Disease Management: The Big Three

Like death and taxes, battling diseases of grapevines can’t be avoided. Managing grapevine diseases is an integral part of tending vineyards; the challenge is due to one simple, modern predicament: many of the diseases are from the New World, and Vitis vinifera is from the Old World, so it didn’t evolve any natural resistance to them. European grapevines and grapevines of European origin have to contend with North American diseases that they don’t have the genetic ability to fight. This problem is not getting any better—most of the varieties that we grow today are propagated by cuttings that date back to an era before grapevines or their diseases traveled across the Atlantic; in some cases, the varieties even predate Columbus. Meanwhile, on this side of the pond native vines have had plenty of time to evolve ways to coexist with our diseases. The Vikings (my Icelandic ancestors) saw it firsthand: Vinland, as they christened North America over 1000 years ago, was full of these eponymous, native vines, hosting the worst of vineyard diseases that we currently battle.

If pruning shears are the primary tool of the viticulturist, the sprayer is a close second. Whether the farmer is organic, biodynamic, or conventional nuke ‘em, until newer disease-resistant grapevines are accepted by the wine world, there is no way to grow the varieties we are accustomed to, like Riesling or Pinot noir, in 99.99% of the world without spraying something. Andy Walker at UC Davis is currently developing non-GMO classically bred mildew-resistant vines, and I am very excited about those possibilities—but they will be entirely new varieties, and as you know, the wine world is quite traditional. Müller Thurgau and Zweigelt are finally gaining some acceptance, so who knows? Maybe they will take off. Maybe not.

Powdery Mildew

Getting away from pie-in-the-sky and back to the trench warfare of disease control, the main organism that viticulturalists fight worldwide is powdery mildew (Uncinula necator). It is closely related to the powdery mildew species you see on roses, melons, squash, and crepe myrtle. It has an over-wintering phase and a summer phase, and as a foliar fungal pathogen it has a very unusual ability during its summer phase—it doesn’t need free moisture to germinate. Its spores can hatch and infect dry leaves or berries. Most fungal pathogens need free moisture to do their nefarious business. Powdery mildew doesn’t actually like to get wet, which is one of it’s weaknesses that we can exploit—part of the killing effect of the sprays is literally the “washing” effect of the water: the fungal hyphae (“hyphae” are thread-like structures that make up the body of the fungal organism) burst due to permeable membranes that create osmotic differential between the solute-filled moisture within their cells and the pure water outside. (Quick chemistry refresher: water is always drawn across a membrane from the side with less solutes—salt, sugar, and amino acids are all solutes—to the side with more solutes, until the two sides are balanced. This is called by osmosis. That is why soaking in epsom salts draws fluid out of our skin, while soaking in pure water puffs our skin up. It’s an important concept for all kinds of processes with plants, fungi, and other organisms, including humans.)

Powdery mildew over-winters as tiny, hard, BB pellet-shaped brown or black specks of dust, just barely visible to the naked eye, called cleistothecia. These little fungal escape pods have evolved to survive a hard winter on the bark and then hatch spores during budbreak. With a moderately warm rain event, they release their ascospores, perfectly timed to infect the new little grapevine shoots that are just appearing from under the bud scales. After this initial infection the organism no longer needs any rain events or free moisture, reproducing asexually during its summer phase as fast as every five days. Its spores, called conidia—which are like little pearls on vast forests of teensy-tiny little white stalks—give the leaf a powdery appearance, and hence its name. Powdery mildew was accidentally introduced into Europe from North America in the mid-1800s, in the same general timeframe as phylloxera and downy mildew. It’s less deadly than these other two pests, but makes up for it with its ubiquity.

Powdery mildew has exquisitely co-evolved with grapevines. As such it doesn’t kill the vines, but it does significantly weaken them. On the leaves and canes it reduces photosynthesis, cold-hardiness, and carbohydrate storage; it slows ripening, and retards leaf growth. On the berries it causes cracking (leaving them susceptible to bunch rots), slows or stops ripening, and gives the wine a distinct mushroom-like aroma. You can sometimes detect an outbreak in a vineyard by noticing the mushroom odor as you walk down the rows.

