​Against phytotoxicity

Problems with phytotoxicity have been reported by farmers in southern Brazil, in soybean crops, with compromised leaf area

02.08.2016 | 20:59 (UTC -3)

Soybean production in Brazil remains in the growing search for more productive levels, by increasing the planted area or even repeating cultivation in the same harvest. This search for productivity has put agronomic techniques under pressure in order to find sustainable alternatives to make this practice possible, which may not always produce satisfactory results. The larger the area to be treated with pesticides, the greater the logistical problem within the farm, leading to applications with a high risk of compromising the effectiveness of chemical control, in addition to negative impacts on the crop being worked.

This harvest, there have been reports of several farmers in the south of the country with phytotoxicity problems due to the application of fungicides from the triazole group, compromising the soybean leaf area (Figure 1). Phytotoxicity is the excessive concentration of the active ingredient on the surface of the leaf, which may be under stress (potentiates) or not, causing burning or destruction of cells due to the difficulty of the plant metabolizing such a quantity of active ingredient. It is important to highlight that any chemical product applied to plants is toxic, whether it is herbicide, insecticide or even fungicide. A practical example is the manganese deficiency generated in transgenic soybean plants by the application of the herbicide glyphosate. Thus, the plant needs to have mechanisms for “detoxification” of the presence of that external product in its cellular interior, also called xenobiotics. Therefore, the longer the “detoxification” period, the greater the residual within the plant. However, the biggest challenge for products is to keep this active ingredient inside the cell without compromising its performance.

Figure 1 – Symptoms of phytotoxicity on soybean foliage

How to identify in the area

Fungicides can cause different levels of phytotoxicity, however, different agronomic circumstances surrounding the plants can enhance this effect, making it apparent and of high severity. Locations in the crop that show the first symptoms:

Therefore, the problem caused by phytotoxicity in plant tissues due to the application of the product is related to physiological factors and application technology.

Physiological factor

The phytotoxicity of some triazoles is related to the combination of high temperatures, water stress – or not – and the genetics of the cultivar used. It is clear that some cultivars have greater “sensitivity” to the application of this chemical group, which is enhanced by these conditions. This may be related to factors such as cuticle thickening and the physiological activity of the plant to “detoxify” its tissues, varying the speed of absorption of products by the leaf.

The speed of fungicide absorption in soybean cultivation depends on the characteristics inherent to the active ingredients applied, as well as the constitution of the leaf epidermis at the time of application (Lenz, 2010). All primary aerial surface that a chemical will encounter in vascular plants will be a thin surface film, the cuticle, which is composed of soluble and polymeric lipids (Jeffree, 1996). The cuticle's main function is to protect the living tissues of plants against water loss (Schönherr, 1982), but it also constitutes a barrier to the absorption of chemical products applied via the foliar route (Balardin, 2010).

The normal physiological activity of a plant depends on sufficient water supply. The decrease in water supply leads to greater energy expenditure to maintain turgidity or promote the growth of new roots to increase water absorption capacity. Kissmann (1998) reports that below the cuticle is the cellulose wall, which is hydrophobic. Between the cuticle and the cellulose wall, there is pectin, which in turn is hydrophilic; therefore, it absorbs water like a sponge. In leaves of plants under water deficit, the cuticle is thicker, making it difficult for pathogens and products to penetrate.

Research data has indicated that plants subjected to water stress have greater retention of active ingredients in the epidermis or plant cells. The increased retention of active ingredients, combined with a greater concentration in the upper portions of the plant canopy, tends to enhance the phytotoxic action of the applied product. Water adheres to cellulose microfibrils and other hydrophilic components of the cell wall. As it evaporates from the cells, the remainder is sucked into the interstices of the cell wall. Due to high surface tension, the more water that is removed from the cell wall, the greater the increase in negative water pressure. With the increase in the concentration difference between the droplet sprayed on the surface and the amount of water inside the leaf, greater diffusion of the fungicide into the leaf is observed. This increase in fungicide concentration in the cell enhances both its control effectiveness and the phytotoxic effect it may present.

Application technology factor

As previously noted, the genetic predisposition of the cultivar to phytotoxicity is fundamental for damage to occur. However, the conditions imposed by the application will increase this damage to the foliage, potentially making it severe. Therefore, the cause can vary depending on the association or not of the following factors.

Climate

Observations of wind, temperature and Relative Humidity (RH) are essential to minimize the risks of phytotoxicity on the leaf. Mainly the last two need to be monitored during and after the application of the fungicide, avoiding exceeding 30°C and 55% RH as much as possible. High temperatures accelerate the dehydration of the sprayed droplet before it reaches the target, or even those that have reached it and there was not enough time for absorption by the leaf. Thus, in the latter case, the droplet will be quickly evaporated, forming crystalline deposits of the active ingredient on the leaf surface (Figure 3), which in case of water or thermal stress (Figure 4), can increase tissue burning. Under appropriate conditions, hydration of the cuticle will facilitate product absorption. Pederson (2007) states that the toxicity of fungicides is enhanced in adverse environmental conditions or in periods of plant water stress.

