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Refining spray recommendations

tractor driving through grape field

We’ve come a long way in the last 10 to 15 years towards better refining application technology and product mixing recommendations for canopy sprays.

Good spray results rely on applying the correct product to the correct target at the correct rate at the correct time. It is also recognised that the area of canopy in a vineyard will vary dramatically depending on canopy configurations, row widths, varieties and plant spacing. Canopy sizes can vary by up to 10-fold from bud burst to harvest and vary considerably between different grape growing regions and climates.

A key change has been to recommend product label rates in terms of rate per 100 litres of water (mL/100 L), rather than rate per hectare (L/ha). The change has been a success, particularly as a rate per hectare did not take into account canopy differences.

It is, however, worth taking some time to refresh your understanding on why this change was made, as well as ensuring those responsible for providing recommendations or applying these products are aware.

The question you need to ask is, “Is the volume of spray being applied appropriate to the target, growth stage and the product being used?”

Inadequate spray volumes may result in too lower dose rate, which may lead to poor results. On the other hand, application of systemic chemicals in very high water volumes can cause the product to wash-off the vine leading to poor disease control and potential environmental risks.

Dilute spraying refers to the water volume needed to cover the total plant surface with spray solution to the ‘point of run-off’. The spray volume is key to ensure enough chemical is applied to control the disease or pest. The amount of water needed to achieve point of runoff is often referred to as the ‘theoretical volume’ and in grapevines, it depends on the size of the canopy. For example, at the start of the season when vines are breaking out of dormancy, low water volumes, say 300 L/ha, are recommended. Whereas, large canopy vines such as those in some vigorous table grape vineyards, may require up to 1500 L/ha.

When calculating or spraying to the point of runoff, coverage of the whole canopy, including inner foliage and bunches, needs to be considered, not just the outer leaves closest to the sprayer. Issues arise when the assessment determining the ‘point of runoff’ is inadequate, especially with large canopy vines. Usually, the outer leaves of the canopy is the first instance where you will see the spray running off. However, if you have a good look in the inner sections of the canopy you can often find leaves or bunches where spray coverage is considerably less. Runoff has not been reached if the inner canopy is mostly dry even if spray solution is dripping from outer leaves.

It is important that the sprayer has been set up to ensure even coverage of inner foliage and bunches, not only outside leaves. Utilising tools such as Surround (a vivid white sun protection product) can assist in making spray coverage assessments in a very visual way. Also, having in-vineyard discussions with a trusted adviser throughout the season and at various canopy growth stages will assist in making the correct adjustments to the spray volume.

Concentrate spraying is more complicated because it involves applying an amount of water less than the point of runoff, but a sufficient volume to achieve acceptable coverage. Acceptable coverage may be achieved at a lower volume of water if the air volume is high enough and is well directed. Adequate coverage at lower volumes may be possible by increasing the number of smaller droplets with a narrower size range. The important thing to remember is that the same amount of chemical (per hectare) is sprayed that would have been applied if dilute spraying had been used. For this to be achieved, the concentration of product in the spray mixture increases proportionally as the volume of water is decreased from the theoretical volume. The amount that this must be adjusted is termed the ‘concentration factor’.

This article only scratches the surface of application technology. I hope it is a catalyst for you to discuss this subject with your adviser, contractor and your wider network. The industry is to be commended on strong progress over the last 10 to 15 years, but we need to continue to move forward and strive for continuous improvement.