Gary’s Corner

Photo by Ian McCausland

Viruses and Wine

By Gary Hewitt, DipWSET, CWE, FWS, Sommelier

It is no surprise that the topic of viruses has been on my mind—and the topic of wine is always on my mind. So, I started musing about viruses and wine.

You likely know that viruses are tiny infectious agents that hijack host organisms to make more viruses. Viruses are incapable of self-replication and are entirely dependent on their hosts, yet they are highly specific in their choice of host—whether a bacterium, yeast, fungus, plant, or animal—and they occasionally migrate between related species. Sometimes viruses employ a vector organism that carries them with impunity to their target host.

The most obvious impact of viruses in the wine world occurs in the vineyard, where 86 viruses are known to infect grapevines. Some have colourful names such as fanleaf virus, red blotch, and mosaic virus. The vectors that transmit them are a nefarious group of mealybugs, soft-scale insects, aphids, dagger nematodes, treehoppers, and mites. Viral infections often manifest as leaf or stem deformities, yellow or red colouration (that can be quite attractive!), lack of vine vigour, and decline unto death. The impact can range from inconvenient low yields of grapes to the complete destruction of a vineyard. Sadly, there is usually no remedy, and vineyard management relies primarily on preventive rather than curative measures. For example, a nematode vector capable of infecting vine roots with a virus may be thwarted by the use of nematode-resistant rootstock. Often the only recourse is vine removal and replanting with virus-free stock, and this is always expensive.

Less obvious, more esoteric impacts of viruses occur in the winery, where viruses can affect the yeast and bacteria involved in winemaking. An example is the “killer yeast” phenomenon first observed in the 1970s that helped explain why some fermentations become “stuck,” in that they slow down or stop. It emerged that certain yeasts produce toxins lethal to other yeasts, even of the same species, but not to themselves. Only later was it discovered that viruses create the killers by installing genes for the toxins and for protecting the host. It seems a precarious balance for the virus, but it works. Another example is the attack on malolactic bacteria by specialized viruses called bacteriophages that destroy the beneficial bacteria and prevent the softening of acidity desired in many wines.

But not all is doom and gloom regarding viruses in the wine world. For example, viruses have been harnessed to introduce beneficial genes into vines, yeasts, and bacteria, although more recent technologies are making such methods dated. The process is much the same as an infection, but instead of inserting viral genes, the virus is modified to insert beneficial genes, such as disease resistance, or to enhance flavour characteristics. Another positive effect of viruses is the 2020 finding that specific bacteriophages target spoilage bacteria responsible for off-putting vinegar and nail-polish remover smells. Their use could eliminate the need to use sulphur dioxide as a preservative—eliminating sulphite headaches in sensitive consumers in the process.

Finally, to turn the tables, wine appears to protect against several viruses, including rhinoviruses and coronaviruses. The effect “appears reflected in the reduced incidence of the common cold in moderate alcohol consumers […] particularly those drinking red wines,” according to Ron Jackson in Wine Science. This good news was recently supplemented with the announcement that tannic acid, a form of tannin present in oak-aged red wines, “suppresses” the COVID-19 virus. No one is advocating copious consumption of Cabernet, but a little may not hurt.

Whether good, bad, or ugly, the world of viruses is complex and compelling. My musings led me to discover that viruses are the most abundant biological entities on Earth and that viruses are major drivers of evolutionary change—by one estimate, a startling 30% of all protein adaptations since humans’ divergence with chimpanzees are due to viruses. David Enard of Stanford University posits: “When you have a pandemic or an epidemic at some point in evolution, the population that is targeted by the virus either adapts, or goes extinct.” So, it seems best that we learn how to live with ’em because we sure ain’t gonna outlive ’em.