Luckily there are good organic materials to prevent powdery mildew. It reproduces so fast, however, that if you let your guard down for more than a couple of weeks, it will roar to life in what seemed like a completely clean vineyard. For this reason, it can’t currently be managed using the Integrated Pest Management (IPM) principles of monitoring and early detection; instead, the vineyard needs to be sprayed at regular intervals preventatively (there is new technology in the pipeline to detect powdery mildew, which should drastically cut down on spraying, but it’s not ready for widespread adoption yet). This means that grape growers—pretty much anywhere in the world—have to get used to climbing on the tractor wearing a respirator and a raincoat and driving up and down the rows every week or two during the months from budbreak to véraison. If the temps are between 70-85° for much of the day the intervals need to be tightened, and if hotter or cooler than that range it is okay to space the sprays out a bit.

The main tool to fight powdery mildew for the past 150 years has been sulfur. As a spray or a dust, sulfur kills the spores as they germinate, preventing infection. It doesn’t do much once the fungus has gotten a foothold, so it must be used preventatively. Sulfur is used both by organic and conventional growers. You have to be careful with sulfur, though: if used too late in the season it can persist on the grape skins, providing a substrate for the yeast during fermentation to make sulfur off-aromas (rotten eggs/H2S, etc). Late-season sulfur applications in areas with very high powdery mildew pressure can be one of the causes for “flintiness” or other reduced mineral-type aromas in the wine. Sulfur can also burn the leaves and the fruit if applied before a hot spell. 

Oil can be sprayed on the vines to smother powdery mildew, and is a good organic option. It works well, especially early in the season when the shoots haven’t grown much, and are easy to thoroughly cover with the spray solution. Oils are hard on the vines, though, especially if the weather is very hot or very cold. They smother the vines too, reducing photosynthesis; and they will burn the vines if applied too close to a sulfur application—those two materials don’t mix. Oil, sulfur, and copper (copper is for downy mildew, not powdery mildew) work on the principle that they are less toxic to vines than they are to the disease. In other words, they kill leaves just as easily as they kill the disease if they are applied at a high-enough rate. So it never feels good to spray them—it’s always a teeth-gritting operation.

There are some newer organic materials based on fermentations that work okay—not great—but free from the problems associated with the toxicity of sulfur or oils (Serenade, Sonata, and Actinovate are the most commonly available products—they are made by fermenting organisms like bacillus subtilis). Some growers are experimenting with milk and whey, which create a competitive environment for the disease organism and produce free radicals that kill the diseases.  Nutrients like calcium, potassium, and silica help the plants ward off the disease on their own (not a stand-alone strategy, but an adjunct), and they can be sprayed on the leaves preventatively—the leaves can use the nutrients to strengthen themselves. The biodynamic spray horsetail is a good source of silica for this purpose.

If you leave organic farming behind, powdery mildew is fairly easy to control with the suite of new synthetic fungicides that are now available. Many of them are systemic, and they all have a much longer-lasting residual effect on the plants. New research about the unintended consequences of these materials is coming out all the time. Some of them are potent endocrine disrupters. They may be causing problems with bees. There are serious concerns with these materials, and so with our farming we choose to restrict ourselves to the organic materials that break down harmlessly in the environment.

 
Powdery Mildew

Downy Mildew

The next most important disease worldwide is downy mildew. Regardless of its similar name, downy mildew is a completely different organism than powdery mildew. Downy mildew originated in the Eastern United States, just like powdery mildew and phylloxera (which is an insect, not a disease, so we’re not covering it here). It’s technically in the class of fungi called water molds, and it requires warmth and free moisture to germinate. As such it’s not always a big problem in the dry Mediterranean climates, and is almost unheard of in California, but it’s a major problem anywhere that gets much warm summer rainfall—regions throughout France, Northern Italy, the Eastern United States, Western Australia, Germany, etc. Unlike powdery mildew, downy mildew will completely defoliate the vine if left unchecked. It is a devastating disease, killing any and all living tissue is a short period of time.

