Is tank mixing site-specific premixes and multi-site fungicides effective and economic for managing soybean rust? a meta-analysis
Introduction
Soybean rust (SBR) is one of the most damaging fungal diseases of soybean (Glycine max (L.) Merr.) caused by the obligate biotrophic pathogen Phakopsora pachyrhizi Syd. & P. Syd. (Goellner et al., 2010; Li et al., 2010). In Brazil, SBR is present in all soybean growing areas and where severe epidemics may develop and lead to around 80% yield losses under disease-conducive conditions (Dalla Lana et al., 2015). A couple years after the first reports of SBR epidemics in Brazil, a mandatory soybean-free period was implemented to reduce early-season inoculum. The wide adoption of early-maturing cultivars and earlier sowing dates, more recently, have greatly helped to reduce the impact of the disease (Godoy et al., 2016a). None of the current commercially grown soybean cultivars are fully resistant to all P. pachyrhizi races (Childs et al., 2018; Hartman et al., 2005) and hence soybean farmers should rely on routine fungicide applications for managing SBR (Beruski et al., 2020; Dalla Lana et al., 2018).
Several options of active ingredients have been made available during almost twenty years of disease management in Brazil using fungicides (Godoy et al., 2016a). Updates in fungicide programs have been frequently made based on information generated by field research to evaluate the performance of currently used and newly available products in the market. For instance, a few years after the sole use of demethylation inhibitor (DMI) fungicides, the premixes of DMIs with quinone outside inhibitors (QoI) gained attention up until the first ten years of chemical control of soybean rust. Following 2013, commercial premixes of succinate dehydrogenase inhibitors (SDHIs) and QoIs have also become available for SBR control. Recently, SDHIs have also been included in triple mixtures with QoIs and DMIs (Godoy et al., 2016a). However, a decline in control efficacy of soybean rust has been observed in the cooperative trials over the past years (Dalla Lana et al., 2018). First the DMI alone, then DMI + QoI mixtures and, lately, QoI + SDHI fungicides are no longer as effective as they were during the first years of use (Barro et al., 2021). Such decline was not only reported for single-a.i. fungicides but also for premixes after at least four years (Barro et al., 2021; Dalla Lana et al., 2018). Declines in efficacy have been linked to reports of less sensitive populations of Phakopsora pachyrhizi to DMI, QoI, and SDHI fungicides (Klosowski et al., 2016; Schmitz et al., 2013; Simões et al., 2018).
FRAC (Fungicide Resistance Action Committee) published a statement about the increasing importance of multi-sites for managing fungicide resistance, including soybean rust (FRAC 2018). Since 2015, multi-site fungicides, e.g. chlorothalonil, copper oxychloride, and mancozeb, have been recently evaluated in tank mix with commercial premixes to improve SBR control efficacy and reduce the risk of resistance (Godoy et al. 2015, 2016b, 2017, 2018). Chlorothalonil, a multi-site fungicide that belongs to the chloronitriles group, was first registered in 1966 and is still in use as a protectant fungicide (Battaglin et al., 2011; Miles et al., 2007). Copper oxychloride (3Cu (OH)2.CuCl2) is another multi-site fungicide that has been recently evaluated against soybean rust (Chechi et al., 2020; Juliatti et al., 2017). Mancozeb, which belongs to the dithiocarbamate group (FRAC 2018), was introduced in 1962 and still has a significant importance in the fungicide market worldwide (Gullino et al., 2010; Thind and Hollomon, 2018). As a typical cost-effective multi-site protectant-only fungicide, mancozeb requires relatively high rate and frequency of application compared to modern fungicides to achieve satisfactory protection (Gullino et al., 2010; Thind and Hollomon, 2018).
In Brazil, fungicides have been evaluated annually since 2003/2004 in a network of uniform field trials (UFTs) conducted by the anti-rust consortium (CAF) with the goal of monitoring and comparing the effectiveness of a range of fungicides (Dalla Lana et al., 2018; Godoy et al., 2016a). Considerable variations in control efficacy have been observed between fungicide chemistries over the years and across regions. Meta-analysis, a statistical technique that combines results from previous studies selected by defined criteria, have become standard for summarizing and comparing fungicide performance in plant disease management (Barro et al. 2019, 2020; Edwards-Molina et al., 2019; Machado et al., 2017; Paul et al. 2008, 2018) including SBR (Barro et al., 2021; Dalla Lana et al., 2018; Delaney et al., 2018). In the present study, we used meta-analysis to: 1) estimate the improvement in both efficacy and yield return provided by tank mixing multi-site fungicides with premixes of QoI plus DMI or SDHI fungicides evaluated from 2014/15 to 2017/18 across all major soybean regions in Brazil; 2) evaluate whether the gains in the responses vary across regions; and 3) estimate the probability of breaking-even on costs for a range of scenarios of soybean prices and multi-site fungicide costs.
Section snippets
Data source and experimental procedures
The data used in this study were obtained from 45 UFTs, which have been published as yearly reports (Godoy et al. 2015, 2016b, 2017, 2018). The experimental design was a randomized complete block with four replications. Replicated plots were at least six rows wide and 5 m long. Three fungicide sprays were applied at label-recommended rates across crop seasons using a CO2 backpack sprayer with a volume of 120 L ha−1. Sprays initiated at around 50 days post plant emergence (before canopy closure)
SBR severity and yield
SBR severity varied across treatments and trials. In the non-treated plots, it ranged from 40.3 to 100% and was higher than 98% in one-quarter of the studies. As expected, SBR severity was higher in the non-treated check (median 80%) compared with premixes alone or with the multi-site fungicides (Fig. 2A). Soybean yield in the non-treated check ranged from 904 to 3760 kg/ha. In general, mean yield was lower in the non-treated check plots (median 2364 kg/ha) than in the plots sprayed with either
Discussion
This study summarizes the benefits of tank-mixing a few selected premixes including QoI + DMI or SDHI fungicide with three different multi-site fungicides evaluated during three growing seasons (2014/15, 2015/16, and 2017/18) across a range of environments in eight Brazilian states. Most importantly, we found that the benefits tended to be highest for the least effective dual premix PICO + CYPR, and the use of +MANC, given its lower price in the scenario evaluated, was more likely to be
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.
Acknowledgements
The authors thank the Programa de Pós-graduação em Fitopatologia (UFV) and CNPq-Conselho Nacional de Desenvolvimento Científico e Tecnológico for providing a graduate scholarship to F. J. Machado and J. P. Barro and the same agency for a research fellowship for E. M. Del Ponte.
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These authors contributed equally.