Vineyards are a front line in the fight against many diseases. Downy mildew, powdery mildew and grey mould are the best-known grapevine diseases, but growers deal every day with far more than these.
Among them, diseases caused by viruses have recently come up more and more often.
When people think of viral diseases, influenza or hepatitis may come to mind first. COVID-19, still fresh in memory, is another example of an infection caused by a virus.
Viral diseases are familiar to us, and the same is true in the plant world. Wine grapes are no exception. Viruses sit close by as a serious threat to the healthy growth of the vine and to the quality of its fruit.
In Europe and North America, the spread of viruses is held in check by strict control of the young vines that are planted. Other regions, however, still lack such systems, and there the threat from viruses grows year by year.
This article explains the viruses present in wine grape growing, the diseases they cause and the options for countering them.
What is a virus?
Viral diseases, or virus-free young vines: when you hear these phrases, can you picture concretely what a virus is?
Take influenza as an example. People are vaccinated against influenza, and every year there is talk of whether the vaccine “matched” the circulating strain. Influenza comes in several types, and a vaccine is ineffective unless it matches the type. That is why the vaccine can match or miss. In other words, there are many kinds of viruses, and even viruses classified as the same influenza virus are not necessarily identical in content.
A virus is an infectious particle so small that it can be seen only with an electron microscope. Whether viruses are living or non-living is said to be still debated, but they are generally classified as non-living.
Viruses have no cellular structure. A virus usually consists of single- or double-stranded nucleic acid carrying its genetic information, protected by a protein coat. Viruses cannot multiply on their own and depend on living host cells. They multiply by manipulating the cell’s metabolism so that the cell mainly produces the virus’s own components. Nor can viruses move by themselves; their spread requires a vector that carries them.
Viruses in wine grape growing
More than 70 different viruses have been isolated from grapevines worldwide, and they are classified into more than 20 groups.
Not all of these viruses threaten every grape variety, because some cause symptoms only in particular varieties.
For some diseases, a virus is suspected as the cause from the way symptoms appear, but the virus itself has not yet been isolated and identified. In addition, how symptoms appear and how severe the damage is vary greatly with the type of virus, the infected variety and climatic conditions. As a result, some diseases once considered viral have turned out to have other causes and have been reclassified.
Diseases caused by viroids
Some diseases long thought to be caused by viruses have recently been found to have a different cause: viroids, infectious particles with an even simpler structure.
A viroid consists only of a circular, single-stranded nucleic acid (RNA) and, unlike a virus, has no protective protein coat. Viroids were discovered in 1971 and are known as the smallest plant pathogens.
Five viroids have so far been detected in grapevines. On their own, they are generally considered to be of little practical importance in wine grapes. At the same time, viroids are known to influence the expression of symptoms in combination with certain viruses, so they cannot be ignored entirely.
Economic damage from viral diseases and the main diseases
As the number of viruses already detected in grapevines suggests, there are many grapevine diseases caused by viruses. Among them, the ones known for especially large economic damage are grapevine fanleaf, grapevine leafroll and the rugose wood complex.
On the other hand, not all viral diseases of the grapevine, including these, necessarily cause severe symptoms. Some viral diseases also cause indirect damage, which makes it difficult even to estimate the economic losses accurately.
In nurseries, for example, using an infected vine as a grafting partner can lower the take rate of the graft and impair rooting. In ordinary vineyards, reduced growth leads to lower yields and a shorter productive life of the vineyard, cutting profitability. Delayed ripening of the grapes and the resulting lower sugar content of the juice can also be counted among the damage caused by viral infection.
One of the most important consequences of infection is that a vineyard contaminated with viruses can no longer supply material for producing young vines.
Grapevine fanleaf
Fanleaf is caused by several viruses. The main causes are a group of viruses in the genus Nepovirus, about 30 nm in diameter. This group includes grapevine fanleaf virus (GFLV), arabis mosaic virus (ArMV) and the cherry strain of raspberry ringspot virus (RpRSV-ch). Viruses that are relatively rare and confined to particular regions are also known to be involved in the disease, including the grapevine strain of raspberry ringspot virus (RpRSV-g), strawberry latent ringspot virus (SLRSV) and tomato black ring virus (TBRV).
