| Deposition and retention of pesticide sprays on leaf surfaces of hydrophobic plants is a major challenge in agriculture.In recent years,the deposition of liquid medicine on the hydrophobic surface caused by crop leaf diseases is a major agricultural problem.The surface of cucumber leaves modified by powdery mildew pathogens at different developmental stages is more hydrophobic than the surface of healthy cucumber leaves,which would hinder the accurate deposition of pesticides on cucumber powdery mildew leaves and may lead to a decrease in pesticide utilization.Therefore,it is of great practical significance for agricultural production to clarify the performance matching between powdery mildew modified cucumber leaf surface and pesticide droplet.The matching properties between the surface microstructure of cucumber powdery mildew leaves and the wettability of pesticide liquid are systematically studied by using the surface morphology,physical and chemical properties,and droplet wettability.Many recent studies have focused on the evaluation of adding surfactant in water rather than in pesticide solution to increase the deposition and retention effect of spray system,so the relative properties of pesticide solution adding surfactant on the surface of cucumber leaves need to be studied.Under the strategy of increasing efficiency and reducing application in China,the mechanism and common technology of efficient and accurate target deposition of chemical pesticides are the key development direction.Therefore,adding a surfactant to pesticide diluent to improve the wetting behavior and deposition behavior of pesticide solution is an effective means to improve the efficiency of pesticide on-target deposition.However,how to further balance the relationship between wetting behavior and deposition behavior of pesticide droplets on the surface of cucumber leaves needs to be studied and clarified.The main results are as follows:1.The surface structure of cucumber leaves at different developmental stages matched with the properties of surfactant solutionAccording to the texture characteristics of cucumber powdery mildew leaves,the developmental stages of powdery mildew on cucumber leaves can be divided into four developmental characteristic stages(NO.1,NO.2,NO.3,NO.4).The three-dimensional(3D)surface structure of the powdery mildew layer on cucumber leaves has specific characteristics in different developmental stages,among which the structure of the powdery layer on cucumber leaves in the first developmental stage is simple and the spores are not easy to spread.The structure height of the powdery mildew layer was the highest(Sa=425.35?m)at the NO.2 developmental stage,and the spores of powdery mildew were scattered and easily spread at NO.3 and NO.4 developmental stage.The surface free energy,static contact angle,and contact angle hysteresis effect of cucumber powdery mildew are closely related to the surface characteristics of powdery mildew at different developmental stages.When the concentration of surfactant reached the critical micelle concentration,the wetting state of AEO-4,AEO-5,and Silwet618 droplets on cucumber leaves surface is easier to reach Wenzel state.The wettability of droplets on the leaf surface depends on the monomer and micellar structure in the surfactant solution and the leaf surface characteristics covered by powdery mildew.This division provides valuable new insights into the matching of the structure of powdery mildew-covered cucumber leaves with the properties of surfactant solutions.2.Optimization strategy for inhibiting pesticide droplet bouncing on the powdery mildew leaf surfaceIt was found that the maximum adhesion area and the maximum rebound height of the pesticide droplets added with surfactants increased,and the trend of rebound to the highest point was relatively weak.In other words,pesticide droplets(fluopyram,pyraclostrobin,hexaconazole)added with surfactants(AEO-4,AEO-5,Silwet618)have a stronger adsorption effect on the surface of cucumber powdery mildew leaves,so the probability of the droplets bouncing is greatly reduced.By studying the effect of surfactant on the behavior of pesticide droplets,the kinetic analysis of three important states(initial state,maximum state of spreading,and highest state of bouncing)of pesticide droplet contacting cucumber powdery mildew leaves is carried out to clarify the direction of energy conversion of the droplet.When the pesticide droplets are in the highest rebound state,the energy dissipation rate of pesticide droplets and pesticide droplets added with RQA on cucumber powdery mildew leaves are lower than 81%,while the energy dissipation rate of pesticide droplets added with non-ionic surfactants AEO-4,AEO-5 and Silwet618 are higher than 81%.According to all the analysis results,when the energy dissipation rate of pesticide droplet reaches more than 81%,the hydrophobic surface formed by the development of powdery mildew layer will not affect the droplet's impact behavior on the surface,but will only have a slight retraction behavior,which will not lead to the droplet's rebound behavior.The decrease of pesticide droplet rebound and spatter is analyzed by combining the pinning site and viscosity dissipation rate formed in the droplet retraction stage.There are obvious pinning sites on the edge of droplet in retraction stage,which is mainly caused by the interaction between the contact line of the droplet edge and the gullies in the powdery mildew layer,where the energy consumption is high.In the retraction stage,as long as the retraction rate of the droplet is lower than 0.12 and the recovery coefficient is lower than 0.62,the droplet