Heping Zhu

Influence of spray additives on droplet evaporation and residual patterns on wax and wax-free surfaces

Evaporation time and wetted area of single droplets sizing from 246 to 886 µm at relative humidity (RH) ranging from 30 to 90% were measured with sequential images under controlled laboratory conditions. Droplets were placed inside an environmental-controlled chamber under a stereoscope and a high definition digital camera. The spray mixtures used to form droplets included different combinations of water, a polymer drift retardant, a surfactant, and two insecticides. The droplet evaporation was investigated on the surfaces of crabapple leaf surfaces, hydrophilic and hydrophobic glass slides, respectively. Adding surfactant into spray mixtures greatly increased droplet wetted area while droplet evaporation time was greatly reduced. For a 343 µm droplet on the crabapple leaf at 60% RH, the evaporation time decreased from 70 to 50 s and the wetted area increased from 0.366 to 0.890 mm2 after the surfactant was added into the spray mixture containing water and insecticide. Adding the drift retardant into the spray mixture slightly increased the droplet evaporation time and decreased the droplet coverage area. Also, changing the target surface from the hydrophilic slide to the hydrophobic slide greatly increased the droplet coverage area and reduced the droplet evaporation time. Increasing RH could increase the droplet evaporation time greatly but did not change the coverage area. The droplet evaporation time and coverage area increased exponentially as the droplet size increased.

Surfactant droplet evaporation and deposition patterns on waxy leaf surface

Leaf surfaces are often altered when attacked by various insects and diseases. To evaluate the effect of droplet deposition on pesticide application efficiency, evaporation and deposit pattern formation of single droplets deposited at various locations on waxy leaves were investigated under controlled conditions. The locations on leaves included the intervenial area, midrib and secondary vein on both adaxial and abaxial surfaces. Tests were conducted with 300 and 600 µm droplets containing water and a nonionic surfactant. The ambient temperature was 25oC and relative humidity was 60%. Evaporation time and wetted area of droplets varied with where droplets deposited on leaf surfaces. The difference in evaporation time of 300 µm droplets without the surfactant on the intervenial area, midrib and secondary vein of adaxial surface was 30% while the difference in wetted area was 39%. The wetted area was significantly larger on the adaxial surfaces than on the abaxial surface but the evaporation time between both surfaces was not significantly different. For the whole leaf, the average evaporation time of 300 µm droplets was decreased by 44% and the average wetted area was increased by 202% when 0.25% nonionic surfactant was added into the spray solution. The total mean evaporation time increased 279% and the total mean wetted area increased 166% without the surfactant, 452% and 229% with the surfactant when the droplet diameter was changed from 300 to 600 µm. The largest spread area of droplets was appeared on the midrib of adaxial surface with the surfactant. The 300 µm droplets had longer evaporation time per droplet volume and greater wetted area per droplet volume than the 600 µm droplets. Increased pesticide application efficiency could be achieved with smaller droplets containing the surfactant to enhance the droplet spreading at different positions on waxy leaves.

The effects of spray application rate and droplet size on applications to control soybean rust

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.

Evaporation and spread of droplets with various types and concentrations of adjuvants on waxy and hairy leaves

Adjuvants have been used to improve pesticide application efficiency and effectiveness for many years. However, knowledge on quantitative reactions of adjuvant-amended pesticide droplets on foliage is lacking. Evaporation rate and wetted area of 500 µm droplets with four different adjuvants on waxy and hairy leaves were measured inside an environment-controlled chamber. The adjuvants included Crop Oil Concentrate (COC), Modified Seed Oil (MSO), Nonionic Surfactant (NIS), and Oil Surfactant Blend (OSB). Droplet evaporation rate and spread on either waxy or hairy leaves varied greatly with the type of adjuvants. On waxy leaves, droplets containing COC had significantly smaller wetted area than those containing MSO, NIS or OSB while the evaporation rate was opposite. On hairy leaves, droplets containing COC remained on top of hairs and did not reach the epidermal surface of leaves. When the relative concentration was 1.50, the wetted area of droplets with NIS was 9.2 times lower than that with MSO and 6.1 times lower than that with OSB. The wetted area increased as the concentration increased. Droplets with MSO or OSB spread extensively on the hairy leaf surface until they dried completely. Results of this study demonstrated that addition of MSO, NIS or OSB into spray mixtures with proper concentrations improved homogeneity of spray coverage on both waxy and hairy leaf surfaces, leading to pesticide reduction strategies.

Comparison of handgun and boom spray delivery systems for greenhouses

Handgun applications are popular general purpose greenhouse sprayers. However, there are few recommendations on how to use them to treat plant benches. Handgun applications made from one side can also result in significant differences in deposits between the side of the plant facing the spray stream and the opposite side. The objectives of this research were to determine how handgun spraying and broadcast applications using twin-fan sprays and air-assistance affect underleaf spray deposits. A bench-top trial was conducted using a mature poinsettia canopy in a

Adjuvant effects on evaporation time and wetted area of droplets

Appropriate adjuvant selection for pesticide applications is central to improve spray performances on waxy leaves and to reduce off-target losses. Evaporation and deposition patterns of 500 µm sessile droplets with five classes of adjuvants on five different waxy plants were investigated. Droplets generated with a single droplet generator were deposited on target leaves placed in a controlled environmental chamber at 60% relative humidity and 25oC temperature. Adjuvants tested were two types of oil-based Crop Oil Concentrate (COC) and Modified Vegetable Oil (MSO), two types of surfactant Nonionic Surfactant (NIS) and Silicone Surfactant, and a type of mixture Oil Surfactant Blend (OSB), plus the water-only droplets for comparison purposes. The Silicone Surfactant was removed from the test because its various properties were inconsistent with time. The five waxy plants were difficult-to-wet with the water contact angle greater than 90o. The single water-only droplets did not spread with time and formed extremely tiny wetted areas on the leaf surface. The addition of the adjuvant to the spray solution significantly reduced the contact angle and increased the wetted area, but the change varied with the plant specie and the adjuvant class. In general, the MSO and NIS had obvious effects on enhancing droplet spread and maintaining the droplet in solution on the waxy leaf surface. The residual pattern of droplets after evaporation was formed with “coffee ring”. Droplets with oil-based adjuvants had more uniform residual pattern than the droplet with the surfactant adjuvant. Results of this study demonstrated that selection of the appropriate class of adjuvants can significantly influence deposit formation on the waxy plants leading to effectiveness of agrochemicals.

The effect of spray volume and quality on handgun delivery of pesticides to greenhouse plants

Greenhouse pesticide labels lack specific recommendations on the spray volume and spray droplet sizes which will provide the most efficacious pest management. A greenhouse trial was established to determine differences in spray retention in a poinsettia canopy between applications using three different spray volumes and three different spray qualities using single nozzle handgun applicators. For the same areas of the canopy, there were few differences in spray deposit between treatments. Fronts and upper areas of the canopy received more deposits than the backs and lower areas of the canopy. There were no significant differences in recovery of fungicide from leaves between treatments. The high volume application produced the highest deposits on artificial targets across all spray qualities. There were no significant differences in overall spray deposit between the low and medium volume treatments.