Of these, GFLV occurs only in grapevines and is widely distributed in the traditional wine-growing regions of the world. Depending on the infected variety and the vintage, crop losses can exceed 50%, and GFLV is regarded as one of the most dangerous grapevine viruses. ArMV has been detected in many plants worldwide; in grapevines, it is particularly widespread in the wine-growing regions of Europe.
The disease generally progresses insidiously. For several years after infection, symptoms are barely visible or remain very mild. The length of the latent period is known to vary greatly with the variety, the virus involved and the stage of the disease. The rate at which infected vines decline also varies widely.
Nepoviruses are transmitted by nematodes living in the vineyard soil (Xiphinema, Longidorus and Paralongidorus). Beyond that, infection also spreads easily through grafting with infected material. Pruning, on the other hand, is not considered to transmit the disease.
Typical symptoms of fanleaf caused by GFLV include delayed budbreak, poor growth and dwarfing, deformed shoots, shortened internodes, zigzag growth and double nodes. Shoots may also fork into two from a single node. The leaves become smaller, sharply serrated, deeply incised at the margin and distorted. The disease takes its name from the strongly puckered, fan-like appearance of the leaf blade. Other characteristic symptoms include dot- or spot-like discolouration of the leaves, small berries in the cluster, and coulure, in which the inflorescence begins to shed soon after flowering.
European grapes, American grapes and interspecific hybrids can all be infected by fanleaf.
In infected vines, shoot growth and inflorescence formation are not affected at first. As the disease progresses, however, typical symptoms appear almost without fail, including poor growth and mosaic, line or spot-like yellowing of the leaf blade. Varieties also differ greatly in susceptibility: Gewürztraminer reacts very sensitively, whereas Kerner and Riesling are known to be relatively less susceptible. In rootstock varieties, symptoms are generally considered to appear only once the disease is well advanced.
Several viruses associated with fanleaf have been identified, but not all of them are equally problematic in every grape-growing region. The RpRSV and SLRSV strains show a fairly strong regional distribution, and for TBRV, infection has been found to follow the range of the nematodes that transmit the virus. In some cases the same causal virus triggers a specific reaction in particular varieties: when Kerner is infected with ArMV, it develops symptoms distinctive enough to be called by a separate name, Kerner disease.
Kerner disease
Kerner disease is regarded as a special form of fanleaf, and its causal virus is arabis mosaic virus (ArMV). As with fanleaf, infection is caused by the nematodes that transmit ArMV.
With ordinary fanleaf infection, Kerner shows only some symptoms on the leaves. With Kerner disease, the situation is entirely different. Severe damage can occur in vineyards where the disease is widespread, and the risk rises sharply. In Germany, the disease raged in some growing regions from the early to mid-1980s. Between 13% and 84% of the vines died, and this is said to have been one of the reasons the planted area of the variety stopped expanding.
The disease has been confirmed only in the combination of ArMV and Kerner. Other varieties can also be infected with ArMV, but in that case the disease stays within the range of ordinary fanleaf and the reaction is not as extreme as in Kerner. Riesling, for example, is said not to suffer serious damage even if infected.
The most conspicuous symptom of Kerner disease is a lack of water and nutrients. The leaves are small and yellowish-white, fruit set is very weak and the clusters are extremely small. Vine growth also weakens, and the graft union with the rootstock becomes noticeably swollen.
The most frightening aspect of the disease is that affected vines die within one to three years. Particularly when the vines are under stress, the disease often leads to sudden death.
Grapevine leafroll
Leafroll is a well-known grapevine disease and is said to be the most widely distributed viral disease of the grapevine in the world. Affected vines grow poorly and become less profitable, making leafroll a cause of major economic losses.
The disease is caused mainly by viruses in the genera Closterovirus and Ampelovirus. These viruses are relatively long and filamentous, about 12 nm in diameter and 1,800 to 2,200 nm long. Nine associated viruses (grapevine leafroll-associated viruses, GLRaV) have been found worldwide so far and given identifying numbers.
