Research PaperAnalysing the preference for pesticide spray to be deposited at leaf-tips
Introduction
Currently, chemical pesticides remain the main method to protect the crops from pests, weeds and diseases. In 2017, over 40 million tons of pesticides were used world-wide with the average pesticide consumption in China being around 13.06 kg ha−1, which is 5 times higher than the mean world-wide value (FAO, 2016, 2017). Pesticide spraying, is a complex process including liquid atomisation, spray dispersion and droplet deposition. Much research has been reported on these processes including the movement of droplets following their impact onto leaf surfaces (Dorr et al., 2015).
The deposition characteristics of droplets on rice leaves were investigated by (Tu, Lin, & Zhang, 1984) who pointed out that spray deposition at the tip of the leaves was slightly, but nevertheless obviously, higher than other locations on leaves and proposed the idea that there was a “leaf-tip preference” (Tu, Lin, & Zhang, 1986). They found that whatever the position of the rice leaves and the application volume, a leaf-tip preference always occurred. This was demonstrated and confirmed by the water sensitive paper and pot grown rice. Leaf-tip preference could be caused by a variety of factors including droplet trajectories during dispersion and also retention and impaction.
The dynamic process of droplet impaction can be considered analogous to a ball hitting a spring-like system surface (Xu, Liu, He, & Wang, 1998). Based on this model, the main factor of impaction procession is the droplet energy. This conclusion could also be proved on other plant canopies where deposition decreased more with larger droplet diameter and higher droplet speed (Taylor & Shaw, 1983). Regarding the process of droplet impaction, three kinds of behaviour - adhesion, rebound and shatter can be observed (Mercer, Sweatman, & Forster, 2010). Many simulation models focusing on the impact process have been developed to calculate the deposition and distribution of spray in crop canopies (Dorr et al, 2008, 2014, 2016, 2014; Nairn, Forster, & Van Leeuwen, 2013, 2014), but most research in this area uses the static leaf models and ignores the movement of leaves following droplet impact.
Tu et al. (1986) suggested the perceived leaf-tip preference was caused by the droplet trajectory changing as spray approached the leaves. There have also been many models studying the movement of droplets in the air (Butler Ellis et al., 2002; Miller & Hadfield, 1989). Research has focused on spray movement outside the crop, including spray drift (Baetens et al., 2007, 2009; Holterman, Zande, Porskamp, & Huijsmans, 1997; Reichard, Zhu, Fox, & Brazee, 1992), and also micrometeorological factors within the crop canopy (Da Silva, Sinfort, Tinet, Pierrat, & Huberson, 2006; Gil, Sinfort, Guillaume, Brunet, & Palagos, 2008). There are also studies investigating the movement of droplets movement near to leaves.
High speed cameras (Castanet, Dunand, Caballina, & Lemoine, 2013) and particle droplet imaging (Kashdan et al., 2003, Kashdan et al., 2004, Kashdan et al., 2007) have been used to observe droplet impaction onto leaves (Boukhalfa, Massinon, Belhamra, & Lebeau, 2014; Dong, Zhu, & Yang, 2013; Xiang; Dong, Zhu, & Yang, 2015; Zwertvaegher et al., 2014; Šikalo, Wilhelm, Roisman, Jakirlić, & Tropea, 2005). However, there are only a few studies tracing of droplet movement near the leaf and no research clearly demonstrating that the leaf-tip preference effect observed is caused droplet trajectory.
Some research has shown that leaf surface structure influences deposition. The difference in leaf-wettability was shown to be mainly caused by the wax content on the leaf surface, and the droplets bouncing and more easily rolling off the surface of the waxy leaves (Watanabe & Yamaguchi, 1992). The leaf roughness also is a major factor and is mainly affected by the bristles. Different plant leaves have different surface characteristics and waxy forms (Hall & Burke, 1974) and different parts of leaves also may have different characteristics (Ren et al., 2007).
Therefore, based on the factors proposed in previous studies as well the hypothesis that leaf elasticity can be a factor the possible reasons for a leaf-tip preference can be summarised as follows:
- 1)
Aerodynamics - droplets may be selectively deposited toward the leaf-tip by the local airflow because of the air resistance near the leaf tip is much less than other parts of leaves.
- 2)
Leaf surface microstructure - droplets may have higher adhesion probability on the leaf-tip due to the different microstructures of leaf surface in that area.
- 3)
Leaf elasticity - droplets may reside longer on leaf surfaces with less elasticity.
The aim of this research was to clarify the main factors concerning leaf-tip preference and establish their influences. Three experiments investigating, aerodynamics, leaf surface microstructure and leaf elasticity, were designed to clarify the main factors. The leaf-tip preference on the wheat and maize leaves were measured experimentally and the effect of the leaf tilt-angle on the amount of leaf-tip preference was evaluated.
Section snippets
Exploration of the main factor of leaf-tip preference
In terms of aerodynamics, a high-speed camera was used to record droplet trajectories and determined whether droplets are deflected due to the local airflow. To examine the role of leaf surface microstructure, natural leaves and leaf-like shapes were compared. The effect of elasticity was studied by supporting the leaf-like shapes with a metal foil layer.
Aerodynamic effects
An example droplet tracking diagram is shown in Fig. 7 in which the red circles highlight droplets. The angle of movement showed no significant change during this process, the p-value was 0.09 which is higher than 0.05 (Table 1). The trajectory of the droplets is shown in the left half of the figure, where the average deviation angle decreased by −0.259°, while that of droplets on the right half changed 0.399°. There was only one camera angle focused on the deposition distribution of different
Conclusion
A leaf-tip preference for pesticide deposition, which could be caused by the leaf elasticity, was confirmed by measurements on maize and wheat. There exists no significant evidence that this effect is due to either the of airflow near the leaf or microstructures on the leaf. Leaf-tip preference appears to occur up to a distance of 10 mm from the leaf-tip and preference is not obvious elsewhere.
The leaf-tip preference increases with the decreasing elasticity and it changes rapidly when the leaf
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work could be carried out due to the financial support of the National Natural Science Foundation of China (No. 31761133019), National Key Research and Development Plan of China (No. 2017YFNC040014), National Modern Agricultural Industrial Technology System of China (No.CARS-28-20).
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