Fungicide efficacy of nanocrystal-based formulations against peanut smut
Graphical abstract
Introduction
Peanuts (Arachis hypogaea L.) are a high-value crop in Argentina playing a significant role in the local economy. Argentina is the leading exporter of crushed and edible grain peanut products worldwide. In fact, more than 80% of production is exported to several countries, showing the importance of the Argentine industry in the world peanut market. More than 92% of Argentine peanuts are produced and processed in the central region of the country, mainly in Córdoba province (Agüero, 2017).
Peanut smut caused by the soil-borne fungus Thecaphora frezii Carranza and Lindquist is one of the most economically important fungal diseases affecting peanut production in Argentina. This fungal pathogen produces teliospores which can overwinter in soil and plant debris without the presence of live hosts. Colonization of peanut tissues occurs during peg and pod development. Infections initially take place when peanut pegs penetrate the soil and release exudates that trigger spore germination. The mycelium colonizes the peanut gynophore in the soil and replaces the grain cells with teliospores. During shelling, these teliospores are dispersed from peanut processing factories by wind to nearby fields (Rago et al., 2017). Peanut smut causes an annual yield loss that represents 3.15% of the total peanut production. In some fields, with a high inoculum density in the soil, the pathogen can cause a tremendous yield decrease with losses of more than 35% (Paredes, 2017). Currently, peanut smut in commercial peanut crops has been reported only in Argentina. The suggested management strategies to mitigate the impact of this pathogen include pathogen-free seeds, sowing in fields with low inoculum density, crop rotation, and chemical control. Fungicide applications aimed at protecting peanut tissues during peg and pod development are desired for control of peanut smut (Cazón et al., 2018; Rago et al., 2017). Fungicide programs that consist of soil-directed applications at the beginning of crop pegging and seven days later have been described as the most effective treatments. Mixtures of strobilurins and triazoles applied at night, when the peanut leaves fold up and the soil surface is exposed, have shown the best results in field experiments. However, control efficiencies occasionally reach acceptable values in this pathosystem even when the fungicides are sprayed at rates 2.5-fold higher than those recommended for leaf spot diseases (Cazón et al., 2018; Paredes et al., 2021). Against this background, increasing the efficacy of fungicides against peanut smut is an important challenge.
A significant number of the fungicide products available on the market are formulated with active ingredients that are poorly soluble in water (Chin et al., 2011). A classical formulation approach to overcome solubility problems involved the use of micronization to obtain a suspension concentrate (SC) with a mean particle size commonly in the range of 1–3 μm. During the manufacturing process, auxiliary ingredients need to be added to the formulation in order to obtain commercial products suitable for spray application in the field (Zhao et al., 2018). Despite this, the exceedingly low solubility of some active ingredients results in a low bioavailability that typically leads to an ineffective delivery of fungicides at the biological level (Chin et al., 2011). At present, it is estimated that the biological target uptake is only 1% while the remaining pesticide is lost during or after application (Garg and Gill, 2018). Therefore, new formulations that enable fungicides to be applied more efficiently are urgently needed.
Nanotechnology is the study and application of materials at nanoscale which display fundamentally new properties and functions from micrometric materials as a result of their size (Chhipa, 2019). In agriculture, nanotechnology is an emerging technology that has been used to develop numerous tools with immense potential to overcome the aforementioned problems (Chhipa, 2019; Worrall et al., 2018). Formulations developed using concepts of nanotechnology have shown great potential for enhancing the efficacy and safety of pesticides (Chhipa, 2019; Grillo et al., 2016). A successful nanotechnological approach for crystalline active ingredients involves producing nanocrystals (NC), defined as drug particles with a mean particle size below 1 μm (Chin et al., 2011; Paredes et al., 2016, 2018; Peltonen and Hirvonen, 2010). Nanocrystallization increases the dissolution rate of a poorly soluble compound and, subsequently, its bioavailability (Paredes et al., 2018; 2020; Peltonen and Hirvonen, 2010). Pesticide formulations based on nanocrystals have been reported to improve the efficacy of numerous agricultural pesticides (Camiletti et al., 2020; Chin et al., 2011; Yao et al., 2018). However, the increased dissolution rate of nanocrystals may lead to an unstable formulation with a more rapid release into the environment, which may not be desirable in some pathosystems (Yadav and Yadav, 2018). In order to overcome this limitation, biodegradable polymers with slow degradation in soil are added to pesticide formulations in order to protect active ingredients and to maintain them in a concentration level for a specified period of time (Roy et al., 2014).
In the present study, self-dispersible nanocrystals were explored as innovative formulations to increase the efficacy of azoxystrobin (AZY) and cyproconazole (CYP) fungicides against T. frezii. The nano-sized formulations were compared with commercially-available formulations in experiments under laboratory, semi-controlled, and field conditions.
Section snippets
Materials and fungicide selection
The active ingredients (AZY and CYP) were selected for this study based on their demonstrated superior efficacies for controlling peanut smut in Argentinean fields (Paredes et al., 2021; Rago et al., 2017). Unprocessed materials with 95% purity were donated by Laboratorios Peyte S.A. (Venado Tuerto, Santa Fe, Argentina). Two commercially available products formulated as SC were included in this study and, according to label information, the amounts of active ingredient were: 200 g a. i. L−1 of
Particle size distribution of nanocrystals
The particles size distributions in batches prepared during the first year of experiments were determined. The mean particle size and PDI values of AZY nanocrystals in reconstituted suspensions were 371.35 ± 13.90 nm and 0.357 ± 0.034 nm, respectively. After redispersion of CYP, the mean particle size and PDI of nanocrystals were 397.53 ± 11.68 nm and 0.377 ± 0.032 nm, respectively. The subsequent batches prepared in 2020 did not present significant differences (P > 0.05) in particle size
Discussion
The fungicide program using commercially-available products formulated with the active ingredients AZY and CYP has been previously described as one of the most effective active treatments to control peanut smut in field production. However, control efficiencies of these molecules are inconsistent among growing seasons and commonly fall below acceptable values (Paredes et al., 2021). To our knowledge, this was the first study to investigate nano-based formulations as an innovative technology to
Conclusions
Based on this study, NC have demonstrated potential to enhance the performance of fungicides currently used to control peanut smut. However, their final formulation should be further developed to verify successful disease control in field production. Fungicides formulated as NC exhibit enhanced efficacy against peanut smut and, subsequently, may be utilized to reduce the amount of fungicide sprayed and number of applications required to control other pathogenic fungi affecting crop production.
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.
Acknowledgments
The authors are grateful to N. Camacho, S. Palma, and D. Allemandi from UNITEFA-CONICET for their technical assistance in preparation of self-dispersible nanocrystals. We would like to acknowledge Laboratorios Peyte S.A. for the kind donation of unprocessed fungicides and Fundación Maní Argentino for the financial support on field trials. B. X. Camiletti holds a Postdoctoral Fellowship launched by the Argentinean National Scientific and Technical Research Council (CONICET).
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Present affiliation: Department of Plant Pathology, University of California, Davis, Kearney Agricultural Research and Extension Center, Parlier, CA 93648.