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Section II - Winegrowing
Chapter 8 - Overall Approaches
Chapter 9 - Soil and Topography
Chapter 10 - Vineyard Management for Wine Quality
Chapter 11 - Soil Management and Nutrition
Chapter 12 - Disease and Injury Management
Chapter 13 - The Growing Year
Chapter 14 - Terroir
About almost every one of these winegrowing chapters I say to my wife as I’m getting ready to publish them, “No one is going to read this.” She replies, “But you say that about all of them.” In this instance I believe I may be correct. No one likes to read about death and disease, yet both remain a reality for us and grapevines.
I know many of you are skipping the footnotes (endnotes unfortunately on Substack). Maybe the best strategy for the general reader with this chapter is to skip the main text and go directly to the footnotes. As a lover of footnotes, I think these are particularly delicious.
Chapter 12.
“Against the disease of writing one must take special precautions, since it is a dangerous and contagious affliction.”
-Abelard
Disease and Injury Management
Grapevines are susceptible to a wide range of diseases and pests. Left uncontrolled, these can result in loss of both quantity and quality of the fruit, and viral diseases may destroy the vineyard entirely. In this area of winegrowing, it is foolish to let nature take its course.[1] I received my master’s degree in Plant Pathology from UC Davis in 1974, about five years before I began making wine. So, the subject of plant disease fascinates me more than it will the average winemaker.[2] To temper my obsession with this and all things microbial, I will limit my comments to scenarios where a winemaker is most likely to be involved.[3]
I believe that a healthy vine supplied with a regular and moderate supply of nutrients and moisture is more resistant to disease than a stressed vine. Certain macroclimates are less prone to disease than others. For example, the typical Mediterranean climate with rain-free summers takes far less pest control than continental climates with regular summer rains. Many climates have warm moist environments that preclude the growing of wine because of the high fungal disease pressure. In all climates, however, there will be some disease or pest pressure that must be mitigated, and, insofar as possible, controlled. The primary diseases of the vine are fungal and viral, with a small minority caused by bacteria. Animal pests are mostly insects, with some vertebrates. Frost and freeze injuries are not uncommon. Each will be addressed individually. While you may lack the special training needed to diagnose and treat a disease, all professionals are capable of careful observation. That old French adage that “The boots of the farmer are the best food for the vine” applies here as well. The more time you spend with your vines, the better the chance of detecting a problem before it causes irreparable damage.
Fungal diseases
It is curious that the fungal kingdom contains both the yeasts without which wine would not be possible, and organisms that will attack every part of the vine and, in some cases, kill it outright. In the soil fungi are also integral to both soil formation and maintenance. Nature, it seems, does not lack for irony. Generally speaking, fungi prefer humid environments, so modifying the vine’s microclimate can be an effective tool in lowering humidity. Activities such as shoot thinning and leafing are regularly employed to improve the vine’s microclimate and discourage fungal growth. Cultural controls such as these are always preferable to chemical controls. An integrated pest management strategy is the wisest and often the most economical. This ecosystem-based method uses a wide variety of techniques to achieve long term control of pests. It strives to eliminate or minimize chemical inputs by using cultural and biological methods, as well as resistant species, to combat pests. It should be noted that doing nothing is often more costly than prevention, as you risk losing the crop itself or jeopardizing some aspect of its quality.[4]
Powdery mildew (Uncinula necator)
This pervasive worldwide fungal disease is unusual in that it does not need free moisture or high humidity to thrive. Because of this, it must be controlled throughout the growing season, not just during wet periods. It will grow on all green tissues of the vine and will impart an off-flavor to the wine.[5] Fungi can produce some of the most strongly aromatic compounds known, which makes their prevention critical in wine production. The damage powdery mildew causes also opens an avenue for infection of the berries by other microbes. Under optimal conditions for growth and reproduction, its life cycle can be as short as five days. This is far shorter than the average fungus, and so mandates rigorous monitoring and control methods.[6] Its growth is encouraged by shade and moderate temperatures. Cultural practices such as leaf and shoot removal, to open the vine canopy and fruit zone, are essential. tools. Because this disease is easily spread by the wind from adjacent vineyards, controlling it requires cooperation between growers at the regional level. It also effectively overwinters and infects the following year’s shoots as they emerge. Therefore, controls must focus both on suppressing infection and on eradicating it if an outbreak occurs.
