Elsevier

Crop Protection

Volume 30, Issue 4, April 2011, Pages 495-501
Crop Protection

Rain-induced removal of copper from apple leaves: Influence of rain properties and tank-mix adjuvants on deposit characteristics at the micro scale

https://doi.org/10.1016/j.cropro.2010.11.028Get rights and content

Abstract

Application of copper fungicides is an integrative component in the control of apple scab. As shown in the past, tank-mix adjuvants might reduce the rain-induced removal of active ingredients. The aims of this study were a) to characterize the impact of defined rain amount on the removal of Cu-oxychloride (Cu-Ox) and Cu-hydroxide (Cu-Hyd) from apple seedling leaves, b) to examine the influence of ethoxylated seed oil adjuvants on deposit properties at the micro scale and their possible influence in reducing the fungicide erosion from the leaves, and finally c) to evaluate the influence of previous adjuvant application on the rainfastness of copper ions. Losses of Cu-Hyd and Cu-Ox from seedling leaves exposed to 5 mm heavy rain reached almost 80% of the original Cu load. Wash-off extent of Cu-Ox could be reduced by using the rapeseed oil ethoxylate RSO 5 (47%), the linseed oil ethoxylate LSO 10 (47%), or the soybean oil ethoxylate SBO 10 (40%). However, in case of Cu-Hyd, none of the adjuvants was able to increase the tenacity of the copper on the leaves. Rainfall removed the bulky deposits and induced changes in the typical shape of the crystals. Furthermore, a reorganization of the spatial distribution of the remaining active ingredient (a.i.) was induced, which was very often embedded in a thin layer of adjuvants. In general, the adjuvants had a significant impact on droplet spread, whereas the area effectively covered with the a.i. was affected to a lesser extent. For both Cu-Hyd and Cu-Ox rewetting of the original deposit under controlled conditions resulted in an increased spread area but reduced the area effectively covered with Cu. Our results showed no clear relationship between the area covered by Cu ions and their adhesion to the surface, i.e. rainfastness. Finally, our studies indicated that previous spray treatments might influence the rain-induced removal of copper.

Research highlights

► We study the impact of rain on the removal of copper from apple seedling leaves. ► We proof the influence of adjuvants on deposit properties at the micro scale. ► There is no clear relation between the area covered by Cu ions and rainfastness. ► Rainfall removed the bulky deposits and changed the typical shape of the crystals. ► Previous spray treatments might influence the rain-induced removal of copper.

Introduction

Inorganic copper, having a high biological activity against fungi, plays a key role in plant protection programs of several tropical, subtropical and temperate zone plant species. Besides its high biological efficacy (Montag et al., 2006), in the last years the use of copper was strongly regulated by German and European laws due to its poor ecotoxicological profile (Jamar and Lateur, 2007). In this context, substantial investments in research and development aimed to faster the reduction of the use of copper products in both integrated and organic production systems (Mohr et al., 2008b, Dagostin et al., 2010). Strategies to reach this objective whilst maintaining the required biological efficacy include the optimization of product formulation (Mohr et al., 2008b), disease prognosis and spray timing (Jamar and Lateur, 2007, Bangemann et al., 2008, Bruns et al., 2008, Keil et al., 2008), as well as application technology (Kaul et al., 2008, Mohr et al., 2008a).

In apple production, copper is mainly sprayed early in the season to protect young and expanding leaves from Venturia inaequalis (Cke) Wint., the causal agent of apple scab (Gessler et al., 2006). In Germany, this period is characterized by frequent rainfall and therefore the general practice of fruit growers is to renew the fungicide coverage after 20 mm precipitation. From earlier studies it is known that rain removes large portions of those agrochemicals deposited on plant surfaces, whereas the extent of wash-off is related to the surface properties (Hunsche et al., 2006) and characteristics of the pesticide solution (Kraemer et al., 2009b). In addition, a strong influence of both rain intensity and rain quantity on the wash-off of organic fungicides (Cabras et al., 2001, Hunsche et al., 2007a) and herbicides (Spanoghe et al., 2005, Hunsche et al., 2007b) was demonstrated. For those agrochemicals which deploy their activity inside the plant tissue, lower rain-induced losses can be achieved by increasing the penetration rate by using adjuvants (Kraemer et al., 2009b). In fact, the adequate use of adjuvants can improve the overall formation of deposits on leaves and the biological efficacy of the applied products (Steurbaut, 1993, Steurbaut, 1994, Green and Beestman, 2007), but much more research is needed to explore the beneficial effects of adjuvants in fungicide applications (Green, 2000, Spanoghe et al., 2007, Wang and Liu, 2007).

