UGA Extension Pecan Horticulturalist Dr. Lenny Wells shared this demonstration of sprayer coverage at the GPGA Field Day in Byron:
Many growers use a two-sided air blast sprayer to spray pecan trees. Spraying out of 2 sides has its advantages in allowing you to get over the orchard much quicker, but this comes at a cost. On a small tree of 30-40 feet it won’t matter all that much. You get adequate coverage with 2-sided spraying up to this height.
By adding a volute to the sprayer, allowing you to direct all the spray to one side, you obtain better coverage over the whole tree, especially for old, mature trees. Dr. Clive Bock with USDA demonstrated this at the field day by applying Surround, a Kaolin clay-based product used for preventing sunscald on fruit and vegetable crops, to pecan trees with an air blast sprayer. Surround coats the surface of the spray target with a fine, chalky, white substance, which becomes highly visible. This allows you to see just how good your sprayer coverage is.
The image above shows that spraying out of both sides (without a volute) provides good coverage up to approximately 40 feet, with very little coverage above that level.
This image shows that you get good coverage up to the top of the tree when spraying out of one side with a volute directing all the spray to one side.
This image shows the non-sprayed side of a tree sprayed with Surround. Virtually none of the chemical makes it to this side of the tree.
I’ve seen many cases of poor spray coverage in orchards in which the bottom 1/3 of the tree was scab-free but the top 2/3 was suffering severe losses to scab. In this situation we always tell growers that the problem is coverage. However, the growers first response is usually that there is a problem with the fungicide. The images above show convincingly that coverage is the biggest problem in this situation.
In addition, I know of many growers who choose to spray every other middle with a 2-sided sprayer each week. I’ve long been skeptical of this practice. These images pretty well show that when you spray in this manner, you are covering about 1/3 of the tree with fungicide every other week while getting almost none to the other side of the tree. Therefore, in a wet year, scab can be expected to be a problem with this scenario of poor coverage.
If a grower is spraying large trees out of both sides of the sprayer, they are obviously not doing an adequate job. In order to get better spray coverage there would basically be two options: 1) add a volute to the sprayer, and spray up one side of the tree and back down the other side on every spray. If you don’t have enough sprayers to get over the acreage in sufficient time, buy an additional sprayer, or 2) hedge the trees to keep them at about a 40′ height.
Many thanks to Dr. Bock and others at USDA who provided this demonstration.