Association of hydrogen peroxide with commercial fungicide formulations in the control of Asian soybean rust

ABSTRACT: In recent years, there have been reductions in the efficacy of the fungicidal control of Phakopsora pachyrhizi, thereby hindering the management of soybean rust and compromising crop yield. This study evaluated the effects of incorporating hydrogen peroxide (H2O2) in commercial fungicide formulations on the control of soybean rust. We conducted two experiments, one of which was performed in a greenhouse environment and the other under field conditions. In both environments, we examined the following four control programs using commercial fungicide formulations: (I) azoxystrobin + cyproconazole (quinone outside inhibitor [QoI] + demethylation inhibitor [DMI]); (II) picoxystrobin + cyproconazole (QoI + DMI); (III) pyraclostrobin + epoxiconazole + fluxapyroxad (QoI + DMI + succinate dehydrogenase inhibitor); and (IV) water (H2O) (program without fungicide application), combined with the incorporation of (i) H2O2; (ii) mancozeb (positive control I); (iii) chlorothalonil (positive control II); or (iv) water (H2O) alone. Analyses of infected leaf area and grain yield revealed that the addition of H2O2 to the formulations of DMI and QoI fungicides led to a reduction in disease severity of between 33% and 44% relative to the effects of these products used alone, as well as an increase in yield and SPAD values. The use of H2O2 and multi-site fungicides alone failed to provide effective control of soybean rust. In addition to enhancing the efficacy of disease control, the use of H2O2 associated with commercial fungicide mixtures was shown to be a potential tool for the management of fungicide resistance and reduction in losses from Asian soybean rust.

Detection of Ramularia collo-cygni DMI- and SDHI-resistant field populations in Austria and the effect of fungicides on the population and genetic diversity

Winter barley (Hordeum vulgare L.) is the third most cultivated crop after corn and wheat in Austria but one of the most challenging for disease control. The foliar pathogen Ramularia collo-cygni B. Sutton and J.M. Waller, causing Ramularia leaf spots (RLS), is one of the most important diseases in barley. In the recent years, control has only been achieved using fungicide mixtures including the multi-site inhibitor chlorothalonil, however this compound is totally banned in the EU. The objective of this study was to assess fungicide dose-rates and spray mixtures for RLS control. Furthermore, a field monitoring within the main barley growing areas of Austria was carried out, to analyse the current resistance situation to DMI and SDHI fungicides, which are still the backbone in RLS control. The results indicate that only the mixture with chlorothalonil achieved a good RLS control. Prothioconazole or benzovindiflupyr (alone or additively) decrease the severity of RLS but increase the local frequency of Cyp51 and sdhC mutations, especially the high dose rates. Based on a low Cyp51 mutation frequency of 16% in untreated control this frequency increased over 3.8 times following an application with 300 g ha−1 prothioconazole. The cumulative-sdhC mutations were even more increased after an application with benzovindiflupyr. This study showed that Ramularia collo-cygni is present in 91% of barley fields presented in this field survey. Widespread use of chlorothalonil fungicide maintained a low to moderate mutation frequency (Cyp51-I325T, Cyp51-I328L, sdhC-H146R and sdhC-H153R) in Austrian barley regions with no increase between 2017 and 2019.

Why do fungicide mixtures delay the evolution of resistance? An experimental evolutionary approach

Pesticide resistance poses a critical threat to agriculture, human health and biodiversity. Mixtures of fungicides are recommended and widely used in resistance management strategies. However, the components of the efficiency of such mixtures remain unclear. We performed an experimental evolution study on the fungal pathogen Z. tritici, to determine how mixtures managed resistance. We compared the effect of the continuous use of single active ingredients to that of mixtures, at the minimal dose providing full control of the disease, which we refer to as the "efficient" dose. We found that the performance of efficient-dose mixtures against an initially susceptible population depended strongly on the components of the mixture. Such mixtures were either as durable as the best mixture component used alone, or worse than all components used alone. Moreover, efficient-dose mixture regimes probably select for generalist resistance profiles as a result of the combination of selection pressures exerted by the various components and their lower doses. Our results indicate that mixtures should not be considered a universal strategy. Experimental evaluations of specificities for the pathogens targeted, their interactions with fungicides and the interactions between fungicides are crucial for the design of sustainable resistance management strategies.

