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Dilute spraying and under dosing

Vine Talk with Scott Mathew
Vine Talk with Dave Antrobus

By Dave Antrobus, Solutions Development Lead

On many occasions, I come across growers who think they are spraying at the correct chemical rate for dilute spraying, but on further investigation with an in-vineyard test, they are under dosing. I find under application of dilute spray volumes of 10 to 20 per cent is common and in extreme cases, it can be as high as 40 per cent. Less than required spray volumes and resultant under dosing of chemical has clear implications for product performance and resistance development.

Dilute volume spraying is where the vine canopy is thoroughly wet to the point of run off. The volume required to reach this point will depend on the canopy size, which changes with vineyard setup and as the season progresses.

My first recommendation is that growers review their canopy size at three stages of seasonal growth and compare their dilute volume decisions against the APVMA recommendations (See Indicative water volumes for dilute spraying grapevines). If there is a significant variation, you should look to do some in vineyard spray trials to determine your correct rate. Spray volume really is the key in determining the amount of chemical that should be applied to a vine canopy to deposit a correct label dose.

Because every vineyard is different, in-field measurements and observations will most likely deliver the most reliable and accurate results. Your own spray tests are an ideal way to gain the experience you need and it’s not as hard as you may think.

Simply run a series of tests where you increase or decrease the spray volume until a point is reached where you consider the spray liquid covers all parts of the canopy to the point of run off. Your local spray expert may be able to help you with this.

Of course, it’s easy to see when water is at the point of runoff on the outer canopy. Assessing the point of runoff inside the canopy is more difficult. Water and oil-sensitive paper (WSP) is a fantastic resource that has been available to spray operators for many years. The WSP card has a special coating that produces a stain when a droplet lands on it. Put these throughout the canopy to make sure spray droplets cover all parts of the canopy as evenly as possible. If a wetting agent is to be used for spraying, then it should be added to the water before you run your tests.