Managing spray application in the presence of drape nets. By Engela Duvenage.
“Drape netting reduces tree canopy penetration. It influences where and how thoroughly sprays are deposited onto leaves, and the amount of chemicals ultimately deposited on the outside and inside of tree canopies,” said Dr Gideon van Zyl, technical consultant with ProCrop, an agricultural advisory service.
Drape netting helps to protect fruit from being damaged by the sun, hail, birds, bats, and the wind. However, the physics governing the movement of air carrying droplets through netting throws a spanner in the works of producers’ hopes to effectively spray covered orchards with pesticides and fungicides, or to apply supportive sprays.
According to Van Zyl, nets form a physical barrier around the trees. In the process, it increases the effective density of the canopy. “Nets are like perforated barriers that spray droplets have to penetrate,” he explained.
The material complicates air flow and diffusion. It influences the direction in which air moves — this could cause slight turbulence, increasing air resistance in the space between the net and the tree.
Van Zyl said that all this influences where droplets land and ultimate penetration into the canopy. More leaf disturbance can occur, and some leaves can be pressed to the insides of the net. This disturbs deposition. A cushion wave also forms on the outside of the net and disturbs airflow as it moves from the sprayer to the tree. This influences the column of moving air and makes it more difficult for air to penetrate into the enclosed orchard.
Pest and disease control under netting
Van Zyl conducted trials in apple orchards of the Du Toit Group. He learnt that although nets seem to help capture spray drift, one does not get the same level of deposition on leaves and fruit when spraying through netting as in open orchards.
“It’s better to manage and spray for pests and diseases before draping an orchard. A reduction in spray deposition parameters after draping complicates and ultimately reduces pest and disease control,” Van Zyl explained. “Optimal deposition is a necessity for disease control. When you want to prevent or control apple scab and powdery mildew, for instance, you’d ideally want even, comprehensive coverage throughout a tree. This cannot be guaranteed when spraying through a net.”
On the plus side, nets keep certain insects out. However, the material can also trap other insects inside, such as woolly apple aphids and mealy bugs. It complicates the control of these pests.
“If you want to use drape netting, I’d first control the primary fungal infections and pests in an orchard before closing it down. Do so early and effectively. I’d also think twice before closing up an orchard later in the season that has a history of pests and diseases, or where control was suboptimal,” he advised.
Optimising spray deposition
Van Zyl did trials spraying horticultural products such as calcium through netting, and noted a reduction in how much landed on the trees.
“Doing so will ultimately influence the amount of calcium being taken up by the trees. If you want to spray through nets, you will have to consider spraying more often. This might have cost implications,” he underlined.
According to Van Zyl drape netting can influence the breakdown of pesticides. “In theory it is possible because of less light penetration through the material. Another possible issue is the build-up of pesticides on inner and outer netting throughout the season. These factors will be investigated in future studies,” he noted.
Better spray deposition was obtained when higher air volume applicators were used on inner and outer canopy leaves, because of improved canopy penetration. Producers using more progressive application methodologies, such as sprayers producing low air volume and speed, should rather use a higher air volume setting when spraying draped orchards.
Van Zyl’s trials showed that sprayer setup, the type of nozzle used, the actual nozzle setup, and calibration all significantly influence how well sprays are deposited onto leaves and fruit throughout the tree canopy.
Another finding was that spray deposition is not improved when one simply increases the spray volume.
“In fact, it makes it worse,” he concluded. The more water is used, the easier a film of water forms on the outside of the net, from which water simply runs off.