The traditional chemical control for this disease has been elemental sulfur, and it is still commonly used, especially in the early season before fruit set. After set, it is advisable to switch away from sulfur to alternative control methods due to the risk of sulfur residue on the grapes.[7] The chances of residual sulfur are lower in warmer climates where the sulfur sublimates off the fruit more easily. Sulfur is allowed under the guidelines of organic agriculture and has the additional virtue of the fungi being unable to develop a genetic resistance to it.
This fungus, because of its short lifecycle, will develop resistance to other types of fungicides unless they are alternated. Therefore, an integrated management strategy is essential. Stylet oils can be used in some situations for control, as can copper containing fungicides. Biological controls with a competing or parasitic fungus such as Ampelomyces have not been effective as a stand-alone. The biology of this disease is well understood, thanks to the work of researchers such as Doug Gubler at UC Davis and many others. In most major grape-growing areas, there are weather-based predictive models for disease pressure that allow timing of sprays to be more precise, thus saving time and money.
Botrytis (Botrytis cinerea)
This organism is not specific to grapes and it grows on a wide variety of plants. Infection pressure is high during periods of rainfall due to increased spore counts in the air, and spore counts tend to be higher after veraison as well. Infection can be provoked by free moisture or conditions above 92% humidity for as little as 18 hours will encourage infection. Under rare circumstances, the effects of a slow-growing form of Botrytis allow the production of some exquisite dessert wines such as Sauternes.
The fungus will grow on buds, shoots, and flowering parts in the springtime. The infection of the flowers is especially pernicious, as it leaves behind a latent infection in the developing berries. The Botrytis within these infected berries is re-activated as the sugars build during ripening—even in the absence of free moisture. This phenomenon makes it crucial to treat with fungicides if damp conditions favoring the fungus are present during flowering. While all varieties are susceptible, Chardonnay, Pinot Noir, and Riesling are particularly vulnerable. So too are less commonly planted varieties such as Chenin Blanc, Grenache, Gewürztraminer, and Semillon. Infection by Botrytis allows entry of other spoilage microorganisms as well. Microbial counts of all organisms are much higher in juice and wine made from Botrytis-infected grapes than from sound fruit. Botrytis is also the major spoilage factor in table grapes during medium term shipping and storage.
An integrated approach to combatting Botrytis works best, using both cultural and chemical controls in combination. In rainy conditions, canopy management practices such as hedging, leaf removal, and shoot thinning prior to or immediately after the flowering period are required. under rainy conditions. Removing infected tissue after the harvest is recommended, as well as removing thinned clusters if wet weather is expected. Under dry harvest conditions, cultural practices alone can control infection, but they will not suffice on their own in rainy conditions, especially if temperatures are in the ideal range for Botrytis (60º–70ºF). A full program of fungicide treatment for Botrytis consists of four applications. The first takes place between bloom and set, and the second at berry touch. These first two are the most critical under wet conditions, as they prevent latent infections. A third at veraison and a fourth three weeks prior to harvest are employed if late season weather conditions are likely to favor infection.
Botrytis is capable of significant damage to both the quantity and the quality of the vintage.[8] It can taint the wine produced with mycelial character and provokes accelerated oxidation through the production of the enzyme laccase. The loss of color and tannin in reds can be devastating.
Downy mildew (Plasmopara viticola)
This disease, which is indigenous to the northeastern United States, began spreading throughout the winegrowing world in the 19th century. As it needs warm wet weather to thrive, you will rarely find downy mildew in true Mediterranean climates such as California. Leaves are normally attacked first but stems and flowering parts may also be infected. Cultural practices that encourage air movement and lower humidity in the canopy help control it, but a combination of protectant and eradicant fungicides are normally applied. The timing of applications is based upon weather-related predictive disease models. The copper-containing Bordeaux mix was the traditional control for this organism, but it has mostly been replaced with fungicides less toxic to the vine. Left untreated, this disease is devastating, and under ideal conditions for the fungi, the vine may become completely defoliated.