Their distribution is said to show regional patterns. In Germany, for example, GLRaV-1 is overwhelmingly dominant while GLRaV-2 is of little importance. In Japan, GLRaV-3 has been reported to occur frequently.
As with fanleaf, transmission of GLRaV through grafting is considered to cause major damage. Leafroll-associated viruses are also known to be transmitted by mealybugs and soft scales. Unlike fanleaf, however, transmission by nematodes has been ruled out. Mealybugs have been confirmed as vectors of GLRaV-1, -2, -3 and -5.
Leafroll symptoms gradually become noticeable from early summer onwards and can be recognised as clear lesions by autumn. The leaves turn downwards and at the same time begin to change colour irregularly from green to yellow or deep red. In the final stage, this discolouration progresses until only the major veins and the tissue around them remain green. Symptoms generally appear first on the oldest leaves and gradually move up the shoot.
Disease progression is strongly influenced by climate and is faster in warmer regions. In most cases, no symptoms are visible immediately after infection; symptoms of poor growth, such as reduced vigour, begin to appear only several years later. The severity of symptoms is known to depend not only on the variety but also on the virus involved. Symptoms caused by GLRaV-1, for example, have been found to appear earlier and to be clearly more severe than those caused by GLRaV-3.
Like fanleaf, leafroll infects all scion and rootstock varieties. In American varieties, however, with SO4 as an exception, the disease does not show symptoms. Latently infected rootstock varieties may suffer reduced rooting ability.
Leafroll has spread over very wide areas. At the same time, its symptoms closely resemble those of red blotch, a different viral disease transmitted mainly by treehoppers, and the two are often confused. Although the symptoms are similar, the causal viruses are entirely different. Red blotch is caused by grapevine red blotch virus (GRBV).
Vines can also be infected with leafroll and fanleaf at the same time, and in such cases the symptoms of each may become more severe.
Rugose wood complex
As the name suggests, the rugose wood complex is a grouping of several different syndromes. Four syndromes are currently included.
Their pathogens and causes are still only partly understood, and the individual syndromes are distinguished by the reactions they produce in specific indicator grape varieties. The rugose wood complex is thought to spread mainly through grafting, but for some of the viruses involved, various species of mealybugs have been confirmed as vectors.
The syndromes currently recognised as the rugose wood complex are corky bark, Rupestris stem pitting (RSP), Kober stem grooving (KSG) and LN 33 stem grooving (LNSG). Corky bark causes no conspicuous symptoms in most European varieties and remains limited to mild growth reduction. In relatively susceptible varieties such as Gamay, Cabernet franc and Pinot noir, however, the wood near the graft union becomes spongy or rubbery to the touch, and swelling and cracking appear.
The syndromes other than corky bark also appear basically only on American rootstocks, and their symptoms are confined to the wood.
In Vitis vinifera, infection remains latent in most cases, and RSP, for instance, develops only on rootstocks of Vitis rupestris and its hybrids. For these reasons, the actual damage is considered not to be very large, and some countries and regions did not treat these syndromes as quarantine targets until recently. In the past, however, they caused yield losses of about 14–35% and even the total loss of vineyards in Italy, South Africa and Greece. Considering the scale and risk of economic losses, they are not viral diseases that can be safely ignored.
Grapevine virus B (GVB), grapevine rupestris stem pitting-associated virus (GRSPaV, genus Foveavirus) and grapevine virus A (GVA) have been put forward as candidate causal viruses. Except in some cases, however, the cause has not been established, and in some cases no causal virus has been found at all.
There are many other viral diseases of the grapevine. In addition to agents such as the viroids mentioned above, new ones continue to be discovered, such as grapevine Pinot gris virus. How many viruses remain undiscovered is not yet known.
Countering viral diseases
Countering a viral disease means countering the virus that causes it. In human life, some diseases have countermeasures such as influenza vaccines, while others still have none. Viral diseases of the grapevine belong to the latter group: in practice, there is no countermeasure against any of them.
The effect of vaccines given to humans makes the reason easy to understand. A vaccine does not attack the virus itself. A vaccine is, in fact, a weakened virus.
