Improving Late Season Spray Coverage  

close-up of leaf sprayed with water

By: Kirk Williams, Lecturer-kirk.w.williams@ttu.edu

Lots of time is spent selecting products to control late season pests but how much time do we spend fine tuning our sprayer for these applications?  Late season spray applications can be critical in delivering clean fruit and keeping a functioning canopy post-harvest especially in areas where rainfall is common during the growing season.   Effective late season spray applications can be more difficult to achieve due to a larger and denser canopy.   

  Understanding spray coverage requires that you understand how droplets are created.  Hydraulic spray nozzles create a wide range of droplet sizes.   These droplets are measured in microns with droplets from 150 to 300 microns considered the most effective at being deposited in grapevine canopies when used with air blast sprayers.   Larger droplet sizes may bounce or roll off leaves while very fine droplets are more prone to drift.  Theoretical spray coverage based on average droplet diameter is 30% for 100-micron droplets, 15% for 200-micron droplets and 10.1% for 3300-microndroplets.  A greater number of spray droplets per unit area will have a higher probability of controlling the target pest. 

  Evaluating your spray coverage at any time of the season is critical but as the canopy gets larger and denser it becomes even more critical.  One way to evaluate spray coverage that requires minimal effort is with water sensitive cards placed in critical parts of the canopy such as the fruiting zone.  When spray droplets land on the water sensitive card, the card turns blue.   Water sensitive cards should be used when the humidity is below 80% and the canopy is dry because any moisture will turn them blue.  The water sensitive cards will react to moisture in your fingers, so be sure to wear waterproof gloves when placing the cards in the canopy.

  Spray coverage standards do not exist for control of specific insects or diseases.  Recommendations from Syngenta, one of the manufacturers of water sensitive cards, recommend 20-30 droplets/cm2 for insecticides and 50–70 droplets/cm2 for fungicide applications to provide satisfactory results.  A square centimeter is about 1/6 of a square inch.  The water-sensitive cards can be visually assessed for coverage through the provided viewing cards.  Visual observation can help you understand coverage and see droplet size, but it does not give you any numerical information that can be used to improve spray coverage.

  Water sensitive cards can be scanned and analyzed by computer programs such as the USDA’s ARS DepositScan.  The program gives you percentage coverage and deposits per cm2 .  While very accurate, this is a slow process and not well suited for the quick results needed for adjusting sprayers in the field. 

  Apps are now available that will quickly and easily analyze water sensitive cards for coverage.  SnapCard is an app that is available only for Android smart devices.  SmartSpray, is available for both iOS and Android smart devices. Developed for use in field crops such as strawberries, the spray card analysis portion of the SmartSpray app is a useful tool for analyzing percent coverage as well as accounting for small and large spray droplets.  

(Figure 1)

grapevine canopy was sprayed (Figure 1) and analyzed for spray coverage using the SmartSpray app (Figure 2).  The analysis from the SmartSpray app came back with 35% coverage with small droplets and 3% coverage with large droplets.  This quick analysis shows very good coverage for this application and within the range of what we would consider good coverage.   The SmartSpray app does not report droplets per cm2  so to confirm that our coverage number matched up to droplet number the same card analyzed by DepositScan. DepositScan reported 125 droplets per cm2 which is more than the target 50-70 droplets cm2 for fungicide applications.  The SmartSpray app can be a new tool to quickly analyze your spray coverage in the field when using water sensitive cards. 

(Figure 2)  

If your water sensitive cards indicate that you are not getting good coverage there are several ways to adjust your sprayer to improve coverage. 

Increase Air Flow

  Lower fan speeds may have been used in the early part of the growing season to reduce drift.  In the later part of the season many sprayers can be adjusted to increase fan speed.   For increased late season spray coverage in dense canopies, increasing the air flow may be the easiest way to improve coverage.   Deflectors that focus the air flow more horizontally or on the fruiting zone may also improve coverage.  Keep in mind that excessive air flow can increase drift by pushing the spray droplets past the canopy target.   Be sure to confirm coverage using water sensitive cards after increasing airflow. 

Change Sprayer Output

  Increasing sprayer output by decreasing forward speed or by using larger nozzles can improve spray coverage but it does require that you recalibrate your sprayer due to these changes.   A higher volume can improve coverage if your droplet coverage is low based on your water card analysis.  The target is still to have many small droplets in the canopy.   Too much volume can run spray droplets together and cause spray run off of the leaves. 

  There are many action items to do in a vineyard to keep it productive and produce high quality grapes.  Taking time during the season to check your sprayer coverage and then adjust as needed is one of those critical items.  Larger and denser canopies may require that you take the time to look at your coverage later in the season as well.  New and old tools are there to assist you in evaluating and improving your spray coverage. 

Sources:

Bettiga, L.J. ed. 2013.  Grape Pest Management (3rd edition).  Publication 3343. University of California, Agriculture and Natural Resources.  Oakland, CA

Nansen, Villar, G. D., Recalde, A., Alvarado, E., & Chennapragada, K.  2021. Phone app to perform quality control of pesticide spray applications in field crops. Agriculture (Basel), 11(10), 916.

Landers, Andrew J., 2016.  Effective Vineyard Spraying (2nd Edition).   Effective Spraying.  Geneva, NY

Zhu, Heping; Salyani, Masoud; Fox, Robert D.  2011.  A portable scanning system for evaluation of spray deposit distribution.  Computers and electronics in agriculture, Vol.76 (1), p.38-43

Kirk Williams is a lecturer in Viticulture at Texas Tech University as well as being a commercial grape grower on the Texas High Plains.  He can be contacted at kirk.w.williams@ttu.edu

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