will form a certain number of pinning sites on the corresponding surface regardless of the severity of powdery mildew.There are four possible results after pesticide spray droplets contact the surface of cucumber leaves,including droplet rebound and spore ejection,droplet splash and spore ejection,droplet splash and spore ejection,and no droplet splash.During the application of pesticide spray on cucumber powdery powder leaves,the bouncing and splashing of pesticide droplets and the ejection behavior of powdery mildew spores are reduced by adding surfactants.This comprehensive evaluation method is helpful to understand the interaction mechanism between pesticide droplets and cucumber powdery mildew leaf surface.3.Changes of wetting and deposition behavior of pesticide droplets on the surface of cucumber leaves and its regulation mechanismThe three-phase contact line is formed when the droplet touches the solid surface.The position and moving speed of the contact line reflect the wettability of the liquid.The wettability of pesticide droplets without surfactant on cucumber powdery mildew leaves is very poor,and the contact angle is above 100°,indicating that cucumber powdery mildew leaves have hydrophobic properties.For pesticide droplets added AEO-4,AEO-5,and Silwet618,the wetting process on the powdery mildew layer is similar to the infiltration and wetting process of droplets on blotting paper,that is,the solution quickly engulfs the powdery mildew layer.The addition of cationic surfactant RQA does not improve the dynamic wettability of pesticide droplets on the surface of cucumber powdery mildew leaves,which is mainly due to the interaction between micelles formed by the positive charge of RQA molecule in pesticide solution and cucumber powdery mildew layer.When a nonionic surfactant is added,the droplet on cucumber powdery mildew leaves is in Wenzel wetting state,and the pinning force is weakened,the contact line velocity is accelerated,and the adhesion force is increased.The solution droplets containing non-ionic surfactants tend to form hemispherical micelle structures and bind to powdery mildew pathogen,while the solution droplets containing cationic surfactants do not form hemispherical micelle structure.Due to the electrostatic interaction between cationic surfactant molecules and powdery mildew pathogen,the wetting effect of cationic surfactant molecules on powdery mildew pathogen surface is poor.Therefore,it is suggested to add non-ionic surfactant to improve the wetting and spreading performance of pesticide solution on cucumber powdery mildew leaves.Micellar state and aggregation behavior of surfactant molecules in pesticide solution applied to cucumber powdery mildew leaves are analyzed.At the same time,the force balance model of pesticide droplet wetting state is established to calculate the pinning force of droplet and predict the transition direction of droplet wetting state.Finally,combined with droplet impacting dynamics and force balance model,the whole interaction process between droplet and surface is systematically analyzed,and the rational use of pesticide droplet is effectively guided,providing effective guidance for improving the application of pesticide droplets on the hydrophobic plant surface.4.Balancing the relationship between droplet wetting behavior and deposition behavior can effectively improve the pesticide utilization rateThe deposition rate of pesticide liquid does not necessarily increase with the increase of spraying amount.When the spraying amount increases to a certain level,the deposition rate of the pesticide does not increase.With the increase of pesticide spray volume,the deposition rate of pesticide liquid with surfactant is lower than that without surfactant.This result is mainly attributed to the increased wettability of the pesticide solution mixed with the surfactant,which leads to the decrease of the carrying capacity on the leaf surface,and ultimately leads to the loss of the solution.Without reducing the efficacy of pesticide control,we found that the spray volume of Silwet618 or AEO-5 should be adjusted to half of the normal spray volume,while the spray volume of RQA should be adjusted to two-thirds of the normal volume,which can increase the deposition rate of pesticides by about 30%.By comprehensive analysis of the deposition parameters and wetting parameters of the pesticide solution,the surface tension of the pesticide solution with surfactant is several times lower than that of the original solution,and accordingly,the spray volume is several times lower than that of the original solution.Even if the spray volume of the solution is reduced,the control effect of the solution and the original solution can be kept at the same level.Balancing the relationship between droplet wetting behavior and deposition behavior can provide theoretical guidance for the rational application of surfactants,and at the same time,it can reduce the use amount of pesticides,improve the effective utilization rate of pesticides,reduce the risk of environmental pollution,and achieve the strategic goal of increasing efficiency and reducing application.This study only preliminarily draws corresponding conclusions,which need to be verified by field experiments on a variety of crops and diseases in order to get more general conclusions.In conclusion,selecting suitable surfactants and adjusting the whole process of pesticide droplet application on the surface of hydrophobic leaves modified with powdery mildew at different developmental stages can effectively guide the prevention and control of cucumber powdery mildew. |
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