As two related grapevine diseases grow increasingly troublesome to California wine-grape growers, scientists at the University of California are stepping up their research efforts, but stress that the fungal diseases don't pose a major threat to the industry. Black-foot disease and young grapevine decline are both characterized by a slowdown in growth, smaller trunk size and a reduction in foliage. The vine's sap turns a dark brown or black, a symptom that has earned the diseases the nickname of "Black Goo." If the problem is identified early enough, vines can be coaxed back to health, but most growers find it more economical to remove the affected vines and replant. "We are seeing more cases of these fungal diseases in California, however we estimate that only about 10,000 out of 900,000 acres -- slightly more than 1 percent of the vineyard area -- are affected," said Doug Gubler, a Cooperative Extension plant pathologist who has been studying black-foot disease and young grapevine decline for several years. "In the overall picture of grape diseases, black-foot and young vine decline are fairly uncommon." The diseases are part of a complex of vine ailments associated with five different fungi, according to Gubler. Tests indicate that black-foot disease is caused primarily by one species of Cylindrocarpon and that young grapevine decline is caused by one or more species of Phaeoacremonium. "That's what muddying up the picture," Gubler said. "It's not yet clear which fungi get into the vines first and cause the problem, because all of them have the capability of penetrating the vines' vascular tissue, which transports water and nutrients throughout the plant. And we often find more than one species of fungi in affected vines." The Phaeoacremonium fungi have been known to exist in California vineyards for at least 60 years, while Cylindrocarpon is apparently new in California. But there has been a marked increase in cases of black-foot and young vine decline diseases in the past five years, Gubler noted. "Interestingly, it's been during that same period that many vineyards have been replanted to new grape rootstock that is resistant to phylloxera," he said, referring to a tiny louse that threatened to decimate some of California's finest wine-grape vineyards during the past decade. "It is possible that those phylloxera-resistant vines are more susceptible to these fungal diseases." Researchers also suspect that the black-foot and young vine decline diseases are more likely to affect vines that are experiencing some other type of environmental stress, such as insufficient water, improper planting or early fruiting. "Growers should make sure they are supplying adequate water to their young vines in order to encourage proper root development," Gubler said. "It's thought by winemakers that some amount of water stress produces premium fruit on established vines, but that type of management can be devastating to young, developing vines." Furthermore, growers should not attempt to produce a crop of grapes until their vines are at least four years old, he said. Because of the high cost of vineyard land, some growers may be tempted to produce a crop of grapes on younger vines, which can severely weaken the immature vines, leaving them vulnerable to disease. Gubler is working with two UC Davis postdoctoral researchers to determine whether the fungi live in the plant as well as in the soil and whether they work as lethal disease-causing agents or dwell collaboratively with the vine as plant parasites known as endophytes. The researchers also are developing a diagnostic test that will identify the diseases in a growing vine and new techniques for identifying the fungi in the soil. They also are working on new techniques for inserting fungicide into the vines using a process known as "vacuum infiltration," which would allow them to treat grape cuttings before they are sold. Some growers and consultants have theorized that these diseases are spread with plants from nurseries or even with grapevine cuttings. However, until more is known about the life cycles of the disease, it remains unknown whether that is the main source of disease or what control techniques at the nursery will be effective, according to Gubler. Often, the same nursery stock produces healthy vineyards in one vineyard site and problems develop at another, suggesting that management of the vines is an important factor. The university has examined its own Foundation Plant Materials Service, which supplies more than 95 percent of the grape rootstocks to California nurseries. The nurseries, in turn, provide plants to commercial grape growers. The purpose of the service, which dates back to the 1950s, is to keep a collection of healthy, correctly identified vines that can be made available to nurseries. Although the program focuses on virus disease, the general health of its vines is also checked regularly. "We have never seen young vine decline or black-foot diseases symptoms in our vines," said Deborah Golino, a Cooperative Extension plant pathologist and director of Foundation Plant Materials Service. "Furthermore, we have seen none of the characteristic black streaking and sap in the cut trunks of hundreds of mature vines that we removed from our vineyards in recent years. We do have a rigorous program to maintain our vines in good health, including state inspections twice a year. Until more is known about how to control this disease, there is little we can do except provide as much information as possible about the management of our vineyards." Golino added that the majority of young vineyards planted to vines that originated at the university plant service are thriving. "Our stock is the foundation of today's California grape and wine industry," she said. "Obviously the causes of black-foot and young vine decline disease are much more complicated than the original source of wood." "There is no doubt that the spores of the fungi that can cause these diseases are present in some vines at the Foundation Plant Materials Service vineyard," said Gubler. "But colleagues and I haven't found any vineyard in the world that doesn't have these fungi present at some level." While some in the industry have suggested that the spread of black-foot disease and young vine decline can be stopped using a hot-water treatment, Gubler warns that a yearlong study found such treatments to be ineffective. The study showed that even 51-degree Celsius (123.8-degree F) water applied to grape wood for 30 minutes didn't kill the disease-causing fungi in the wood. Gubler and Golino urge growers who suspect their vines may be infected with black-foot or vine-decline disease to contact their county farm advisors for an accurate diagnosis. "Young grapevines can lack vigor for a number of reasons related to farming practices," said Gubler. "We'd hate for growers to pull vines that aren't actually infected with these fungal diseases."