Although the destructive phylloxera insects have been reported feeding on grape rootstocks in several declining northern California vineyards, the cause of the damage appears to be fungal activity rather than the loss of rootstock resistance, report researchers at the University of California, Davis.
The scientists suspect that disease-causing fungi, spread by phylloxera, are responsible for the weakened vines at some previously infected vineyard sites. The research results suggest these fungi are spread by phylloxera as they taste and probe among feeder roots and older storage roots of the grapevines, leaving the roots more vulnerable to fungal decay.
The scientists, who are conducting studies at several field sites in Napa, Sonoma and Mendocino counties, presented their findings at the Lake and Mendocino Winegrape Day earlier this year.
“There is no reason to question the utility of either the 101-14 Mgt. Rootstock or the Teleki 5C rootstock,” stressed Jim Wolpert, chair of UC Davis' viticulture and enology department. “Nor is there any indication that any other phylloxera rootstock with pure American species parentage is failing.” However, the researchers noted that populations of phylloxera were higher than normal on the immature roots of the strongly resistant rootstocks, and that such populations are not normally associated with vine damage.
Phylloxera are aphid-like insects that feed on grapevine roots, eventually killing the vines. During the later part of the 1800s, phylloxera destroyed two thirds of Europe's vineyards. The pest arrived in California about the same time, either directly from its native range in the eastern United States or indirectly via Europe.
California viticulture adopted the European solution to the problem, using rootstocks developed from resistant American species. These rootstocks performed well with the exception of a short but unfortunate bout with the poorly resistant rootstock AXR#1 at the end of the 20th century.
For the past year, UC Davis professors Jeff Granett and Andy Walker, with expertise, respectively, in entomology, and grape rootstocks and breeding, have been investigating several sites where phylloxera-associated damage was reported. Their research is funded by the American Vineyard Foundation and the Viticulture Consortium.
Several of the vineyards studied had been planted on rootstocks that are not resistant to phylloxera. At other study sites where vines were damaged, phylloxera were rarely found.
Phylloxera were present at four sites where the declining grapevines were planted on the phylloxera-resistant rootstocks 101-14 Mgt. And Teleki 5C. The researchers observed that shoot growth was reduced on these vines and phylloxera populations were unusually plentiful, but not overwhelming, on young feeder roots. However, phylloxera were not found feeding on the vines' mature roots, suggesting that those rootstocks were still resistant to the pest.
The researchers did find that plant-feeding nematodes — tiny soil-borne worms — were at the research sites, but their numbers were the same among both the strong and the weakened vines.
Disease-causing fungi also were found among the vineyards studies, appearing much more virulent in the soils of the damaged vines.
Deep roots left over from vineyards where phylloxera-susceptible vines were removed many years before can continue to provide a food source for phylloxera and the fungal strains, as well as for nematodes associated with the decline of roots.
This would explain the phylloxera-associated decline observed in vineyards that had been previously planted to the rootstock AXR#1, known for its poor resistance to phylloxera. AXR#1 was widely removed in the 1990s from California vineyards due to phylloxera and associated fungal activity.
Granett and Walker suspect that phylloxera populations are elevated somewhat by the effect of irrigation on the initiation of susceptible feeder roots, and that the previously killed AXR#1 roots, now buried deep in the replanted vineyards, have helped to maintain virulent fungal strains that have subsequently been transferred to the mature storage roots by phylloxera probing.
They note, however, that an alternative scenario could be that the virulence of the phylloxera has changed.
Research is being planned for the next calendar year to explore these two possibilities. Since phylloxera appear to partially adapt at infesting immature feeder roots of most rootstocks now in commercial use, the results do not suggest that one of the strongly resistant rootstocks is better or worse than the others under these circumstances. In addition, they note that many acres of vineyards have been replanted with the 5C and 101-14 rootstocks, and symptoms of decline in these replanted vineyards are rare.
Granett and Walker stress that phylloxera have been observed for many years feeding on root tips of resistant vines, but reports linking root-tip feeding to decline of the grape vines are rare.
They urge vineyard managers to be vigilant and notify them of any unusual phylloxera activity.
“The university takes seriously all reports of phylloxera and declining vines and is in the process of evaluating these sites to determine what is happening,” Wolpert said “There is no evidence of phylloxera-related decline on the vast majority of the replanted vineyards.