Delayed Development of Resistance to QoI Fungicide in Venturia inaequalis in Israeli Apple Orchards and Improved Apple Scab Management Using Fungicide Mixtures

Quinone outside inhibitors (QoI) fungicides group were introduced for commercial use against apple scab (Venturia inaequalis) in Israel in 1997. Unlike other regions in the world, in which resistance of V. inaequalis to QoI fungicides was observed within 3–5 years of use, in Israel it only occurred after 14 years of use. Field trials conducted between 2007 and 2017 showed a significant reduction in susceptibility to QoIs in northern Israel only since 2011. The delay in the development of resistance is related to limited fungicidal sprays resulting from unfavorable conditions for the pathogen. Of the 28 isolates collected from infected leaves or fruits of commercial orchards in northern Israel, 27 were resistant to the QoI fungicide Kresoxim-methyl. Amplification of the CYTB gene and sequencing of the G143A mutation region confirmed the resistance of all 27 isolates to QoIs. Resistance is demonstrated in the orchard, in vitro and molecular-based study, which forced the growers to avoid using QoIs against apple scab. We show that foliar applications of tank mixtures of systemic fungicides plus captan or prepacked fungicidal mixtures improved efficacy and can be used as a strategic approach in fungicide resistance management, including in orchards in which resistance to QoIs has been detected.

Performance and Profitability of Rain-Based Thresholds for Timing Fungicide Applications in Soybean Rust Control

Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, is the most damaging disease of soybean in Brazil. Effective management is achieved by means of calendar-timed sprays of fungicide mixtures, which do not explicitly consider weather-associated disease risk. Two rain-based action thresholds of disease severity values (DSV50 and DSV80) were proposed and compared with two leaf wetness duration-temperature thresholds of daily values of infection probability (DVIP6 and DVIP9) and with a calendar program, with regard to performance and profitability. An unsprayed check treatment plot was included for calculating relative control. Disease severity and yield data were obtained from 29 experiments conducted at six sites across four states in Brazil during the 2012–13, 2014–15, and 2015–16 growing seasons, which represented different growing regions and climatic conditions. The less conservative rainfall action threshold (DSV80) resulted in fewer fungicide sprays compared with the other treatments, and the more conservative one (DSV50) resulted in fewer sprays than the DVIP thresholds. Yield was generally higher with the increase in spray number, but the economic analysis showed no significant differences in the risk of not offsetting the costs of fungicide sprays regardless of the system. Therefore, based on the simplicity and the profitability of the rain-based model, the system is a good candidate for incorporating into the management of SBR in soybean production fields in Brazil.

The Efficacy of Ethaboxam as a Soybean Seed Treatment Toward Phytophthora, Phytopythium, and Pythium in Ohio

Phytophthora, Phytopythium, and Pythium species that cause early-season seed decay and pre-emergence and post-emergence damping off of soybean are most commonly managed with seed treatments. The phenylamide fungicides metalaxyl and mefenoxam, and ethaboxam are effective toward some but not all species. The primary objective of this study was to evaluate the efficacy of ethaboxam in fungicide mixtures and compare those with other fungicides as seed treatments to protect soybean against Pythium, Phytopythium, and Phytophthora species in both high-disease field environments and laboratory seed plate assays. The second objective was to evaluate these seed treatment mixtures on cultivars that have varying levels and combinations of resistance to these soilborne pathogens. Five of eight environments received adequate precipitation in the 14 days after planting for high levels of seedling disease development and treatment evaluations. Three environments had significantly greater stands, and three had significantly greater yield when ethaboxam was used in the seed treatment mixture compared with treatments containing metalaxyl or mefenoxam alone. Three fungicide formulations significantly reduced disease severity compared with nontreated in the seed plate assay for 17 species. However, the combination of ethaboxam plus metalaxyl in a mixture was more effective than either fungicide alone against some Pythium and Phytopythium species. Overall, our results indicate that the addition of ethaboxam to a fungicide seed treatment is effective in reducing seed rot caused by these pathogens commonly isolated from soybean in Ohio but that these effects can be masked when cultivars with resistance are planted.

