A successful phytosanitary treatment demands that the product must be uniformly distributed over the foliar surface of the tree. Modern trends are about the doses reduction of the active product, through the technological upgrade of the terrestrial sprayer vehicles. The traditional ways to apply these products over the field are using mechanical continuous rate sprayers. This yield a continuous application, even in undesired empty spaces such as between plants or missing plants. The Victoria de Girόn Fruit Company of Jagüey Grande, Cuba, were suited with a lot of air assisted mechanical sprayer of the TEYME manufacturer. This paper is focused on the design of a low cost hardware and software architecture, addressed on the automation of the variable application of pesticides, through tree detection in citrus plantations. The designed system is composed by two distributed subsystems: the one embedded in the tractor is responsible of the positioning and tree inventory update, the other, embedded in the air-assisted sprayer detects the tree presence using ultrasonic methods. The performed experiments, showed a suitable acquisition of the position of the system. It was confirmed that the designed prototype allows the detection of plants with an accuracy of 98%. The proposed architecture makes it possible to reuse existing vehicles by improving spraying. This will make it possible to save pesticides by reducing costs and pollution. These results are extendable to other fruit fields that employ a spraying system.