Experiments with the organic fungicide prochloraz revealed that the rain-induced fungicide redistribution and reallocation on wheat foliage might have a positive effect on the control of the eyespot disease (Cooke et al., 1989). In case of the surface active copper-based products, a good understanding on how Cu deposits are influenced by rain, and how the arrangement and mobility of Cu ions are influenced on the leaf and at the micro scale, is essential. As shown, the persistence of fungistatic concentrations following exposure of leaves to rainfall is directly correlated to the active ingredient (a.i.) deposition (Neely, 1971). The structure of deposits on leaves depends strongly on the complex interaction between the agrochemical and the plant surface, and is impacted by both application technique and environmental factors (Hartley and Graham-Bryce, 1980). Thus, adjuvants have a strong impact on droplet spreading and deposit formation (Clay and Miksis, 2004, Baur and Pontzen, 2007, Faers, 2007, Ryckaert et al., 2008, Spanoghe et al., 2007). In this context sophisticated techniques such as electron scanning microscopy (SEM) and energy dispersive x-ray microanalysis (EDX) might contribute significantly to a better understanding of the interaction of pesticide residues and plant surfaces. While SEM pictures provide qualitative information about the structure and micromorphology of the pesticide deposit (Hess and Falk, 1990, Nalewaja and Matysiak, 2000), the EDX recordings indicate the spatially resolved distribution of specific elements over the analysed surface (Bukovac et al., 1995, Kraemer et al., 2009a, Kraemer et al., 2009b, Kraemer et al., 2009c, Krause et al., 2004).

Former studies report a clear impact of rain quantity on the weathering of copper oxide and copper hydroxide deposits, and the redistribution of Cu ions when leaves were exposed to worst case rainfall situations with extremely high intensity (40 mm min−1) and precipitation ranging from 6 to 400 mm (Rudgard et al., 1990). In a different approach, a rotating shear device was developed and optimized (Paradelo et al., 2008). By using this system it could be shown that the removal of copper is controlled by the detachment and transport of Cu particles from the testing surface (Pose-Juan et al., 2009). Aside, the importance of the simulation of rain events having similar properties to natural rainfall is highlighted elsewhere (Hunsche, 2006, Dunkerley, 2008).

In this context, the first aim of our studies was to evaluate the impact of defined rain precipitation on the wash-off of copper oxychloride (Cu-Ox) and copper hydroxide (Cu-Hyd) from apple seedling leaves. Based on the previous results with organic fungicides (Hunsche et al., 2006) we hypothesised that the tenacity of Cu can be increased by tank-mix-adjuvants, which might reduce the extent of changes in the microstructure of the deposit. For this purpose, ethoxylated seed oil adjuvants produced from linseed, soybean, and rapeseed oils having distinct ethoxylation and propylation degrees, and a strong impact on the a.i. distribution pattern inside the spread area (Kraemer et al., 2009a, Kraemer et al., 2009b, Kraemer et al., 2009c), were selected. Thereby, we hypothesised that modified seed oils with a lower ethoxylation degree and a rather hydrophobic property induce a more homogeneous distribution of a.i. within the droplet spread area, resulting in a bigger contact area between Cu and plant surface, and lower rain-induced wash-off. In a following stage, leaves with surface characteristics modified by residues from a previous spray application were used in rainfastness experiments, with the hypothesis that altered surface properties will influence the removal of Cu by rain.

Section snippets

Plant material

Apple (Malus domestica Borkh.) plantlets raised from seeds extracted from Golden Delicious fruits (Eichenberg und Co. Gehölzsamen GmbH, Miltenberg, Germany) were grown under controlled environmental conditions (T = 20 °C; RH = 60%; photosynthetic active radiation (PAR) = 180 μmol m−2 s−1). Dormancy of the seeds was broken by immersion in water for one week at room temperature (approx. 20 °C) followed by four week storage at 4 °C in a refrigerator (RH > 90%). Seeds were sown in a tray filled

Cu wash-off as influenced by rain quantity

Copper residues on seedling leaves which were subjected to 1, 2, 3, 4 and 5 mm heavy (5 mm h−1) rainfall indicate a significant removal of the a.i. after few millimetres of rain. The a.i. losses after 1 mm rain reached 35% in case of Cu-Hyd, and about 60% in case of Cu-Ox (Fig. 1). After 2 mm rain, losses of 60% or more of the total available Cu-Hyd and Cu-Ox on the leaves before rain onset were registered. After 5 mm rain, the accumulated Cu wash-off was 75% for both Cu-Ox and Cu-Hyd.

Cu rainfastness and deposit characteristics as influenced by ethoxylated seed oil adjuvants

The

Discussion

Our results show a strong erosion of copper deposits from apple seedling leaves, reaching 35% and 58% of the original Cu-Hyd and Cu-Ox deposit, respectively, after 1 mm heavy rain. Irrespective of Cu-salt, a.i. losses amounted more than 60% after 2 mm rain, and more than 70% after 5 mm rain (Fig. 1). In previous work with the organic fungicide mancozeb losses of more than 90% were registered after 5 mm rain (Hunsche et al., 2007a). As reported previously by Rudgard et al. (1990), a strong

Acknowledgement

The authors acknowledge the technical support of Knut Wichterich, Libeth Schwager and Ira Kurth for sample preparation and analysis, and Anton Berg and Wilfried Berchtold for their valuable help for establishment, calibration, and technical maintenance of the rain simulator. Seed oil exthoxylates were kindly provided by Cognis Agrosolutions. Anastasia Alexeenko received a one-year scholarship from the German Federal Foundation for the Environment (Deutsche Bundesstiftung Umwelt, DBU), which is

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    Scholarship holder of the German Federal Foundation for the Environment (Deutsche Bundesstiftung Umwelt, DBU).

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