Compatibility of herbicide-insecticide-fungicide combinations and their phytotoxicity on soybean

Introduction

In contemporary agriculture, effective pest, weed, and disease management is crucial for achieving optimal crop yields and ensuring food security. This involves the strategic use of insecticides, fungicides, and herbicides, each tailored to combat specific threats to crop health. However, when these agents are combined, interactions may arise that affect their efficacy and safety on crops. CIBRC (2024) [2] has registered combinations of insecticide + insecticide (92), fungicide + fungicide (82), herbicide + herbicide (48), and insecticide + fungicide (6). Notably, combinations involving all three-herbicide, insecticide, and fungicide-have not been extensively studied. Hence, this study aims to explore the physical and chemical compatibility of such combinations, crucial for understanding their stability and effectiveness under varying environmental conditions. The preparation of pesticide spray solutions in water involves complex chemical processes that can alter pH and other key parameters. This becomes particularly significant when mixing active ingredients with diverse properties in compatibility experiments. Changes in pH can potentially trigger pesticide hydrolysis, compromising their absorption by pests and thereby impacting pest control effectiveness. Achieving compatibility between herbicides and insecticides requires maintaining stable pH conditions to avoid incompatibilities and ensure optimal performance. Factors influencing alkaline hydrolysis include pH levels, temperature, solubility, concentration, humidity, application timing, and the presence of adjuvants, all of which influence the chemical and physical properties of pesticide combinations. Therefore, assessing physical and chemical compatibility is essential for evaluating the hydrolytic stability at room temperature (25 °C) building upon previous research by Stanley et al. (2010) [13] , Jayasekharan et al. (2018 a and b) [6][7] , and Madhuri et al. (2021) [9] .

Materials and Methods

The physical and chemical compatibility of two insecticides viz. emamectin benzoate (Super Yodha 1.9 EC) and flubendiamide (Flue 39.35 SC); two herbicides viz. quizalofop ethyl (Targa Super 5% EC) and imazamox + imazethpyr (Odyssey 70% WG); and one fungicide i.e., fluxapyroxad + pyraclostrobin (Prioxar 167 + 333 g/L) at recommended concentrations (CIBRC, 2021) [2] , along with their four combinations, were evaluated in laboratory conditions at 25 °C using tap water. Individual solutions (60 ml) and combined solutions (20 ml each in a 1:1:1 ratio) were prepared in stoppered measuring cylinders and shaken vigorously. Observations for pH, total dissolved solids (TDS), electrical conductivity (EC), solubility, colour, cream volume, and sediment volume were recorded at 0.5, 2, 8, and 24 h. (Stanley et al., 2010; Jayashekharan et al., 2018b, Raju et al., 2018, Madhuri et al., 2021) [13,7,11,9] . The pH and TDS were measured with a Hach HQ40D portable multi-meter, and EC was calculated using following formula: [8,5,1,4,16,9] . Observations on yellowing, stunting, necrosis, and epinasty/hyponasty were recorded at 1, 3, 7, 9, 11, and 14 days after each spray. Leaf injury and phytotoxicity were rated as per Suneel Kumar et al. (2016) [14] .

Results and Discussion

Physical compatibility

From the experiment of physical compatibility, it was found that when two insecticides viz. emamectin benzoate and flubendiamide; two herbicides viz. quizalofop ethyl and imazamox + imazethpyr; and one fungicide i.e., fluxapyroxad + pyraclostrobin; were serially diluted in tap at the room temperature (25 °C), individual market formulations were cloudy in colour, except imazamox + imazethapyr which was clear, over a period of 24 h. and were readily soluble without any precipitation or sediment formation. When insecticide + herbicide + fungicide combinations were prepared in tap water at 25 °C, they exhibited turbidity due to interactions with their respective market formulations of insecticides, herbicides, and the fungicide. Importantly, despite this turbidity, they remained readily soluble without any precipitation or sediment formation.