Eutypa (Eutypa lata) or Dead Arm
This fungus is not specific to grapes. It infects a wide variety of fruit trees, and as a result sources of the airborne spores are widespread. It infects through pruning wounds, which are particularly susceptible during the first day after pruning. Pruning wounds should be kept small, and any larger cuts should be treated with a fungicide at the time of cutting. In areas with dry summer and fall conditions, such as California, the first fall and winter rains are particularly heavily laden with spores. Because of this, fall pruning in these areas should be eschewed at all costs. Spore counts remain elevated during any rain event, so pruning should be avoided in the days immediately before, during, and after rainfall. Late pruning is the single best preventative practice for controlling this disease. This is paramount in young vineyards, to prevent Eutypa’s introduction. so that the disease does not become established.
Eutypa and several other associated fungi are slow-growing organisms that attack the woody parts of the vine, including arms and trunks. Spur-pruned vines, because of their greater mass of permanent trunk and cordon arms, are most prone to become infected. Cane pruning should therefore be considered for the most susceptible varieties. No symptoms are apparent during the first few years, but by the time symptoms appear, several inches of cordon or trunk will already have been infected. If detected early enough, the diseased parts of the vine can be removed and the vine will regrow healthy replacement wood. The mycelia grow several inches ahead of any visible symptoms, so the wood must be removed well past any signs of the disease.
Minor rots
As with minor surgery, minor rots are those that happen to someone else. To call them minor only identifies them as having a smaller impact on the general endeavor of growing wine. Any of them can have major impacts when they arise. Most of these are more common in areas and seasons with abundant moisture. The most common are Anthracnose, black rot, and Phomopsis, but they are by no means the only rots that can arise.
Nematodes
These soil-dwelling worms cause direct damage to the root system and can also be vectors for viral diseases. Though soil fumigation prior to planting is the best control, this option is less and less available because of environmental concerns. Enhancing soil organic material with nematode-antagonistic green manure or vermicompost can help reduce levels of some nematodes, as can certain soil microorganisms; mycorrhizal and bacteria-like organisms have been shown to be effective in suppressing nematodes. The development of resistant rootstock would be the most desirable control of these pests.
Viral disease
Viral diseases are devastating. Once established in the vine, they are untreatable and incurable. Over time they destroy the vines’ ability not only to ripen fruit, but also to flourish at any level. Preventative and cultural controls are the only strategies for combatting them. First and foremost is selecting only the most sanitary plant material during establishment of when establishing the vineyard. The advantages of disease-free scion wood and rootstock cannot be overemphasized.[9] Field selection of scion wood, also known as selection massal, is strongly discouraged, because of the risk of spreading viral diseases. Viral infections inevitably limit the productive life span of the vineyard. This truncating of the vineyard’s lifespan is an argument against field selections based on the known benefits of mature vineyards on wine quality. If the vector of the virus or mode of disease transmission is understood, all precautions should be taken to limit its spread. While the virus itself is not controllable, the insect vector, such as a mealybug, may very well be. If certain parts of the vineyard are more infected with virus than others, the cleanest blocks or portions should be worked first, and the most diseased last. This will lessen the risk of inadvertently spreading the disease with pruning shears or other tools.
Bacterial disease
Grapevines are susceptible to two major bacterial diseases. Crown gall, caused by Agrobacterium tumefaciens, is found worldwide. It was first identified in France in the 1850s and is most common in regions that combine a moist climate with winter freeze injuries. It can also be spread through nursery material that has not been properly handled. The best way to manage this disease is by preventing vine injuries and sourcing clean stock from the beginning. Pierce’s Disease is a bacterial infection caused by Xylella fastidiosa. It is spread by sharpshooters, a leafhopper type of insect. It has been known in California since the 1880s, and there have been cycles of outbreaks ever since. The habitats for the insect vector are understood, but habitat removal or modification is costly. There is a range of susceptibility to the disease among grape varieties. Effective management of the disease involves choosing more resistant varieties in areas prone to the disease. Insecticide treatments of bordering areas and the perimeters of the vineyard can also be effective. However, chemical control of insects is problematical in general terms, as is discussed in the next section.
