An apparent mix-up in wine-grape rootstocks distributed to commercial agricultural nurseries in the late 1980s may have a limited impact because of the relatively low numbers of mislabeled vines, according to a survey completed last week by the Foundation Plant Materials Service at the University of California, Davis. The survey examined a total of more than 14,000 rootstock vines in nursery vineyards and found that about 8 percent of those were not the disease-resistant variety they were thought to be. It is anticipated that all of those misidentified plantings will have been eliminated from California nursery sites by the next grafting season, when cuttings will be collected for making rootings and grafted vines. The rootstock is the base of the grapevine plant, consisting of all the vine's roots, onto which any variety of grapevine can be grafted. Nurseries purchase rootstocks from the Foundation Plant Materials Service to establish "increase blocks" -- plantings of rootstock vines from which they take cuttings to produce plants for sale to grape growers to plant in their own vineyards. The hardiness of the rootstock portion of the plant often determines the success of the entire vine. The 039-16 rootstock -- the variety at the heart of the mix-up -- is resistant to phylloxera, a tiny aphid-like insect that weakens grapevines by feeding on its roots. As phylloxera attacks the root system of the vine, it interferes with the vine's uptake of water and nutrients. Because there is no effective control for phylloxera, a grower's only recourse is usually to remove the infested vine and replace it with a rootstock that is more tolerant of the pest. Last fall, Andrew Walker, an associate professor in the UC Davis viticulture and enology department, discovered that more than a decade ago some of the 039-16 rootstock had gotten confused with the 043-43 variety, which is not resistant to phylloxera. Unfortunately, these two rootstocks are so similar that only a few experts can tell them apart. Walker found that he needed to develop a new DNA test this fall so that he could conduct a survey to determine the extent of the problem. "The 039-16 rootstock is used primarily in certain North Coast vineyards that are susceptible not only to phylloxera, but also infected with the grapevine fanleaf virus and the nematode that carries it," said Deborah Golino, director of the Foundation Plant Materials Service. The service is a self-supported operation that screens and propagates disease-free grapes, fruit, nuts, strawberries and roses for release to commercial nurseries. "Very little use of 039-16 is reported outside Napa and Sonoma counties," she said. Golino noted that the recently completed survey and DNA testing of suspect vines by Walker revealed that the level of contamination of nursery rootstock plantings with the phylloxera-susceptible 043-43 rootstock ranged from zero percent to 50 percent in individual blocks at nurseries. The purity of the statewide stock of 039-16 is probably a little higher than that found in the nursery survey, according to Golino. Because newer nursery plantings and those propagated from cuttings rather than whole plants are likely to be the correct rootstock variety, some of those vineyards were not included in the survey. The survey found no rootstock mix-ups in nursery plantings propagated after 1990 or in blocks propagated from cuttings. She noted that the level of contamination in individual growers' vineyards may vary tremendously because of the way nurseries propagate and distribute vine cuttings. "Scores of cuttings can be collected from an individual vine, bundled together and supplied to the same grower," Golino said. "This means that the contamination rate in a grower's vineyard could be either significantly higher or lower than the contamination rate in the nursery vineyard that supplied the cuttings." Furthermore, the impact on individual vineyards may vary according to soil type, cropping history and vineyard practices, she added. Some vines planted on the non-resistant 043-43 rootstocks may not fare as poorly as do vines planted on the same rootstock but grown under different conditions. Golino stressed that it is not economically practical for individual growers to conduct genetic testing on their vines. "Although the DNA tests used in the survey to help distinguish the resistant variety from the non-resistant are very accurate, it costs about $100 to determine the identity of a single vine," Golino said. "It would be prohibitive for an individual grower to even consider testing each vine in a vineyard." Instead, growers are advised to contact their nurseries to find out the source of the vines in their fields. If the vines came from an uncontaminated source, it is unlikely that there will be future problems. If the vineyard was planted to 039-16 rootstocks from nurseries found to be affected by the mix-up, growers should keep an eye on their vines. If any of those vines appear to be failing, growers should get in touch with their county UC Cooperative Extension farm advisor. "Growers who suspect their vines are declining can contact us for help in diagnosing the cause of the problem," said Ed Weber, viticulture farm advisor for Napa. "In most cases the mix-up in the rootstock varieties would produce problems in only a few scattered vines, rather than a vineyard-wide decline," added Rhonda Smith, viticulture farm advisor for Sonoma County. "The farm advisors will be monitoring the situation in their individual counties and alerting us to the scope of the problem," said Jim Wolpert, chair of the UC Davis viticulture and enology department and a Cooperative Extension viticulture specialist. "We, on campus, will be prepared to work in concert with the farm advisors to assist growers in any way we can." Golino said the procedures used by the Foundation Plant Materials Service to propagate and distribute rootstocks changed dramatically in 1993 when new procedures were put in place. "Highly qualified staff now collect and individually label all cuttings before they are placed in the greenhouses," she said. "And before those cuttings are sent out they are double checked by experts in grapevine identification to make sure that the nursery is receiving the plant material it ordered. In addition, FPMS strongly urges all nurseries to have professionals inspect their increase blocks before any wood is used for any growers' orders to catch possible mix-ups at FPMS or the nursery. "It's highly unlikely that a mix-up like this could occur with the multiple checks we have built into our current operations," she said. Media contacts: -- Deborah Golino, Foundation Plant Materials Service (530) 754-8102, dagoino@ucdavis.edu (Golino is out of the country during July.) -- Jim Wolpert, Viticulture and Enology, (530) 752-0381, jawolpert@ucdavis.edu -- Ed Weber, Napa Co. UC Cooperative Extension, (707) 253-4221, eaweber@ucavis.edu -- Rhonda Smith, Sonoma Co. UC Cooperative Extension, (707) 527-2621, rhsmith@ucdavis.edu -- Patricia Bailey, News Service, (530) 752-9843, pjbailey@ucdavis.edu