Chemical compatibility

The chemical parameter, TDS, of insecticides, herbicides, and the fungicide remained below 500 mg/L over 24 hours. The pH of insecticides (emamectin benzoate and flubendiamide) decreased by 0.17 and 0.16 units, respectively. Quizalofop-ethyl (herbicide) remained stable at pH 7.61, while imazamox + imazethapyr increased by 0.06 units. The fungicide (fluxapyroxad + pyraclostrobin) also showed a pH increase by 0.7 units. Combinations of emamectin benzoate and flubendiamide with quizalofop-ethyl + fluxapyroxad + pyraclostrobin or imazamox + imazethapyr + fluxapyroxad + pyraclostrobin exhibited pH decreases by 0.42, 0.50, 0.56, and 0.37 units, respectively, over 24 hours. The control (tap water) showed a pH decrease of 0.36 units over 24 hours. The spray solutions of individual market formulations and their combinations maintained near-neutral pH (7-7.9) and TDS below 500 mg/L over 24 hours. Physically, all combinations were cloudy but readily soluble without precipitation or sediment formation. This indicates that the combinations did not undergo alkaline hydrolysis and remained stable, making them suitable for field spraying within 24 hours of preparation.

Phytotoxicity

These combinations were sprayed on soybean plants in polyhouse conditions and none of the combinations at the recommended (x) as well as at the double dose (2x) could cause any phytotoxic symptoms such as injury to the leaf tip and leaf surface, vein clearing, wilting, hyponasty, epinasty, and necrosis. The results proved that all the treatments tested were compatible and did not cause any phytotoxicity to soybean plants. Furthermore, none of the treatments including the control displayed any sign of disease on soybean plants. Therefore, it could be concluded that the herbicides, insecticides, and the fungicide caused a slight change in the pH of spray fluid when added to water. There was no sedimentation or precipitation, and turbidity remained consistent across combinations. However, market formulation like imazamox + imazethapyr was clear but turned cloudy when mixed with insecticides and the fungicide. Tharp and Sigler (2013) [15] noted that most pesticides are weakly acidic or neutral, suitable for pH ranges of 4 to 7. If water pH exceeds 7, pesticides may undergo hydrolysis, particularly alkaline hydrolysis, which accelerates in pH ranges of 8 to 9. Alkaline or acidic water has higher concentrations of OHˉ or H + ions compared to neutral water. The rate of hydrolysis depends on the water's buffering capacity (Seaman and Riedl, 1986) [12] . Weak acid pesticides, like ammonium salt of imazethapyr, break down in alkaline solutions, reducing pest absorption and effectiveness (Whiteford et al., 1986) [17] . Conversely, weak alkaline pesticides, such as sulfonyl urea, degrade in acidic solutions (Mckie and Johnson, 2002) [10] . Our observations also indicated a slight shift towards acidity in the tap water pH over 24 hours. *All the treatments were readily soluble in Tap water. There was no volume of cream (ml) and sediment at bottom (ml) **All the concentrations taken were based on the dose recommended by CIB&RC, 2021, on soybean; 1: Insecticide; 2: Herbicide; 3: Fungicide; 6 to 9: Herbicide + Insecticide + Fungicide combinations, the combinations were prepared by combining the recommended concentration of herbicide, insecticide, and fungicide in 1:1:1 ratio, respectively. *All the treatments taken were based on the dose recommended by CIB&RC (2021) on soybean; EC = TDS/0.67; µS/cm = micro siemens per centimetre; 1: Insecticide; 2: Herbicide; 3: Fungicide; 6 to 9: Herbicide + Insecticide + Fungicide combinations, the combinations were prepared by combining the recommended concentration of herbicide, insecticide, fungicide in 1:1:1 ratio.

Conclusion

The present study brings out a concrete understanding for combining the insecticides, herbicides, and fungicides by assessing the physical and chemical properties. The combinations of emamectin benzoate and flubendiamide with quizalofop-ethyl + fluxapyroxad + pyraclostrobin or imazamox + imazethapyr + fluxapyroxad + pyraclostrobin exhibited pH in neutral range (7-7.9) over 24 h. and were found readily soluble without any sedimentation or cream formation. Therefore, the test combinations were found physically and chemically stable. The phytotoxicity experiments also commemorate the stability of the combinations. Due to the limited research on insecticideherbicide-fungicide combinations limits our ability of compare the results of pH and TDS or EC. The tested combinations can be further studied for toxicity against various insect pest, diseases, and weeds of soybean crop and other pulse crops.

Acknowledgement

AcknowledgementsThe authors are thankful to Dr. R.M. Srivastava (Professor Entomology) for providing facilities to conduct experiments on physical and chemical compatibility in the Molecular Entomology Laboratory, and Dr. Ranjan Srivasatava (Professor Horticulture) for providing the polyhouse facility in the Model Floriculture Centre of the University to conduct experiments on phytotoxicity.

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