Effects of grapevine leafroll disease on wine quality (and when is a disease a disease?)

Gut reaction: Viruses cause disease. Disease is bad. Viruses are bad.

Gut reaction muted by a lot of recent genetics research: Viral DNA seems to be embedded in genomes all over the place. We’re not sure why a lot of it is there, or stayed there, or what it does while its there. Some viruses cause disease. Some don’t. Viruses are complex, and we probably don’t know the half of it yet.

A name like “grapevine leafroll-associated virus” gets you thinking about negative consequences. Rolled leaves don’t collect light efficiently, which means that they won’t contribute to the plant’s photosynthetic metabolism efficiently, which means that the plant may be malnourished, grow slowly, and/or not have enough energy to ripen fruit. Rolled leaves are bad. A virus that’s associated with rolled leaves is bad. But the virus is only associated, not causative. Some viruses in this general family of leafroll-associatedness aren’t associated with vine symptoms. And infected vines only show symptoms post-veraison (the stage of ripening at which grapes change color), even though they carry the virus in detectable quantities year-round.

Ergo, a group of vine and wine scientists headquartered in eastern Washington state designed an experiment to ask (published in PLOSOne, and therefore open-access to everyone): do grapes from vines with grapevine leafroll disease, and carrying one of these viruses (GLRaV-3), lag behind their undiseased counterparts throughout ripening, or only when vines show symptoms? Being particularly conscientious*, they also improved on existing studies of grapevine leafroll disease by collecting data for three consecutive years from a commercial vineyard, sampling grapes throughout the season but also harvesting grapes at the typical time and making wine from diseased and undiseased pairs, and subjecting those wines to (limited) chemical and sensory analysis. They also used own-rooted rather than grafted vines, which eliminates some potentially confounding variables.

Their conclusions, after collecting data over the 2009, 2010, and 2011 growing seasons:

  • Grapevine leafroll disease decreases vigor (as measured by cane pruning weights) and fruit yield in own-rooted Merlot vines in Western Washington.
  • Grapes from diseased/infected vines have lower total soluble solids (TSS) and higher titratable acidities (TA) (and, to a less dramatic degree, lower anthocyanin concentrations) than grapes from undiseased/uninfected vines, but only after vines begin displaying symptoms post-veraison.
  • Wines made from diseased grapes were browner and less intensely colored, earthier and less fruity, and more astringent compared with their undiseased counterparts.
  • However, panelists only correctly distinguished diseased from undiseased wines when served side-by-side in black glasses, removing the notable color differences from consideration and forcing them to differentiate on smell and taste alone.
  • Soluble solids, TA, and pH were all more dramatically affected than anthocyanins in diseased vines, which reflects the decoupling of anthocyanin development and sugar accumulation that happens late in ripening during which environmental conditions play heavy in anthocyanin development.

These conclusions probably do more for plant scientists than for commercial growers: data from one Merlot vineyard near Prosser can’t be precisely extrapolated to you, wherever you are, and thresholds for usable fruit are always a matter of context (the authors note that future studies should document the effects of grapevine leafroll disease on specific sensorily-important compounds). The study does add data points to a collection of statistically robust data that might help large companies make judgments about what they can include in their generic red blends before pH or some other parameter becomes a problem. But maybe the most interesting line of thinking here has to do with the nature of disease, and of relationships between viruses and diseases and symptoms. Do vines have leafroll disease before they exhibit symptoms? Where do we want to draw lines between normal or acceptable variation and disease symptoms? If a vine looks sad but makes grapes that make wine indistinguishable from happy-vine wine, and if genetic testing says that the plant also happens to have a virus, does that mean that the vine has a disease, or is it healthy?

Disease” can mean something different to the plant pathologist who looks at a vine, a geneticist who looks at the DNA of a vine, a commercial grower who looks at the fruit of the vine or a winemaker who looks at the juice it makes. That vine may be infected with viruses. Is the virus bad?

 

*Full disclosure: I know and think highly of several of the scientists on this team.