Downy mildew has an interesting spore that causes infection, called a zoospore—it actually swims with little flagella, like a sperm. Powdery mildew spores, on the other hand, just blow around in the wind. The swimming action of the downy mildew spore necessitates moisture to germinate and infect.

Downy mildew is controlled organically with various preventative copper sprays (Bordeaux mix, copper hydroxide, copper oxide, etc.), but copper builds up in the soil—if sprayed over enough years it can accumulate to toxic levels. This is in contrast to sulfur, which is absorbed by nature in other benign ways: sulfur is eaten by soil organisms, taken up by plants as a fertilizer, and incorporated into soil humus.  Because of the soil buildup, copper sprays must be minimized, and growers struggle to control downy mildew organically. There is a lot of work going on in Europe to find cocktails of plant extracts, compost teas, and biodynamic preparations that will control downy mildew with the toxic impact of copper.

There is an environmentally benign—but not certified-organic—control for downy mildew: phosphorous acid. Use of this tool represents one of the many challenges raised in meshing totally organic farming with sustainable farming. Phosphorous acid is a synthetic product, and therefore not organically approved, but it is harmless in the environment: it breaks down into phosphorus fertilizer. In most cases organic farming and sustainability overlap, but not always. There are gray areas. As time goes on we will hopefully able to fine-tune our organic methods to mesh completely.

 
Downy Mildew

Eutypa and Botryosphaeria

The third most important disease challenge is the devastating trunk-rotting duo of Eutypa and Botryosphaeria. They are two different organisms with virtually identical behavior. Also called Dead Arm and Eutypa Dieback, these two suspects are the number-one reason we don’t see more old vineyards. The spores shoot into the air during rain events, and infect vines through fresh pruning wounds, slowly working their way into the vines, choking off the flow of sap, destroying the productivity, and eventually killing the vine. You start to see the disease when the vineyard is around ten years old, and it can be devastating by the time the vineyard is twenty years old. In many cases, by the time the vineyard is twenty years old the diseased wood is so bad that the vineyard needs to be replanted. Without dead arm, the same vineyards could last for one-hundred or more years.

Some varieties, like Zinfandel, are less susceptible. So you see more old-vine Zinfandel vineyards. Also, with head pruning you make fewer big cuts in the vine, which helps to reduce the number of new infections—the bigger cuts, into thick old wood, are much easier pathways for the disease.

Over the past decade or two we have learned how to minimize the disease, but it’s an added cost, and presents some challenges. In California, pruning in early spring rather than midwinter makes a big difference because the spores typically exhaust themselves earlier in the winter rainy season. More spores are flying around in the first fall/winter rains; less are out in the spring rains. This is a tough to do, though: it significantly reduces the winter pruning window, leaving you only a few weeks to prune prior to budbreak rather than the entire winter. A labor nightmare as you might imagine, but it’s become the norm for a lot of growers.

Painting the pruning wounds with fungicide is another option. These materials have historically been pretty toxic, and don’t last long on the pruning cuts, leaving organic farmers to make their own homemade wound treatments, with ingredients like compost, manure, clay, or whey. These concoctions work by encouraging competitive fungi. There is finally an effective and non-toxic wound treatment available called Vitiseal, based on cinnamon oil. We’re hopeful that this new Vitiseal is going to make a big difference in vine longevity.

In many vineyards, once Eutypa starts showing up enough (you can see the dead spurs and cordons), you cut back the vines and retrain them. If this is done diligently you can maintain the vineyard in pretty good shape, but it’s a lot of work, and the vines are out of production until they are retrained.

 
Eutypa

These are the big three diseases. There are many more diseases that we fight, like Botrytis, Esca, and Armillaria, but none of them are as ubiquitous and potentially devastating as powdery mildew, downy mildew, and Eutypa/Botryosphaeria. As a grape-grower you need to think about mildew every day in the summer, and Eutypa/Botryosphaeria every day in the winter. Fighting them is a constant battle, and it’s one of the core aspects of growing grapes, right up there with pruning and soil fertility. It’s another reason, though, that the art of viticulture is so interesting.