US grape specialists warn nursery vines are not automatically disease-free

A group of 23 grape-sector specialists warned on November 1 that vines purchased from nurseries should not automatically be considered free of damaging diseases. Writing in WineBusiness Monthly, contributors associated with the grape industry and the National Clean Plant Network’s grape program said growers establishing vineyards or replanting blocks should require recent negative test results and clear traceability to carefully screened mother plants.¹

Clean planting stock is available in the United States, but nurseries may handle both tested and untested material. The specialists said infected vines can remain in the ground for years, making an unsupported assumption about plant health an expensive mistake.¹

What qualifies as clean stock

The authors defined clean planting stock as scion or rootstock propagated from clean foundation vines that have undergone extensive testing and were found free of economically important viruses and other pathogens. Testing covers grapevine fanleaf virus, leafroll viruses, red blotch, Pierce’s disease and phytoplasmas.¹

The system has limits. In the United States, clean foundation vines are not tested for Allorhizobium vitis, the bacterium associated with crown gall, or for wood-colonizing fungi linked to trunk diseases. Buying clean stock therefore does not establish that a vine is free from every possible pathogen.¹

Clean foundation vines are generally produced and maintained at Clean Plant Centers affiliated with the National Clean Plant Network, a program of the USDA Animal and Plant Health Inspection Service. Centers operate in California, Florida, Missouri, New York, North Carolina and Washington, supported by industry funding, federal money and participating institutions.¹

A long and costly process

Producing one clean foundation vine typically takes five to 10 years and costs about $10,000 to $15,000. The process begins with intensive screening that can include high-throughput sequencing, bioinformatics and polymerase chain reaction assays. Accepted vines are then monitored annually for reinfection by pathogens transmitted by insect vectors and other pests, including mealybugs, soft scales, leafhoppers, treehoppers, planthoppers, psyllids, spittlebugs and dagger nematodes.¹

Clean Plant Centers hold only small quantities of each foundation accession, so nurseries multiply the material for commercial use. The original foundation plants are designated G1; subsequent propagation in nursery increase blocks produces G2, G3 and sometimes G4 material used for grafted or own-rooted vines. This expansion is necessary to meet demand, but each step away from the G1 source creates additional opportunities for reinfection.¹

Oversight of these later generations is not consistent nationwide. Later-generation vines are generally monitored at vineyard level rather than individually, while state certification programs differ substantially in their testing schedules and protocols. The National Clean Plant Network does not administer nursery-level certification.¹

Checks growers should request

The specialists recommended ordering vines at least one year before planting and asking which pathogens were tested in the mother vines used for propagation. Buyers should obtain recent negative results from a reliable laboratory and establish who conducted the tests, when they were completed and which methods were used.¹

Growers were also advised to visit nursery mother blocks in autumn, when symptoms of some diseases are more visible. Where possible, they should confirm that both scion and rootstock come from G2, G3 or G4 vines that tested negative for unwanted pathogens and pests for at least two consecutive years.¹

If doubts remain, growers can take samples directly from the mother vines intended for their order and send them to a laboratory whose accuracy has been independently verified. The authors’ recommendation is to treat traceability and test documentation as integral parts of the purchased product.¹

Economic and wine-quality consequences

The warning arrives as US grape growers contend with unusual weather, changing demand, new national policies and unstable international trade. In California, thousands of vineyard acres have been removed during an industry restructuring. The specialists argued that future investment should begin with healthy material rather than introduce avoidable disease into new plantings.¹

Infected stock can shorten vineyard life, weaken vine growth and reduce yields, increasing replacement costs and undermining long-term supply. Economically important viruses may also diminish fruit quality and change a wine’s sensory characteristics, extending the consequences from vineyard management to production costs, grape quality and finished wine.¹

The authors estimated that viruses of economic concern can cause losses of as much as $4,000 per acre annually through reduced vigor and production, poorer fruit quality, and failed or underperforming plantings. Their article compared blocks established with cleaner and less clean material, with the weaker block showing visible leaf reddening associated with leafroll disease.¹

Clean vines still require protection

Clean planting material is a starting point, not a complete safeguard. After planting, growers must continue sound vineyard management, regular scouting and control of disease vectors to protect vines from later infection.¹

The contributors included university scientists, extension educators, nursery executives, state agriculture officials, a winery representative and other specialists, many of them members or affiliates of the National Clean Plant Network’s grape board. Their conclusion was that growers must actively select verified clean stock: asking detailed questions before purchase can determine whether a new block becomes a productive investment or develops persistent problems originating with its first vines.¹