A review of applicable methodologies for variable-rate spraying of orchards based on canopy characteristics

Variable rate spray applications using proportional control systems can greatly reduce pesticide use and off-target contamination of environment in orchards. Variable rate spraying of the canopy allows growers to apply pesticides only to the target, only use the correct quantity according to canopy size, season and growth stage and to apply plant protection products in an economic and environmentally sound manner. A major challenge is the reduction of agrochemicals used as Plant Protection Products (PPP) while achieving suitable deposition on the canopy. Spraying efficiency can be improved by reducing the spray losses associated with deposition on the ground and off-target drift. Adjustment of application rate proportional to the size and shape of tree crops has shown high potential for reducing agrochemicals in automatically controlled sprayers. In recent years target detection methods have been developed by using advanced techniques such as vision and laser scanning systems or simpler ultrasound, infrared and spectral systems. These systems have made it possible to develop geometric maps of trees allowing site-specific management of orchards. Variable rate spraying can thus be utilized as a methodology for applying the required amount of PPPs to the canopy while preventing over dosage as well as drift. Utilization of sensors to monitor canopy, distances and location ensures better use of expensive inputs, resulting in a sustainable approach to an important practice. This paper discusses various methodologies available for determination of canopy structural parameters and introduces some applicable commercial systems while pointing out their similarities and differences.