SPRAY APPLICATION PLAYS A DETERMINING ROLE IN PERFORMANCE OF SYSTEMIC COMPOUNDS APPLIED TO THE FOLIAGE OF FRUIT PLANTS

Spray application factors are discussed relative to effective delivery of foliar applied nutrients to fruit trees. The application process can be viewed as a series of complex interdependent events, including: formulation of the active ingredient; atomization of the spray liquid; transport of the spray cloud to the tree; spray droplet impaction; retention; and residue formation and penetration of the nutrient. Any significant change in one event has profound effects on others, and each may be affected by application parameters, plant factors and environmental conditions. Only limited data have been generated specifically on effects of application parameters on performance of nutrient sprays. Thus, we have drawn heavily on studies of application of aqueous solutions of pesticides. Nutrient sources are not extensively formulated, but since nutrients are commonly incorporated in programmed pesticide applications, the spray additives (mostly adjuvants) used in the pesticide sprays will impact on the performance of the applied nutrients. The lack of uniform deposition over the fruit tree is a critical limitation; the lower quadrant next to the spray lane is frequently overdosed while the top and center are under sprayed. Fruit tree surfaces, except for some fruit surfaces, are not difficult to wet, thus droplet reflection is not a limiting factor. Surfactants, when used, reduce reflection, increase spreading (coverage) and, at high volume spraying, reduce the total dose retained per unit surface area. Residues from previous pesticide sprays and the presence of natural products (e.g., extrafloral nectar) on some plant surfaces modify spreading and retention. Droplet reflection, spreading, retention and nature of droplet residues are determined primarily by the physico-chemical properties of the spray solution together with plant surface morphology, fine structure (microroughness) and chemistry. The nature and distribution of spray droplet residues are important factors in performance of systemic compounds because penetration not only takes place from the droplet, but also from the remaining residue.