Dr Gordon Brown reviews the results of season-long pesticide application through a fixed system compared to an airblast sprayer.
Traditional airblast sprayers are designed to push insecticides and fungicides several metres through a thick canopy to ensure excellent coverage and distribution of crop protectants throughout the tree. Modern tree training systems, however, are focussed on producing thin canopy structures not suited to traditional airblast sprayers which blow crop protectants straight through the fruiting wall where they can be lost to the environment. Hence there is a need to adopt new technologies to ensure efficient and efficacious application of fungicides and insecticides with minimal loss to the environment.
One possibility is the use of a fixed spray system permanently installed within the canopy which can rapidly apply crop protectants without worker or heavy tractor and spray equipment entering the orchard. This allows for timely pesticide application without damaging the soil structure, trees or trellis system and minimises exposure of orchard workers to pesticide spray drift. It even allows for potential automated application of pesticides, evaporative cooling of the crop as well as frost protection.
Applications of crop protectants through fixed systems such as those designed for overhead frost protection or irrigation have been historically studied, however, they have proved unreliable for a range of reasons. More recently the focus of research has moved to studying the efficiency and efficacy of fixed systems using micro sprinklers permanently installed in the canopy specifically for crop protectant application.
While there have been several research papers that demonstrate that this approach does not lead to even spray coverage on the foliage the system has comparable efficacy for controlling insect pests and fungal diseases. The most recent publication by Owen-Smith, Wise and Grieshop, based in Michigan, USA, reports on results of season long pesticide application through a fixed system compared to an airblast sprayer. They studied pesticide coverage, distribution and deposition as well as black spot and insect control.
Their fixed system consisted of a pesticide supply line that looped back at the end of the row and returned back to the spray tank. Attached to this supply line was non drip spray valves which supplied pesticide to three dripper tubes when the line pressure exceeded 241 kPa. These dripper tubes fed pesticide to Hadar 7110 series micro sprinklers with 0.08 mm nozzles. One micro sprinkler was placed 2.6m high in the canopy, over the top of a tree and the other two were placed 1.2m high between the trees (Figure 1).
Figure 1. Schematic of the canopy delivery system
To operate, the return line was left open, maintaining the line pressure below 241 kPa and the pesticide pumped through the lines until it was returning to the spray tank. Due to the pressure the non-drip valves prevented the micro sprinklers from operating. Once ‘primed’ the return line was shut off, rapidly increasing the system pressure such that the micro sprinklers operated. After 10 seconds the return line was opened to cease application and this applied 655 L/ha at 415kPa. Upon completion the supply line was cleaned with compressed air to push residual pesticide back to the spray tank and when clear the return line was turned off again to push air through the micro sprinklers to clean them of residual pesticide. This system was tested against a tractor mounted-airblast sprayer which also delivered 655 L/ha. A total of 15 insecticide and 15 fungicides were applied on 12 dates during the growing season. Pesticide coverage on low, middle and upper canopy leaves was measured on 4 occasions and assessment of pest and disease control was made mid-season as well as just prior to harvest.
Results
The mid-season spray coverage and deposition results are presented in Figures 2 and 3. For both spray systems the surface area covered in spray was highest on the upper leaf surface at all heights in the canopy. While the spray coverage on the upper leaf surface was always slightly lower for the solid set spray system compared to the airblast sprayer it was markedly lower on the lower leaf surface, particularly higher and lower in the canopy. This difference with the airblast sprayer was due to the lack of air movement as well as larger droplets. These results for spray coverage demonstrate that the solid set system had poorer leaf coverage of pesticide.
For many years now there has been a focus on designing airblast sprayers to maximise their coverage of pesticides onto the leaves to improve pesticide efficacy. As a result this aspect of a sprayer’s performance has been used by marketers to promote their sprayer over the competitor’s strengthening the perception that spray coverage is critical for efficacious spray application. It must be remembered, however, that compared to older pesticides, modern crop protectants usually have trans laminal translocation potentially making complete spray coverage less important.
This research also identified that despite the poor spray coverage with the solid set spray system the quantity of pesticide per leaf deposited was about double that of the airblast sprayer, potentially due to the larger droplets (Figure 3). This indicates that less pesticide was lost to the environment and more deposited on the target increasing the potential for pest control within the orchard. In their conclusion the authors state that in a later study it was found that there was a two-fold reduction in the off-target drift confirming less contamination of the environment with the solid set spray system.
Figure 2. Foliar coverage on upper and lower leaf surfaces of spray material, mid growing season for a solid set application system versus an airblast sprayer on leaves low, middle and high in the canopy.
Figure 3. Spray residue (deposition) on foliar surface, mid growing season for a solid set application system versus an airblast sprayer on leaves low, middle and high in the canopy.
The impact of this interaction between foliar coverage and quantity of pesticide deposited on the leaves makes a study of pest control essential. In this trial the nearby unsprayed trees had more than 90 per cent of the leaves and 80 per cent of the fruit with visible black spot symptoms. This compared to less than 2 percent for both spray systems which were not statistically different from each other indicating both had excellent and equivalent control of the disease (Figure 4).
A similar story was found for insect control where the fruit from the nearby unsprayed plots had more than 19 per cent of fruit with insect damage. Again the two spray systems had statistically identical control with less than 2 percent of fruit with insect damage (Figure 4).
Finally the authors measured the temperature in the orchard during operation and it was found that the 10 second spray application dropped the temperature within the orchard by 2 to 3°C confirming the potential of the system to cool the crop.
Figure 4. Incidence of black spot on mature leaves, terminal leaves and fruit as well as insect damage on fruit for a solid set application system versus an airblast sprayer. Incidence on nearby unsprayed trees was 98, 89, 80, 19 and 29% respectively.
Take home messages
This study has demonstrated that a solid set sprayer system in a modern thin-walled orchard can provide equivalent protection of the crop from pests and diseases to traditional airblast sprayers. This system has the advantages that applications can be made rapidly in small windows of opportunity and there is no need to drive heavy equipment into the orchard with the associated negative impacts on the soil. On the safety front the spray workers have less exposure to pesticide spray drift and there is less pesticide lost to the environment with more hitting the target. There is also potential to use this system for frost control as well as evaporative cooling the crop to assist in prevention of sunburn damage on the fruit. This is a new technology that will improve over time as system designs are refined and specialist equipment for the method developed.