Taking it into the body stimulates the immune system that the body already has, and that is what counters the virus. In other words, what fights the virus is the immune system of the infected organism, not the vaccine. For a vaccine to help a grapevine fight a viral disease, therefore, the vine would first need an immune system of that kind. And grapevines do not have one.
This also means that once a vine is infected, there is no way within the vine to drive the virus out.
Without such immunity, a grapevine cannot recover naturally from a viral disease. At the same time, no direct countermeasure using plant strengtheners has been found so far. This is closely related to the fact that viruses lack the metabolic machinery of living organisms.
In short, once a vine is infected, it is already too late. Such vines can only be grubbed up, roots and all, and destroyed. In the fight against viral diseases in viticulture, preventive or indirect measures are decisive.
One such measure, and the one considered most important, is the use of healthy propagation material. The production of so-called virus-free young vines falls under this measure. It consists of running a cycle in which young vines are produced from material guaranteed to be 100% free of infection, and those young vines in turn supply material for further grafting.
Making the vines alone virus-free, however, achieves little. Many viral diseases spread through vectors. If vectors remain in the vineyard, virus-free young vines will be reinfected immediately after planting, and the effort will be wasted.
For this reason, it is extremely important to eliminate in advance the nematodes and insects such as mealybugs that act as vectors from vineyards where virus-free young vines are to be planted.
Alongside eliminating vectors, two further measures are effective. One is to remove infected vines from an early stage through phytosanitary measures and similar means. The other is, when replanting, not to plant varieties that are highly susceptible to the viral diseases previously found there. In a plot where Kerner disease has been found, for example, one would not replant Kerner but plant Riesling, which is less susceptible to the disease.
Summary | An accurate assessment of the current state comes first
Viral diseases are troublesome in two ways. The first is that the true state of a vineyard can hardly be judged from its appearance alone.
A vineyard with late-stage leafroll, for example, can be roughly identified by how it looks. Even then, it is impossible to tell whether it is really leafroll or in fact red blotch, all the more so at a stage when symptoms have not fully developed. Accurate diagnosis requires genetic testing by PCR.
ELISA is a simpler method for identifying viruses, but its simplicity comes at the cost of some accuracy. Because ELISA sometimes fails to detect viruses, PCR is currently indispensable for reliable assessment.
Portable devices for simpler PCR testing and DNA sequencers have recently been developed. Even so, it is still too early to say that wineries routinely test their own vines. Apart from testing devices, rootstocks with high virus resistance are also being developed. How much easier it will become to check vineyards for viral infection remains to be seen.
The second is that some growers do not necessarily regard viral diseases as harmful.
Vines with viral diseases often grow poorly. In the past, poor growth led directly to lower juice sugar levels and ultimately to lower wine quality. Recently, however, this situation has begun to change.
With climate change, the vine’s development keeps advancing earlier. Growers are therefore eager to delay ripening even a little. In one respect, viral diseases grant this wish.
In vines infected with fanleaf or leafroll, ripening is delayed. In reality, ripening is not delayed so much as the vine is physiologically impaired, but at least the apparent rise in juice sugar is held back. Under recent weather conditions, on the other hand, it is not impossible to make up for this delay by harvesting later than before and still reach adequate sugar levels. In a sense, then, viral infection creates a convenient situation, and that situation delays countermeasures.
Viral diseases are erratic and hard to predict or foresee in every sense. Because symptoms may not show, growers may not notice an infection until most of the vineyard has gone past the point of no return. In the worst case, vines may even die suddenly in large numbers, and this possibility should not be forgotten.
As we have seen, once a vine is infected with a viral disease, nothing can be done from that point on. Not bringing viruses into one’s own vineyard, and not letting them be brought in, is the most important measure of all. And where viruses have already entered, they must be removed.
Eliminating vectors, nematodes in particular, takes time. To gain that time and to carry out countermeasures effectively, knowing the current state is more important than anything else. Moreover, viruses move, and unfortunately a single test cannot guarantee peace of mind for the future.
Regular testing, and the consideration and implementation of countermeasures, are required.