Molecular Tracking and Remote Sensing to Evaluate New Chemical Treatments Against the Maize Late Wilt Disease Causal Agent, Magnaporthiopsis maydis

Late wilt is a destructive disease of corn: outbreaks occur at the advanced growth stage and lead to severe dehydration of susceptible hybrids. The disease’s causal agent is the fungus Magnaporthiopsis maydis, whose spread relies on infested soils, seeds, and several alternative hosts. The current study aimed at advancing our understanding of the nature of this plant disease and revealing new ways to monitor and control it. Two field experiments were conducted in a heavily infested area in northern Israel seeded with highly sensitive corn hybrid. The first experiment aimed at inspecting the Azoxystrobin (AS) fungicide applied by spraying during and after the land tillage. Unexpectedly, the disease symptoms in this field were minor and yields were high. Nevertheless, up to 100% presence of the pathogen within the plant’s tissues was measured using the quantitative real-time PCR method. The highest AS concentration tested was the most effective treatment, and resulted in a 6% increase in cob yield and a 4% increase in A-class yield. In the second experiment conducted in the following summer of the same year in a nearby field, the disease outbreak was dramatically higher, with about 350 times higher levels of the pathogen DNA in the untreated plots’ plants. In this field, fungicide mixtures were applied using a dripline assigned for two coupling rows. The most successful treatment was AS and the Difenoconazole mixture, in which the number of infected plants decreased by 79%, and a 116% increase in crop yield was observed, along with a 41% increase in crop quality. Evaluation of the effectiveness of the treatments on the plants’ health using a remote, thermal infra-red sensitive camera supported the results and proved to be an essential research tool.

Performance and profitability of rain-based thresholds for timing fungicide applications in soybean rust control

Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, is the most damaging disease of soybean in Brazil. Effective management is achieved by means of calendar-timed sprays of fungicide mixtures, which do not explicitly consider weather-associated disease risk. Two rainfall-based action thresholds of Disease Severity Values (DSV50 and DSV80) were proposed and compared with two leaf wetness duration-temperature thresholds of Daily Values of Infection Probability (DVIP6 and DVIP9) and with a calendar (CAL) program, with regards to performance and profitability. An unsprayed check treatment plot was included for calculating relative control. Disease severity and yield data were obtained from 29 experiments conducted at six sites across four states in Brazil during 2012-13, 2014-15 and 2015-16 growing seasons, which represented different growing regions and climatic conditions. The less conservative rainfall action threshold (DSV80) resulted in fewer fungicide sprays compared with the other treatments and the more conservative one (DSV50) resulted in fewer sprays than the DVIP thresholds. Yield was generally higher with the increase of spray number, but the economic analysis showed no significant differences on the risk of not offsetting the costs of fungicide sprays regardless of the system. Therefore, based on the simplicity and the profitability of the rain-based model, the system is a good candidate for incorporating into management of SBR in soybean production fields in Brazil.

Relationship between soybean plant defoliation and Asian soybean rust severity

ABSTRACT Data from experiments conducted in the field with eight fungicide mixtures added of five mancozeb levels were used to generate gradients of soybean rust severity, plant defoliation, and soybean grain yield. The data were subjected to correlation analysis between defoliation and severity, defoliation and grain yield, and grain yield and rust severity. All correlations were significant, evidencing a relationship between variables. Analyses conducted in other studies have shown that severity control has greater influence on decision-making, compared to defoliation. Therefore, as defoliation is dependent on severity, evaluation of rust intensity is sufficient, suggesting that defoliation can be excluded from evaluation for this pathosystem.