Insects and mites
It is beyond the scope of this book to discuss the wide range of individual insect pests. However, it is worth considering general philosophies of pest and disease management. Insects and mites can be effectively controlled by chemical pesticides on most agricultural crops, and grapevines are no exception. Chemical controls come with a range of significant costs. The fiscal cost is simple to weigh against the benefits, but it is the environmental and health costs of insecticides which are more difficult to assess— and to bear.
When considering control of insect pests, bear in mind that they are animals. Accordingly, insect biology has much more in common with human biology than with plant or fungal biology. This means that compounds which are toxic to insects are more likely to be harmful to humans than fungicides or herbicides are. Humans have little in common with plants and fungi physiologically, so our sensitivity to their pesticides is low. Conversely, exposure to insecticides poses a significant danger. Residues of insecticides are of a higher order of concern than residues of a fungicide or herbicide, though both are best at zero.[10] So, too, exposing yourself or your workers to insecticides is in general more dangerous than exposure to other agricultural chemicals. Because of this, cultural and biological controls are even more desirable for insects than for other pests.
With all pests, not just insects, a coherent strategy for control must be implemented based on the general principles that follow.
First and foremost, the pest must be identified, and its biology fully understood in relation to the biology of the vine. In the absence of this knowledge, control is severely hampered. Currently, the biologies of almost all major diseases of the vine are well understood. New diseases, especially viral ones, will continue to arise, and new pests will be introduced. So, the scientific study of them will continue.
The next step is to understand both the economic damage and the level of infestation that must be controlled to limit its spread. The goal is rarely elimination of a pest, but rather keeping it below an adverse level. Tolerable levels have not yet been completely determined for all pests in all areas. Once the pest is identified and the economic damage defined, a control strategy can be developed based on one or more of three main approaches.[11]
Cultural control is the first approach and has been mentioned in all pest controls discussed so far. Cultural control covers a very wide range of topics including, but not limited to, planting material, vineyard layout, canopy management, weed control, and soil and water management. The relationship of all vineyard practices to disease susceptibility and control should be considered before decisions are made. Often a cultural practice that benefits the ripening conditions of the vine will also be beneficial in pest control, especially with fungal disease.
Biological controls involve introducing or encouraging competing or predatory organisms to fight the pest, and these can be particularly effective against certain insect pests. The use of disease-resistant planting stock is a long-term biological control. The well-known example of North American rootstocks having resisted the Phylloxera infestation of European vineyards is the most famous example of this.
Chemical controls should always be thought of as the last resort, though instances will arise when they will need to be employed. It is imperative that this be undertaken in a thoughtful and careful manner. My personal experience over many decades of practice has confirmed that there is no more careful and conscientious farmer than the winegrower. Winemakers are focused on the purity of their product and cognizant of the danger of flavor contamination. This creates an inherent reticence towards treatments in the vineyard that might result in a residue on the grape.
Animal pests
Many of the same issues mentioned in connection with insect pests also apply to vertebral pests. It is not a realistic goal to entirely eliminate these pests, but rather to reduce their damage to a tolerable level. There are at least eight possible options depending on the pest.[12] They are:
1. Mechanical exclusion, such as bird netting or deer and rabbit fences;
2. Habitat modifications such as the removal of weedy areas adjacent to vines;
3. Behavioral modifications such as scare devices for birds;
4. Natural controls and predators—either by introducing or by encouraging them;
5. Alternative foods;
6. Trap and release;
7. Repellants;
8. Trap and kill, shoot, or poison.
Small rodents such as voles and gophers are normally controlled by trapping or poison bait. These animals, if left uncontrolled, will cause serious damage. Larger animals such as rabbits and deer are best controlled with fences. Natural controls are often inadequate with animal pests because the stable balanced populations, even after control has been achieved, are still too large for the vineyard’s health.
