ASSESSMENT OF A MATHEMATICAL-MODEL TO PREDICT SPRAY DEPOSITION UNDER LABORATORY TRACK SPRAYING CONDITIONS .2. EXAMINATION WITH FURTHER PLANT-SPECIES AND DILUTED FORMULATIONS

Mean spray depositions onto leaves of five plant species (Zea mays L., Vicia faba L., Sinapsis alba L., Glycine max (L.) Merr., Vitis vinife ra L.) were measured following spraying of an array of 36 solutions of acetone + aqueous 'Triton X-100' varying systematically in compositio n and properties. The spraying was carried out on five occasions using a standard laboratory track sprayer delivering the equivalent of 600 litre ha-1 onto a plane surface around plant height. The results, plot ted as response surfaces, showed that there was little variation in sp ray deposition with solution composition for V. vinifera, there were s ome slight decreases in deposition with increase in 'Triton X-100' con centration on V. faba and S. alba, though not with increase in acetone concentration, and that there were slight systematic increases for G. max and large systematic increases for Z. mays with increase in aceto ne and 'Triton X-100' up to concentrations of 3 50 ml litre-1 and 0.5 g litre-1 respectively. At higher concentrations of these components, there were no further increases in deposition on these latter species. The results were in agreement with those predicted by a mathematical model derived previously, with the exception of the slight decreases i n deposition on V. faba and S. alba and smaller increases in depositio n than predicted on G. max. The decreases in deposition on the former species were attributed to slight run-off from their easy-to-wet leave s at the higher 'Triton X-100' concentrations at the spray volume rate (600 litre ha-1) used. The poor fit of the observed and predicted spr ay depositions on G. max was attributed to the nature of its leaves. T hese are hairy and it is speculated that the fine hairs, rather than t he true leaf surface, played a major role in capturing the small spray drops (115-130 mum) created by the laboratory sprayer used in this wo rk. The observed and predicted spray depositions with a set of diluted commercial-type formulations were in good agreement for Setaria virid is (L.) Beauv., Pisum sativum L., Z. mays, with correlation coefficien ts (r) of 0.985, 0.988 and 0.935 respectively, and also for the more c onstant depositions on the easy-to-wet species Beta vulgaris L., but s lightly less so for Triticum aestivum L. (r = 0.886) in this test. Ove rall the model was well-behaved, giving a good prediction of the varia tion in spray deposition on leaves of a range of plant species, provid ed that these were not extensively hairy, with variation in the dynami c surface tension of the spray solution.