The extent of an apparent mix-up in wine-grape rootstocks supplied to California nurseries in the late 1980s is being investigated by the Foundation Plant Materials Service at the University of California, Davis. The inadvertent switch may have resulted in some wine-grape growers planting rootstocks that are susceptible to "phylloxera," a tiny aphid-like insect that feeds on the leaves and roots of grapevines. Phylloxera weakens the vine, decreasing grape yield, and can eventually kill the vine. "We are working with the UC Davis Department of Viticulture and Enology to conduct surveys at nurseries that supply rootstock to vineyards," said Deborah Golino, director of the Foundation Plant Materials Service. The self-supported service screens and propagates disease-free grapes, fruit, nuts, strawberries and roses for release to commercial nurseries. "Preliminary data suggest that the industrywide level of vineyard contamination by the non-resistant rootstock is relatively low, probably less than 5 percent of the 039-16 plantings," she said. "We should have definitive results available in about two weeks." In the meantime, Golino recommends that nurseries refrain from propagating, distributing or selling suspect rootstocks of the variety involved in the mix-up until they can be checked using molecular DNA tests and expert examination. After the results of the testing are available, growers who have purchased rootstock should contact the nurseries to find out if it is likely that they received the non-resistant rootstock variety. The rootstock is the base of the vine, including its main stem and roots, into which any variety of grapevine can be grafted. The heartiness of the rootstock often determines the vigor of the vine. In the 1860s, phylloxera decimated wine-grape vineyards in Europe and California. Scientists, however, discovered that European grapevines could successfully be grafted onto the rootstock of native American wine-grape vines, which proved to be resistant to phylloxera attacks. Many phylloxera-resistant rootstock varieties were developed and performed well for decades in California. But in the 1980s it became apparent that an important once-resistant grapevine variety, the AXR-1, was falling victim to a new strain of phylloxera. Many growers had to replant large portions of their vineyards to rootstock varieties that had higher levels of resistance to the new type of phylloxera. The Foundation Plant Materials Service at UC Davis screens and propagates 339 different wine-grape varieties, including 40 rootstock varieties. Among these are the 039-16 and 043-43, UC-patented rootstocks originally developed for vineyard sites infested with fanleaf virus and its nematode vector. These two rootstocks were released to the industry together in the mid-1980s. The 043-43 is no longer recommended because of its susceptibility to some phylloxera strains. The 039-16 has questionable phylloxera resistance, therefore is recommended only for sites where fanleaf virus also is a problem. The two rootstocks are closely related and very difficult to tell apart. The 039-16 rootstock is largely used in older North Coast vineyards that have been contaminated with the fanleaf virus and related nematode, Golino said. Since there is no complete chemical control for the problem, use of the 039-16 resistant rootstock is the recommended strategy. However, because of its questionable phylloxera resistance, this rootstock is not recommended for any other vineyard situation, she noted. Last fall, Andrew Walker, an associate professor in the viticulture and enology department, discovered that several nursery vineyards, which produce rootstocks for sale to commercial grape growers, contained the non-resistant 043-43 variety along with the 039-16. He then developed a new DNA-based test to help determine the extent of the mix-up of the two rootstocks. Walker, an authority on wine-grape breeding and genetic identification of grapevine varieties, has been conducting the genetic tests at commercial nurseries around the state. At least three nursery vineyards, known as registered blocks, were found to be contaminated with the non-resistant 043-43 rootstock. Golino sent out a Nov. 6 letter to her nursery customers, alerting them to the problem and cautioning them to check any 039-16 varieties to confirm their identity. "Although the problem came to light last fall, a statewide investigation had to be delayed until spring because new growth is needed to allow both accurate visual inspection and genetic testing," Golino said. She noted that the two-rootstock varieties are so similar that DNA testing is sometimes necessary to confirm their identities. She explained that the Foundation Plant Materials Service changed its propagation procedures in the early 1990s in order to avoid just such a mix-up, which now appears to have occurred sometime in the late 1980s at some point in the distribution to growers. "In 1992 we instated very stringent propagating procedures," Golino said. "Each plant now is checked visually by Dr. Walker before it is sent out to a nursery, and we have developed new procedures for collecting and labeling rootstock cuttings." It may not be possible to pinpoint where and how the mix-up occurred because at least 10 years have passed and the rootstock varieties are so similar, she added. "When we complete the survey we will be able to provide nurseries with the necessary information to help their growers determine the likelihood that they have this problem," Golino said. "We're confident that this will not be a widespread problem."