Deborah A. Golino, director of the Foundation Plant Materials Service at UC, Davis, says, in the past, own-rooted Vitis vinifera could tolerate some common viruses, yet modern rootstocks, selected for nematode protection, growth regulation, or other reasons, cannot.
Speaking at the recent San Joaquin Valley Table Grape Seminar in Visalia, Golino said an epidemic of newer rootstock varieties collapsing from latent virus disease during the 1990s was painful evidence.
Latent viruses, combinations of two, three, four, or more multiple viruses requiring sophisticated laboratory tests to sort out, may be expressed early or years after planting. Nevertheless, she said, they produce "a synergism that is far more than you’d expect."
Symptoms can range from stunting, shortened internodes, leaf discoloration, and leaf rolling to graft-union disorders. In some virus infections, no symptoms are obvious, although losses in yield and sugar occur along the way and the vines eventually die.
"It didn’t matter when wine and table grapes were own-rooted or on AXR-1 or St. George, which are highly tolerant of grapevine viruses."
One example of the new threat in virus problems in recent years relates to the Princess variety, a white, mid-season, table grape released by USDA breeder David Ramming in 1999. "It was so popular that at first growers took any budwood of it they could find," said Golino.
Trials on the variety by Jennifer Hashim, Kern County farm advisor, demonstrated differential effects of rootstock and virus. On own-rooted vines and some tolerant rootstocks, any virus carried by the Princess wood had very little effect. On the other hand, on some rootstocks the effect was fatal.
Golino posed a hypothetical case in which latent viruses could destroy Princess vines after viruses accumulated on succession of infected scions. Say a grower started with an old Thompson Seedless vineyard that had the customary fanleaf virus. The grower grafted over the Thompson Seedless to Flame Seedless 10 years ago. The Flames carried the corky-bark virus and one of the leaf-roll viruses, which are associated with many of the latent viruses.
Later, the grower decided to graft over to Red Globe, which, like all Red Globe in California, carried rootstock stem-lesion virus. That brought the number of viruses present in the vines to four.
In his latest topwork, say the grower decided to graft over to Princess. Even though the Princess has been tested and found to be virus-free itself, when grafted, it picks up all four viruses already present.
Then, Golino added, when budwood was taken from the Princess vine for budding to a rootstock, the resulting vine, doomed by the multiple infestations, can never be as productive as it should be.
"This is not an unlikely scenario. We have tested a lot of vines where there are indeed this many viruses. This is a real shame, especially with a new variety. The virus infection can harm sugar, productivity, and storage, and you may never know whether it’s the variety or not using clean wood."
The FPMS specialists at UC, Davis are researching the latent virus problem and what to do about it, and Golino had some early results of trials.
"We are looking at the effects on all parameters of growth. We have tested 22 different virus isolates and combinations of them. When you have a single virus, the effect on growth isn’t devastating and you may have a vine with nearly the productivity of the healthy control vines," she said.
"But when you have leafroll type 2 virus and one of the vitiviruses, it can have a devastating effect on many of our modern rootstocks, especially Freedom and Harmony."
Discussing other viruses affecting grapes, Golino said the most important group is the nepoviruses, or nematode transmitted, polyhedral viruses.
In California, the chief among these is grapevine fanleaf virus, which reduces yields by poor berry set, in severe cases reducing yield by 80 percent. It is vectored by the dagger nematode. Once the nematode and the disease are in the same vineyard, the cycle is very hard to break, since the virus can remain in root residue of a pulled vineyard for a decade.
The various symptoms of mottling, distortion, banding, and mosaic patterns have more to do with the variety, location and time of year than the virus itself.
While symptoms can be a guide in identifying disease, "the absence of symptoms does not mean the viruses are not present," she said.
Leafroll, caused by any of nine different viruses, produces the brilliant red and orange fall vineyard foliage in photos of tourist publications. It may draw visitors to wineries, but it does nothing good for the vines, restricting the phloem of infected vines and causing poor fruit color and delayed fruit maturity.