow from a less than 0.5 mm to a plantlet about 6 cm.

qPCR Testing

Meanwhile, a panel of qPCR tests is performed on the candidate vine.  The panel includes tests for over 35 viruses, phytoplasmas and Xylella fastidiosa and is continually evaluated and improved to include the latest developments in grapevine virology (view the list of tests). In fact, members of the NCPN-G are world renown grapevine virologists and are the people responsible for discovering, characterizing, and testing technology for pathogen detection.


Test Results

After two years of growth in the field, the results of the woody index are complete and test results from all the different tests are compiled and compared. If all tests are negative, FPS submits a letter to USDA requesting release from quarantine. By the time a selection is released and is planted in the foundation vineyard at Russell Ranch, at least three years have passed since importation.  Now, the vines still need at least two years to grow onto the trellis system and become established before dormant canes are available.


Lab testing is extensive and uses state of the art technology.


Processing petiole samples for testing.

Fast Tracking and HTS

There are several ways used to speed up the process.  First, mentioned above, is that therapeutics are begun before test results are complete.  Secondly, FPS propagates green cuttings from plants before they are planted in Russell Ranch. This can reduce the time from import to availability to two years. Finally, high throughput sequencing (HTS) is used to test the plants. HTS is a powerful sequencing technology that can reduce the time required for testing to 2 to 3 months.  More information is available on the HTS Factsheet on the NCPN website. If a plant tests negative by HTS it can be released under provisional quarantine release status.  Under provisional quarantine release status, nurseries may begin to propagate vines before the woody index is complete.

Approximately half of the selections introduced through FPS are found to be virus infected.  The virus elimination treatment requires about one year and then testing is repeated to determine if the treatment was successful.  Overall treatment success is over 87% but some virus infections are more difficult to eliminate than others. Treatment and subsequent testing may add three more years to the process.


Domestic Selections

The progression of domestic material through the pipeline is similar with the exception of the dip treatments and USDA inspections.



Most foundation grapevine material is sold to nurseries to establish registered increase blocks.  Increase blocks require another 3 to 4 years to mature.  Nurseries harvest cuttings from their increase blocks to make dormant benchgrafts.  Dormant benchgrafts are plants that are grafted, rooted and dormant. It requires two years to produce a dormant benchgraft and they are grown in a block referred to as a certified nursery planting. From here, dormant benchgrafts are dug and sold to growers for the establishment of fruit production vineyards.  Nursery blocks and plantings are tested and inspected by the CDFA on a scheduled as well as as needed basis. 


It’s a long process - the time from when a selection is imported to when it is available to growers as clean, certified material can range from 10 to 20 years.  A presentation entitled ‘The Pipeline: From tissue culture to your vineyard’ with more details and photos of each step was presented as part of a clean plant webinar series hosted by Cornell.


Experience and economic studies have shown that planting with clean material greatly enhances the health of the vineyard and the fruit quality and quantity.  The NCPN-Grapes centers continue to provide clean, virus-tested for the benefit of grape growers throughout the United States.