How Can You Measure Spray Coverage?
When you spray during the growing season, we can see the droplets on the leaves. But how do we know if the droplets we see are enough to do the job? In other words, how do we know the sprayer is giving good coverage throughout the entire tree? Measuring spray coverage during the growing season is a great way to know if the sprayer is applying pesticides uniformly.
Water-sensitive spray cards are used to measure spray coverage. These cards have a yellow-coated surface that turns blue when exposed to liquids. This helps you to see what the sprayer did. The cards can be placed throughout the canopy and on the central leader to achieve good representation. The cards can be attached to branches in the canopy using alligator clips, and to the trunk using simple materials like twine and badge clips. It is helpful to leave the clips on the tree, and measure spray coverage at different times during the growing season to determine spray coverage when the foliage density of the tree changes.
Once the cards are in place, you can fill the tank with water and run the sprayer through the orchard at the same speed used during a typical pesticide spray trip. It is important to let the cards dry before collecting them to avoid smearing the stain and getting an inaccurate reading. It is also important to note the weather--relative humidity above 80% will turn the cards blue! Avoid taking spray coverage data early in the morning when there is a risk of dew on the leaves in the canopy. Once you’ve collected the cards, a business card reader, office scanner, or printer scanner can be used to scan the cards onto a computer. Once the image has been scanned, the USDA-ARS has developed a free, easy-to-use scanning program to help analyze the cards to determine the percentage of spray coverage.