Evaluation of the aerial application of micronutrients and adjuvants to citrus orchards

Abstract

Large farming operations make use of conventional tractor sprayers that spray high volumes of water. This is done to increase coverage and uptake of the active ingredient that is sprayed. However, time is of the essence to make applications at critical phenological growth stages of the citrus trees in order to make the micronutrient application as effective as possible. Due to the need for timeous applications, large farms make use of aerial applications that are more time efficient with lower volumes of water used. Thus, a trial was initiated in collaboration with Villa Crop Protection and the Citrus Research International (CRI) to evaluate the aerial application of micronutrients, and to test the value of an added adjuvant (Masterlock®) to improve application efficacy and nutrient uptake. An experimental citrus orchard of McClean Valencia’s was selected at Letaba Estates in the Limpopo province, and foliar Zinc (MAX-IN™ ZINC) and Boron (MAX-IN™ BORON) products were applied, as well as a patented product Masterlock®. The MAX-IN™ range consists of a surfactant and humectant to increase leaf uptake of the micronutrient, Masterlock® enhances droplet distribution throughout the canopy, as well as increasing the uptake of applied compounds through the leaf surface. The trial was carried out over two years, with micronutrient applications conducted in September of 2019 and 2020. Leaf analyses to quantify total B and Zn concentrations were conducted on leaf samples before application and two weeks after application. Droplet deposition was quantified by using two methods: 1. Water sensitive paper and 2. Dropsight. Deposition measured throughout the canopy to ascertain the efficacy of Masterlock® to improve distribution and wetting levels of the spray mixture. Fruit set was determined for both seasons as well as fruit size and brix for the 2019 season. The aerial application of micronutrients significantly increased the total foliar nutrient concentration in the leaf. Micronutrients can be used to maintain the micronutrient nutrition status of trees throughout the growing season if multiple aerial or conventional applications are made. Compared to the producer’s prescribed programme, where products were applied at a much lower concentration, the MAX-IN™ products, applied at a higher concentration were not more effective at increasing the foliar total B and Zn concentration when applied either aerially or conventionally. Both the MAX-IN™ product range and the farm practice were able to iv increase total Zn and B concentrations inside the leaf within the prescribed norms. Masterlock® significantly increased the droplet deposition and total wetted surface area throughout the tree canopy for aerial applications compared to the treatment were no Masterlock® was included. The better distribution throughout the canopy can be explained by an increase in deposition onto the leaf surface. This study conclusively established that aerial application of foliar nutrients such as B and Zn are effective in citrus and can be used successfully. Masterlock® an adjuvant that consist of drift control characteristics and a surfactant will significantly improve the deposition of the spray mixture of aerially applied nutrient mixtures