One of the most common pests to vineyards all over the globe is powdery mildew (Erysiphe necator Schwein).¬† It is responsible for causing widespread destruction in nearly every viticultural area worldwide, and is a fungus that is able to develop in a variety of temperatures and humidity levels.¬† Though it is present in nearly every corner of the globe, the severity of the infection from vineyard to vineyard is dependent upon a variety of factors, including the variety of grape, the vigor of the vines, the type of protective chemicals applied, and the weather conditions.
Some studies have found that the training system used in the vineyard has a significant effect on powdery mildew development, by altering the microclimate in the cluster area.¬† It appears as though light intensity and UV radiation appear to contribute to the changes in powdery mildew development severity, both of which have also been shown to affect the chemical composition of the grapes themselves.¬†¬† Specifically, work done by the authors of the paper presented today found that there were significantly lower powdery mildew infections in grapes trained in the free canopy system versus grapes trained in the vertical shoot positioned system.
The article presented today is a very short article with the objective of investigating whether or not the difference in infection incidences as described above were due to light intensity itself, the susceptibility of the berries, or both.
The study was performed in June 2003 at an experimental vineyard in the Golan Region of Northern Israel.¬† Grapevines planted in this vineyard were Cabernet Sauvignon and Chardonnay (both with good susceptibility to powdery mildew).
Half of the vineyard was subject to a vertical positioned system, while the other half was left as a free canopy, though topped to one meter in length after fruit set and hedged no more than twice during the growing season.¬† Dates of bud burst, flowering, and fruit set were the same for both training systems.
|Figure 1 from Zahavi and Reuveni, 2012|
Experiment 1: Clusters were picked when the diameter of the berries were 3-5mm.¬† Thirty to forty berries from each training system were selected and placed in plastic boxes.¬† Berries were then inoculated with powdery mildew.¬† Twenty more berries were placed in plastic boxes but not inoculated with the fungus to serve as a control to monitor natural infections from the field.¬† Percent of infected berries was then calculated 7-9 days after inoculation with powdery mildew.
Experiment 2:¬† Clusters were inoculated in the exact same manner as in Experiment 1, however, 1-2 hours after inoculation, berries were returned to the vineyard and either placed on the cluster zone/vine cordon of their original training system or on vine cordon of the opposite training system for 8 hours.¬† After this time, berries were brought back to the laboratory and disease development was monitored 7-9 days after.
- For the first experiment, incidence of powdery mildew was significantly higher on berries originating from the vertical positioned vines than those originating from the free canopy system.
- ¬†For the second experiment, berries that originated from the vertical positioned vines and then incubated in those same vines were significantly more infected with powdery mildew than those berries that originated from the free canopy vines and incubated in either of the vine position systems.
- Also in the second experiment, berries that originated from the vertical positioned vines that were incubated in the free canopy system vines had an intermediate level of powdery mildew disease severity.
- Powdery mildew did not develop on control berries that were not inoculated.
The results of this study found that grapes originating from a free canopy system, which has a greater exposure to light, resulted in lower susceptibility to powdery mildew infection than grapes originating from vertically positioned vines, which have a denser canopy that does not allow in as much light.¬†
By transferring inoculated free canopy grapes into a vertical positioned set up, powdery mildew development decreased as a result of a pre-conditioning effect on the grapes.¬† In other words, this means that the grapes were less susceptible to infection after being exposed to higher intensity of light from the free canopy system.¬† The authors conclude by stating that the conditions in which grapes develop influence the severity of infection by powdery mildew.
Being a short experiment, there are certainly many more questions that these results raised which cannot be answered with the results found.¬† For example, how do the different training systems affect the chemical and sensory characteristics of the wine?¬† Are there any differences?¬† Since it appears grapes grown under a vertical positioned system are more susceptible to powdery mildew than grapes grown under a free canopy system, it suggests that perhaps there are some chemical defense changes within the plant, which may or may not affect the overall sensory characteristics of a wine made from those grapes.
Would the results be the same for each and every variety of grape out there?¬† Or are Cabernet Sauvignon and Chardonnay more susceptible to powdery mildew invasion under a vertical positioned system while say Riesling and Malbec are more susceptible under a different training system?¬† I would think it once again boils down to plant defensive chemistry, but we‚Äôd need some further studies examining many more grape varieties to be sure.
What about other training systems?¬† What is the ‚Äúhierarchy of susceptibility‚ÄĚ for powdery mildew in Vitis vinifera grapes?¬†
The results of this study are not to suggest that one should switch from a vertical positioned system to a free canopy, however without any extra chemical defense (fungicides, etc), it might be recommended that one reconsider the training system that is employed at one‚Äôs vineyard.¬† There are, of course, many other factors that any given training system will affect, thereby requiring one to weigh all the pros and cons before choosing any particular method.
I‚Äôd love to hear what you all think of this study!¬† What questions did this study raise for you?¬† Please feel free to comment below!
Source: Zahavi, T., and Reuveni, M. 2012. Effect of grapevine training systems on susceptibility of berries to infection by Erysiphe necator. European Journal of Plant Pathology 133: 511-515.
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