Birds are pests with regard to the ripening fruit and in some situations can cause extensive damage—including loss of the entire crop. Migratory birds can be particularly troublesome in certain areas and vintages. Most bird species are protected by law, so efforts are normally focused on a combination of mechanical exclusion and behavioral modifications.
Frost and freeze injuries
This damage can affect the vines in the fall, mid-winter, and the spring. Damage to the dormant vine in winter is referred to as winter kill or freeze injury, while damage to green tissue in the spring is called frost damage. The loss of or damage to green tissue in the fall may be termed frost or freeze damage.[13] Each warrants brief discussion here.
Winter freeze injuries
Vines enter dormancy through a process called acclimation brought about by cold and shortening daylight. Once the vines become dormant, and after a certain number of winter chilling hours, warmer weather will then de-acclimatize them, which results in budburst in the spring. Several factors influence how resistant a vine is to freezing, or how cold hardy it is. Genetic differences between the varieties is a factor.[14] Differences also exist between varieties in their phenology that can affect their susceptibility to cold injury. As just one example, Cabernet Franc is more cold-hardy during dormancy than Cabernet Sauvignon. But Cabernet Franc buds out up to two weeks earlier than Cabernet Sauvignon in the spring, making it more prone to damage then. Vines’ resistance to cold varies depending on conditions in the fall and the pattern of temperatures during the winter. In the fall, conditions that promote storage of carbohydrates and the hardening of canes make the vines more cold hardy. Conversely, excessively large crops or other conditions that may delay ripening of the canes will decrease cold hardiness. The pattern of cold during the winter also has a strong effect. Continuous cold conditions below freezing but above damaging levels cause the movement of water out of vine cells, making the vine more cold tolerant. The most damaging circumstances are just the opposite, when injurious temperatures follow a mild spell.[15]
Most of the scientific work on cold hardiness involves bud viability studies, but canes, trunks, and, in some cases, roots can be damaged by winter cold. Roots are most vulnerable in lighter soils with low moisture contents. For grafted vines, cold injury can be so severe that all scion wood is killed, and the vine must be replanted. Own-rooted vines may be killed to the ground but will normally regrow. In areas with regular winter killing conditions, a spur or cane may be encouraged from just above the graft union and buried in several inches of soil to protect it from the cold. If the trunk of the vine is killed, this buried cane can be trained to replace it. Soil may be mounded over the graft union itself to protect it, but must be removed in the springtime so that the scion does not self-root. Other strategies include leaving additional buds at pruning, with the expectation that some will be lost to winter kill. This is only practiced when bud kill is expected or possible at levels approaching 50%, as the vine will naturally compensate for bud kills lower than this by pushing more shoots in the spring.
Frost injury
Frost injury is most common in the spring, though it can occur pre-harvest in the fall as well. If the primary shoot is killed by frost, a secondary shoot will normally grow to replace it. The fruitfulness of this secondary shoot varies by variety. The clusters on the secondary shoot will be smaller, and the ripening pattern later and weaker than with the primary shoot. Frost damage in the fall can cause the loss of all leaves, leading to ripening problems if this occurs before harvest. Vineyards with autumnal frost damage are difficult to machine harvest as the dead leaf material is almost impossible to remove. If these dried leaves end up in the fermentation, off-flavors will occur. The loss of leaves early in the fall also makes the vine more vulnerable to subsequent winter injury and a lack of vigor the next spring, and decreased capacity in the following season.
Inversion conditions—when a layer of cold air settles close to the ground—are the most common cause of spring frost injuries. Advective cooling, caused by the horizontal movement of cold air masses, is less common in the spring but is often the cause of autumnal damage. Frost conditions are very much specific to the circumstances and unique situation of each site and must be dealt with on an individual basis.
