Dr. Carien Coetzee
8 July 2020
No one expects to see machine harvesters in the vineyard in late spring and early summer. However, a new way of disease prevention could put these machines to good use during the pre-harvest period.
After a season of harvesting an exceptionally large Sauvignon Blanc crop (where the yield surpassed the demand), a winegrower from the Marlborough region in New Zealand asked researchers to investigate the use of machine harvesters as a cost-effective method to control crop levels. The initial idea was to reduce the crop load by shaking the vines during the period between fruit set and pre-vériason causing berries/bunches to drop. However, the researchers noticed that the shaking of the vines not only served as an easy method for controlling yields but it also significantly reduced the prevalence of Botrytis cinerea infection in the vineyard.
A formal research project was launched to investigate the efficiency of this treatment in disease control and the results were reported over the course of six years. During these trials, various harvester settings, types and timing of application were tested on around 90 sites in the Marlborough region. Results showed that shaking Sauvignon Blanc vines with the harvester just before bunch closure (between fruit set and pre vériason), reduced the prevalence of Botrytis infection significantly compared to a control site and, depending on the settings applied, there may be no reduction in the crop load.
Mechanical thinning vs mechanical shaking
The harvester settings can be adjusted and factors such as ground-speed, beater speed and beater positioning (one set on the trunk and one or two sets in the canopy about 50 mm above the fruit zone)1 were all adjusted and tweaked to define the amount of matter (crop and/or organic debris) removed from the canopy.
- A ‘heavy’ setting will result in the reduction of larger berries, bunch shoulders and even whole bunches, therefore reducing the crop size (thinning). This was accompanied by the elimination of organic debris located in the canopy of the vine. Thinning the crop in general delayed vériason and the onset of sugar accumulation, however, the lower yields resulted in faster accumulation of soluble solids from vériason and the soluble solids concentrations were generally similar to or higher than those of fruit from the unthinned control2.
- A ‘light’ setting (shaking) will only reduce the canopy debris without inducing a reduction in crop size.3 This setting allowed the harvester to move at a faster pace (5.0-5.5 km/h) and a reduced beater speed (480-490)4. Come harvest, no reduction in berry size or average bunch weight was reported. There was also no difference in ripening and the concentration of soluble solids of the harvested grapes when compared to a control treatment. Therefore, these settings did not induce crop thinning, however, there were still immediate and temporary signs of trauma to the growing tips directly after the shaking.
The results obtained from various degrees of shaking was compared to control sites to which a conventional maximum residue limits Botryticide program was applied at flowering, bunch closure and in some cases at vériason.
- Heavy setting:
During an exceptionally high crop-loaded year, the mechanical shaking treatment reduced the crop with a maximum of 25-30%.3 Together with the crop thinning, a reduction in Botrytis infection of 59% compared to the control site was reported.3
- Light setting
Applying the ‘light’ setting likewise reduced the Botrytis infection (decreasing the infection with 53% compared to the control), while maintaining the crop load.3
How mechanical shaking can reduce Botrytis infection
The plant response towards mechanical shaking may not be fully understood yet, however, the following factors may play a significant role in preventing the disease from setting in.
- Debris removal3
There is a strong correlation between the amount of organic debris trapped within the bunches and canopy and the level of pre-harvest Botrytis. By applying the mechanical shaking, the floral debris, stamens and unset berries in the canopy can be dislodged by passing harvesters and reduced by at least 70%. This will decrease the available material for the fungus to grow on. It will likely also allow better airflow leading to less moisture.
- Berry count3
By applying the “heavier” settings, the bunch is effectively thinned out. The removal of predominantly larger berries and shoulders reduces the yield and results in looser bunches with smaller berries. The “light” setting will have less of an impact on the berry count and yield.
- Berry skin thickness and berry size3
The researchers believe that the trauma applied while hitting the trunk and the canopy delays the cell division and berry size which in turn leads to thicker skins and greater resistance to Botrytis infections. The “light” setting will have less of an impact on skin thickness and berry size.
Spray or Shake…or both?
It is difficult to predict what the level of pre-harvest Botrytis infection will manifest in a specific vineyard, however, by applying the mechanical shaking as a preventative measure the likelihood of infection setting in will be reduced. Combining this treatment with your conventional Botrytis spray programme might result in even more desirable outcomes.
The use of machine harvesters, in conjunction with a conventional botryticide spray programme, was shown to be at least 50 % more effective at preventing Botrytis compared to the use of sprays alone. The researchers also reported that the results of one trial showed that it was possible that machine shaking is just as effective without the use of botryticides5.
The effective coverage of the Botryticide spray pre bunch closure is dependent on weather conditions, canopy density, bunch clumping and the ability to cover the bunch. The advantage of the mechanical shaking is that it can be done in windy conditions when grapegrowers are unable to spray and it is not dependant on effective spraying or timing.
REDUCING THE RISK – IS IT WORTH THE COST?
The risk of infection and fruit losses due to infected bunches should be weighed against the costs involved in applying mechanical shaking as a preventative measure. Varietal susceptibility and vineyard historical data can help to identify high-risk blocks.
In New Zealand, the cost-benefit of mechanical shaking becomes effective if you need to remove by hand more than 1 bunch of fruit per vine by either declumping or cutting off infected bunches3. In South Africa, this calculation might look dramatically different.
The use of modified grape harvesters appears to be a practical and cost-effective alternative to chemical control of Botrytis infection. In New Zealand, the option of a more sustainable, effective, and possibly cost-saving alternative to chemical control was welcomed with an estimated 50% of Marlborough producers currently applying this technique. Whether this is a viable method of disease control in South Africa will need further investigation, however, it certainly adds another option for producers wanting to transition to a more environmentally friendly manner of producing healthy Sauvignon Blanc grapes.
(1) Botrytis Control Gets a Shake Up. Grapegrow. Winemak. Mag. 2018.
(2) Trought, M.; Mundy, D.; Neal, S.; Allen, M.; Pecchenino, D.; Gunson, A. New Opportunities for Sustainable Grape Thinning. New Zeal. Winegrower 2011, 71, 77–78.
(3) Allen, M. Mechanical Shaking for Botrytis Control. Nelson Grape Wine 2016, No. 87, 8–10.
(4) Shaken Not Dropped. New Zeal. Winegrower 2015.
(5) Lewis, O. Grapegrowers Could Save Thousands Using Mechanical Harvesters to Prevent Disease. Marlbrgh. Express 2016.