Figure 3 – Representation of the arrival of the drop on the leaf with and without stress conditions

Figure 4 – Average climatic conditions in soybean producing regions in the state of Rio Grande do Sul, 2014

Adjuvants

The use of adjuvants can increase the risk of phytotoxicity of fungicides if applied under inappropriate conditions on “sensitive” cultivars. It is worth mentioning that the use of adjuvants under appropriate conditions can reduce the risk of extinction of the droplet due to the presence of its oily phase on the leaf, accelerating the absorption of the active ingredient. Furthermore, the use of the recommended adjuvant is essential for the performance of fungicides from the strobilurin group, whose activity on the leaf cuticle layer requires the presence of the adjuvant.

Drop deposit

In addition to the physiological changes in the plant during the reproductive period, there is the closure of the crop canopy, which acts as a physical barrier to the penetration of drops into the plant bottom. As a result, the vast majority of drops are deposited on the leaves in the upper part of the plant's tissues, reaching up to four times the recommended initial dose (Figure 5), adding a very high risk of leaf activity not being able to support such a load without presenting phytotoxicity, especially under conditions of water or thermal stress.

Figure 5 – Relationship between the amount of deposits (drops/cm2) and the dose of fungicide applied to the profile of the soybean plant.

Volume of syrup

With the increase in soybean planting areas and/or the need to increase the farm's logistical performance, a phenomenon of reduction in the volume of spray applied per hectare is occurring without a consistent technical-agronomic basis. Therefore, there is an excessive concentration of the active ingredient within the syrup, which can lead to mixing problems and, mainly, this chemical concentration will remain in the drop that will be deposited on the surface of the leaf. Furthermore, this volume reduction is generally preceded by a decrease in the size of the droplets that will be more susceptible to extinction. Then, the more chemically concentrated droplet will be subject to the same climatic stress factors, further increasing the crystalline deposits under the leaf epidermis.

Another point to note is that volume reduction must be preceded by knowledge of the dosage of the adjuvant per unit, as the variations are huge when talking about dose per hectare or dose per volume of spray applied. In the latter, the reduction in spray volume will reduce proportionally to the dose of adjuvant applied per hectare, minimizing problems. However, this understanding is not homogeneous and, generally, with the reduction of the spray volume, the dose of adjuvant per hectare is maintained, which generates an excessive increase in the amount of oil in the drop, increasing the risks of phytotoxicity.

Concentrations

Excessive reduction of the syrup volume increases the chemical concentration of the syrup, which predisposes to unwanted interactions between the mixed active and inert ingredients. These incompatibilities can be of a physical nature, flocculation, separation or precipitation, or of a chemical nature, ionic dissociation (low pH), hydrolysis

alkaline (high pH) or radical inactivations in the product molecules (Figure 6).

Figure 6 – Mixture compatibility test

With the high risk of interaction between the products inside the syrup, the application system may also be compromised, due to clogging of the precipitates formed inside the tank (Figure 7).


Figure 7 – Retention of chemical compounds formed inside the sprayer tank in the nozzle filter

Formulation

Formulations such as wettable powders and concentrated suspensions tend to be less phytotoxic than emulsifiable concentrates. This fact is not necessarily due to the active ingredient present in the formulation but rather to the solvents used to allow stable interaction in the formulation.

The burning of leaf tissue by the action of crystalline deposits, associated with the production of Reactive Oxygen Species (ROS), is an irreversible process within the plant, given that the necrotic tissue will not be restored. Under normal plant conditions, ROS production is low, increasing with the application of the fungicide. This can trigger lipid peroxidation (destruction of membrane lipids), damaging cell structure and function. However, alternatives are emerging to try to minimize this foliar damage, such as paying attention to the factors already discussed in this article and the use of foliar fertilizers, mainly to reduce the production of ROS.

Foliar nutrition

The objective of foliar nutrition is linked to stimulating increased cell division, absorption and use of supplied nutrients and water (Atiyeh et al, 2002; Chen et al, 2004 ), in addition to acting as a hormonal regulator and increasing stress tolerance (Piccolo et al, 1992). Furthermore, several scientific studies attribute to these amino acids enzymatic activation and protein synthesis (Ulukan, 2008), the increase in photosynthetic pigments (Ali; Hassan, 2013), as well as the reduction of stress generated by glyphosate in soybean plants ( Lambais, 2008).

In search of consistency in these responses, we are working on this new line and achieving promising results. In internal work not yet published, a reduction of up to 29% in ROS was observed in plants that received the application of fungicide and foliar fertilizer, compared to treatment with fungicide alone. Still in these studies, a 7% increase in ROS production was observed in plants with fungicides than in control plants, without any application. Thus, foliar application of amino acid fertilizers can reduce the harmful effects of ROS and improve plant resistance under stress conditions (Bahari et al.

Therefore, with the accelerated production process seeking high production standards, research needs to constantly look for answers to anticipate potential problems in the field. However, it is necessary to respect the physiological limitations of the plant and application technology in order not to encounter unpleasant surprises that will, invariably, reflect on the plants' productive capacity, limiting reaching high production ceilings.

Click here to read the article in issue 179 of Cultivar Grandes Culturas.


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