Prevention begins of course by selecting vineyard locations not prone to frost. In most cases, this means hillsides at least 50' above the level of the valley floor.[16] Cultural practices have a substantial effect. Cleanly cultivated or closely mowed vineyard floors are less prone to frost damage. Extremely late pruning can delay budbreak by up to two weeks and shorten the frost danger window. Therefore, low-lying more frost prone portions of the vineyard should be pruned just before budbreak— in February in the northern hemisphere. Pruning canes long and removing the terminal few nodes after the buds are out 2" or so will have the same effect[17] while allowing pruning to occur at the normal time. Active forms of prevention include wind machines and/or heaters as well as conventional sprinklers and mini sprinklers. An effective system of thermometers and warning devices should be in place as well.
[1] A notable exception being some insect infestations, where the populations will often, but not invariably, be controlled by natural predators. Sufficient time must be given for predators to build their populations and bring the pests under control.
[2] Becoming a winemaker was the first time my theoretical knowledge of plant disease had a practical outlet application: managing a 50-acre vineyard. I vividly recall reading René Bovey’s book on virus diseases of the vine in my first year as a winemaker and being absolutely horrified at the thought of all these potential enemies of my vines. I was still too young and green to have developed the fatalism necessary to work happily in agriculture.
[3] Controlling pests and disease in the vineyard is a highly specialized subcategory of management. Even skilled vineyard managers usually subcontract this aspect of the work, and for good reason.
[4] I recall analyzing the cost benefit of a full Botrytis treatment spray program and discovering that saving just one grape cluster per vine would offset the entire expense. It is always worth considering where it is possible to economize and when it is best not to. For example, a vintage with rain in the springtime around flowering is not the scenario where cutting back your fungal spray program would be wise.
[5] Lighter and more delicate wines show the flaw most obviously. Sparkling wine is particularly apt to expose it. Even a small percentage of infected clusters will taint the wine.
[6] As with most vine diseases, the prevention of powdery mildew is far less expensive than its eradication. The same holds true for the amount and toxicity of the pesticides needed. An ounce of prevention is indeed less costly than a pound of cure in combatting infection.
[7] The presence of elemental sulfur in a fermentation will almost certainly result in the forming of reduced sulfur compounds in the wine.
[8] Botrytis blight refers to the sudden and severe necrosis associated with rapid growth of the organism. I witnessed this in the Edna Valley in the early ’90s with Chardonnay, when a tropical storm during the final ripening period dropped significant warm rain, followed by warm humid weather. Within three days, 60% of the crop was lost to blight. The 40% that was harvested made poor wine due to lack of full ripeness and mycelial taint.
Botrytis rot refers to the slower growth of the organism under more usual conditions. The 2006 vintage in the Central Coast was an example of this, when a cold wet spring allowed establishment of Botrytis in the clusters, which then grew slowly during the late extended ripening period in October. Careful thinning of infected clusters in the vineyard and modification of standard winemaking techniques allowed the production of excellent white wine, and, from the best producers, decent quality red wines.
[9] Tissue culture for isolating and propagating virus-free scion wood has been practiced since the 1940s.
[10] Analytical chemistry has developed to such a degree that traces of pesticides can be detected at levels many orders of magnitude below their regulated or harmful levels. The general public does not understand this concept well and assumes that if any residue exists, it must be harmful. This reality puts the onus on the producer to have wine free from any residues.
[11] Integrated pest management has been the preferred method since my days as a graduate student in the early ’70s.
[12] Many vertebrates, especially birds, are protected by local, state, and federal laws. It is important to correctly identify the pest and to confirm legal remedies before proceeding with any control measures that may result in the death of the pest.
[13] The rare and costly ice wine is produced when grapes are left on the vine to freeze solid in early winter. These are then picked frozen and pressed, effectively cryoconcentrating the juice.
[14] It should come as no surprise that Riesling is one of the most cold-hardy varieties. Pinot Noir holds this distinction as a red wine cultivar.
[15] In Washington State in late 1990, during winter dormancy, a period of 50F was followed by -19F weather. Bud death after this event varied from 60–100%. These conditions were so severe that bud injury even occurred in the normally freeze-tolerant Concord grapes grown for juice in this area.
[16] Gladstones’s discussion in Viticulture and Environments on frost risk and topography is well worth reading.
[17] Removing these first apically dominant shoots will suppress and delay budbreak on the remaining buds on the cane.