Reducing the Use of Pesticides with Site-Specific Application: The Chemical Control of Rhizoctonia solani as a Case of Study for the Management of Soil-Borne Diseases

RESEARCH ARTICLE Reducing the Use of Pesticides with SiteSpecific Application: The Chemical Control of Rhizoctonia solani as a Case of Study for the Management of Soil-Borne Diseases Ronan Le Cointe, Thomas E. Simon, Patrick Delarue, Maxime Hervé, Melen Leclerc, Sylvain Poggi* a11111 INRA UMR1349 IGEPP, 35653, Le Rheu, France * Abstract OPEN ACCESS Citation: Le Cointe R, Simon TE, Delarue P, Hervé M, Leclerc M, Poggi S (2016) Reducing the Use of Pesticides with Site-Specific Application: The Chemical Control of Rhizoctonia solani as a Case of Study for the Management of Soil-Borne Diseases. PLoS ONE 11(9): e0163221. doi:10.1371/journal. pone.0163221 Editor: Sabrina Sarrocco, Universita degli Studi di Pisa, ITALY Received: February 1, 2016 Accepted: September 6, 2016 Published: September 26, 2016 Copyright: © 2016 Le Cointe et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This research was supported by the French National Research Agency (http://www. agence-nationale-recherche.fr) through funding of the SYSBIOTEL project referenced ANR-08_STRA14 (RLC SP TES). Part of this research was also supported by the INRA “Plant Health & the Environment” Division (http://www.spe.inra.fr/) (RLC SP PD ML). The funders had no role in study Reducing our reliance on pesticides is an essential step towards the sustainability of agricultural production. One approach involves the rational use of pesticides combined with innovative crop management. Most control strategies currently focus on the temporal aspect of epidemics, e.g. determining the optimal date for spraying, regardless of the spatial mechanics and ecology of disease spread. Designing innovative pest management strategies incorporating the spatial aspect of epidemics involves thorough knowledge on how disease control affects the life-history traits of the pathogen. In this study, using Rhizoctonia solani/Raphanus sativus as an example of a soil-borne pathosystem, we investigated the effects of a chemical control currently used by growers, Monceren1 L, on key epidemiological components (saprotrophic spread and infectivity). We tested the potential “shield effect” of Monceren1 L on pathogenic spread in a site-specific application context, i.e. the efficiency of this chemical to contain the spread of the fungus from an infected host when application is spatially localized, in our case, a strip placed between the infected host and a recipient bait. Our results showed that Monceren1 L mainly inhibits the saprotrophic spread of the fungus in soil and may prevent the fungus from reaching its host plant. However, perhaps surprisingly we did not detect any significant effect of the fungicide on the pathogen infectivity. Finally, highly localized application of the fungicide—a narrow strip of soil (12.5 mm wide) sprayed with Monceren1 L—significantly decreased local transmission of the pathogen, suggesting lowered risk of occurrence of invasive epidemics. Our results highlight that detailed knowledge on epidemiological processes could contribute to the design of innovative management strategies based on precision agriculture tools to improve the efficacy of disease control and reduce pesticide use. PLOS ONE | DOI:10.1371/journal.pone.0163221 September 26, 2016 1 / 18 Reducing the Use of Pesticides with Site-Specific Application design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Introduction The sustainability of agricultural production requires ecological management practices and, among others, parsimonious use of chemical-based control strategies that should be based on thorough understanding of epidemiological processes [1,2]. The sustainable use of pesticides in cropping systems is an important issue and the development and the adoption of well-reasoned practices for controlling crop pests and diseases depend on technical and sociological issues [3–5]. Institutional indicators, such as the treatment frequency indicator (TFI) have been defined to monitor pesticide use in fields [6,7]. TFI measures the number of "approved doses" sprayed on the whole surface of a plot during a cropping season. This definition highlights three possibilities for reducing the amount of pesticides used: (1) reducing the number of spray applications during the cropping period, (2) decreasing the applied dose (compared to the reference dose), and (3) restricting the treated area. The frequency of spraying can be reduced through the use of decision-making tools [8]. These tools usually rely on predictive models and recommend treating crops at the right time only if necessary and with the most appropriate chemical product. Applying a lower dose is often carried out in cereal production systems for economic reasons, but this practice is controversial with regard to fungicide resistance. On the one hand, the Fungicide Resistance Action Committee [9] recommends limiting the number of applications, using the full recommended dose and combining fungicides with different modes of action. This advice is based on the hypothesis that using lower doses may potentially promote pesticide resistance [10,11]. On the other hand, others argue that lower doses reduce the selection pressure favouring resistant strains and therefore improve fungicide durability [12–14]. The third pesticide reduction strategy is to restrict the treated area by spatially localizing applications. This method called “sitespecific application” allows, through precision farming, to target a specific area accurately and spare the rest of the crop [15–18]. Site-specific applications can be implemented as either preventive or curative control strategies. Regarding preventive control strategies, current site-specific application consists in adjusting the dose across the field surface according to canopy density. With this strategy, denser canopies receive a higher dose of fungicide, assuming that a dense canopy creates a microclimate more conducive to the disease, and contributes significantly to crop yield [19,20]. For curative fungicides, site-specific application is sometimes practised by farmers in fields, e.g. by using a knapsack sprayer on patches of diseased plants. The difficulty lies in accurately differentiating the targeted symptoms from those of other diseases and from abiotic stress [21,22]. In addition, fungal disease symptoms must be detected early enough in the cropping season; for example, roots infections caused by soil-borne pathogens such as Rhizoctonia solani Kühn on sugar beet can result in detectable symptoms only late in the cropping season on (...truncated)