Canopy penetration and thorough coverage are important when applying foliar fungicides to soybeans for the control of rust. The purpose of this study was to examine how spray application rate and spray droplet size affect the efficacy of soybean rust applications. Four treatments were examined: a medium droplet spectrum applied at 47 L/ha (5 GPA) and at 140 L/ha (15 GPA), and a very coarse droplet spectrum applied at 47 L/ha (5 GPA) and at 140 L/ha (15 GPA). Applications were made to soybeans planted with 91 cm (36 inch) row spacing in the R5 growth stage. Spray coverage, deposition, soybean rust severity, and yield were measured to evaluate the effectiveness of these treatments. Spray coverage was measured on Kromekote paper positioned in the upper and lower parts of the canopy. Deposition was measured using a dye and Mylar plates positioned in the upper and lower parts of the canopy. The very coarse droplet spectrum at 140 L/ha (15 GPA) had the highest coverage and deposition in both the upper and lower canopy. Overall the very coarse droplet spectrum performed better than the medium droplet spectrum. There was no difference among the treatments in soybean rust severity or yield. All the treatments had significantly lower soybean rust severity than an untreated control, and all but the very coarse droplet spectrum at 140 L/ha (15 GPA) had a significantly higher yield than the untreated control.
The impact of tank mix adjuvants and a formulated fungicide on spray atomization and in-field movement under aerial application conditions was examined. High speed wind tunnel testing was conducted to determine droplet size resulting from treatments selected for evaluation in the field. These treatments included a "blank" (water plus a non-ionic surfactant) as well as five additional solutions with a formulated fungicide, four of which have an additional adjuvant. The wind tunnel testing measured droplet size using the flat fan nozzles and operational parameters (spray pressure, nozzle orientation, and airspeed) selected for field trials. These treatments were then evaluated in the field for both in-swath and downwind deposition, with a mass balance on the measured results used to compare each of the formulated product treatments to a reference treatment. Wind tunnel results showed the formulated product tank mixes resulted in significantly different droplet sizes than the water and non-ionic surfactant "blank" reference Manuscript sprays. Additional adjuvants resulted in minimal changes in droplet size as compared to the formulated product mixture. However the polymer tested broadened the droplet size distribution. Drift modeling of the wind tunnel droplet size results demonstrated little difference between the formulated product and spray adjuvant spray mixtures. However, all treatment solutions significantly reduced modeled drift as compared to the reference treatment. While the field study results did highlight significant differences between treatments solutions, it also showed a great degree in data variability as a result of meteorological and sampling issues. These results have led the authors to conclude that field testing of potential drift reduction technologies under aerial application conditions will be cost prohibitive and likely would give highly variable results. Wind tunnel evaluations at certified laboratories offer a much quicker and inexpensive method for evaluating large numbers of nozzle and spray formulation treatments.
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