How a millennia old disease destroys whole vineyards and what can be done about it.
Plenty of questions (Credit: Pixabay)
The oldest grapevine trunk disease of the world: Esca
If you make wine, you know that there is sword dangling over you, so you could constantly worry about any of the more or less sudden events that could destroy your hard work. Damage from late frost, hail or heat, pests, fungi or viruses that attack roots, leaves or berries: The list goes on and these threats dangle like a sword over a successful harvest and too often still vintners can do little to avert disaster.
Vine diseases form a large part of these threats, and Esca is one of the most dangerous among them.
A Problem from the Last Millennium
If you haven’t heard of it yet, there’s no need to be embarrassed about it, but you might be surprised how often it comes up when you talk to experts about the current situation in the vineyards during summer, since it has been affecting winemakers more and more for several years.
While it isn’t exactly a modern phenomenon – the disease was already known in ancient Rome and is mentioned in medieval writings of the 12th century from Spain and Italy, too – it is something relatively new in northern wine-growing regions since it has only begun to be in issue there towards the very end of the last century.
What is Esca?
You see, Esca is a rather complex problem. It starts at its biological origins, because various types of fungi are involved in the disease itself and some of them can be found directly in the wood, regardless of the age structure of the vines. That is why they can also appear in young vines in the form of the Petri disease, which then can develop into Esca.
Esca – as in the cases mentioned in Ancient Rome and through medieval times – used to be an issue that occurred predominantly in the vineyards of the Mediterranean. So, because this has not affected northern wine growing regions the question of the origin of the current plague in such places automatically comes up, but is still to be answered conclusively.
Similar to other infections, wood cuts are the gateway into the vine itself, thus increased pruning is one cause, just as early pruning has a negative effect on the resistance of the plant. Other then that a lot is still speculation, but could at least in part be responsible for the spreading of the disease such as increased globalization or milder winters in the context of global warming, since the pathogens prefer it warm, but who doesn’t?
The Difference between Slow and Sudden Disease Evolution
The course of Esca syndrome is not always uniform. The disease can progress slowly, in a chronic evolution, which extends over several years and at times with several years without showing any symptoms of infection. Naturally, this contributes to added infections in the vineyard as it is not recognized and the disease is left to spread unchecked. In most cases though, small and irregular light green and yellowish spots form on the leaf in summer. The leaf dries up from the center and only strong veins remain green. This reduces the supply of the berries, which in turn shrink and dry out.
On the other hand, as part of the acute disease progression that frequently coincide with hot, dry weather during the summer season, dry and inconspicuous grey green spots all of a sudden form on the leaves. The spots quickly grow in sizes and as a result can lead to sudden wilting of the entire plant.
The Esca Enigma
Much of the Esca plague remains a mystery and many questions are left unanswered. Why does the infection in many cases only affect individual vines? Why are some grape varieties apparently less susceptible than others? Why are symptoms completely absent sometimes for years or differ in intensity from year to year? Esca still is an enigma that waits to be solved.
How do you fight Esca?
Equally difficult is the question of how to fight the disease. Since pruning could be a key factor that contributes to the spreading of the Esca, sealing off the cuts is a common recommendation to protect the root against infection, but lack conclusive proof. Forms of soft pruning, i.e. carried out in dry and cool weather, not too early and as careful as possible, apparently minimizes Esca infestation.
In addition, the use of Trichoderma is currently examined in order to combat the fungi that cause Esca. Trichoderma are filamentous fungi that are used as antagonists to the disease, attacking the fungi and depriving them of needed nutrients. However, substances that are based on the process have only recently been approved and the actual effectiveness of this approach has yet to be demonstrated. Until a thorough solution to the problem has been found though, vintners are only left with little choice but take drastic measures: the vineyard has to be thoroughly and continuously examined, infected grapes have to be removed, the vines marked and either immediately cut or replaced by winter at the latest.
Another question arises in respect of the effectiveness of attempts to restore vines. Despite promising results, it cannot be said whether this approach really is successful even if the plant remains free of symptoms in the future. After all, that is a common development of Esca as outlined above. Yet, extensive pruning of diseases vines and rebuilding them from shoots close to the ground has so far shown promising results at least in the test environments.
In any case, experts recommend to remove cut vines from vineyards as quickly as possible and to refrain from storing the wood on the site of or near the vineyard, because it presents perfect surroundings for pathogens to reproduce and spread by wind to the healthy plants.
More than just Disease Control
However, cutting out the infected grapes shouldn’t be considered only to fight the disease in the vineyard. It simplifies the work during the harvest, too. Otherwise, workers would have to be very well trained and work accordingly, because the infected grapes are likely to add undesirable bitterness to must and wine.
Ultimately, the example of Esca disease shows the difficulty and risk that winegrowers are exposed to, and which, according to forecasts, will continue to increase. A reduction in yields due to failing vines and the need to replace old vines are therefore probably a reality we need to learn to live with.