Introduction

The effective control of cereal diseases is increasingly under pressure from the development of fungicide resistance, but this development is highly dependent on local disease pressure and common practices of disease control. Currently, the control of barley diseases typically relies on using 1–2 treatments with fungicide mixtures, which include both QoIs, DMIs and SDHIs. The aim of this study was to evaluate the efficacy of older and more recent chemistries against two of the main diseases in barley across Europe, namely net blotch—(Pyrenophora teres), and Ramularia leaf spot—(Ramularia collo-cygni). Additionally, the effects against brown rust (Puccinia hordei) were evaluated.

In barley, Ramularia is currently of the greatest concern regarding development of resistance, and substantial reductions in sensitivity to QoIs, DMIs, and SDHIs have been seen especially in western Europe (Fountaine & Fraaije 2009; Piotrowska et al. 2017; Rehfus et al. 2019). The disease often arrives late in the season, and diagnosis can be difficult because of the resemblance between the symptoms of the disease and physiological spots. During the last decades, Ramularia has been increasingly recognized as an important yield limiting disease in barley (Havis et al. 2015).

Net blotch was the first cereal disease to develop widespread resistance to SDHI fungicides, with reduced sensitivity observed since 2012 (Rehfus et al. 2016). According to FRAC (2021), the sensitivity is moderate to high in central European countries, the UK, and Ireland, while Denmark has observed only low levels of resistance. While QoI resistance is prevalent in several cereal diseases, net blotch remains sensitive to most QoI fungicides (Jørgensen et al. 2018). DMI-resistant net blotch isolates have emerged with target site mutations like Y137F and S524T, also found in the Zymoseptoria tritici gene. Reduced sensitivity has been observed in specific regions of France and Germany according to FRAC in 2021. Unlike other fungicides, multi-site inhibitors like sulfur and folpet have multiple modes of action, reducing the risk of resistance. However, they generally have lower efficacy than the single-site fungicides and are considered more as supplementary products, especially useful as parts of anti-resistance strategies.

Materials and methods

The results of this study were collected from two trial series carried out in 2021 and 2022. The trials were carried out across Europe by local scientific organizations with the aim of including diverse climate zones and agricultural practices. Trials aimed at net blotch were located in Denmark, Finland, Belgium, and Scotland, while trials aimed at Ramularia were placed in Denmark, Ireland, Scotland, and Germany. Similar procedures and a randomized plot design with four replicates and plot sizes of minimum 10 m2 were used in all trials. Fungicide applications were carried out with a variety of equipment, using, for example, knapsack sprayers or self-propelled sprayers, pressures of 1.7–3.2 bar, and water volumes of 200–300 L/ha. Trials aiming at net blotch and Ramularia were sprayed at growth stage (GS) 37–45 and 47–51, respectively. In conditions with high disease pressure, a cover spray was applied to all plots including untreated to eliminate diseases like rust and net blotch, no later than 2 weeks before the estimated time for GS 37–39. Disease severity assessments were carried out following the EPPO guideline 1/26 (4)) (Oepp/Eppo, 2014). Individual leaf layers were assessed as percent symptomatic leaf area approximately every ten days after application. Grain yields were measured for each plot, which were adjusted to 85% dry matter. BASF supplied the fungicides, which were applied at full or 60%, 67%, or 80% recommended dose (Tables 1 and 2). The treatments included co-formulations of DMIs, SDHIs, QoIs, and multi-site inhibitors, and the same treatments were tested in both years.

Table 1 Protocol for trials aimed at net blotch carried out in 2021 and 2022. Treatments were carried out at growth stage 37–45
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Table 2 Protocol for trials aimed at Ramularia carried out in 2021 and 2022. Treatments were carried out at growth stage 47 – 51
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Results

Field effects—net blotch and brown rust

In the net blotch trial series, only three trials developed sufficient levels of net blotch to rate the fungicide effects. Net blotch disease pressure was high ranging from 43.8–55% in the three trials (Table 3). The field effects were also quite similar across the trials with good effects on average above 80% for all treatments except prothioconazole, which also had the most variable effects ranging from 51–81% (avg. 66%) (Fig. 1). Nearly complete control was achieved by many of the solutions, and especially the solutions containing three active compounds gave excellent control, but excellent control was also seen from solutions with combinations of two active compounds fluxapyroxad + pyraclostrobin (95–100%) or fluxapyroxad + mefentrifluconazole (97–99%). However, the effects from the mixture with mefentrifluconazole and pyraclostrobin were a bit more variable ranging from 79 to 97%.

Table 3 Disease severity in untreated plots on flag leaves − 1 in the net blotch trial series
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Fig. 1
figure 1

Net blotch control (%) on flag leaf -1 in three field trials from 2021 and 2022 in Denmark (2) and Finland (1). Assessments were carried out at 34–41 days after application, growth stage 75–87. Fungicide products were applied at full rates in most cases. Active compound dose rates are presented in g/L. FLX Fluxapyroxad, MEF mefentrifluconazole, PYSTR pyraclostrobin, PTH prothioconazole, BNZ Benzovindiflupyr, BIX bixafen, FLU fluopyram, FLSTR fluoxastrobin, TRSTR trifloxystrobin, MTY metyltetraprole

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Considerable levels of brown rust developed in five trials from the net blotch trial series. The rust severity varied across the trials from 5.3 to 76.9% (avg. 26%) (Table 3). The control effects were high with averages above 80% from most treatments (Fig. 2). The solo solutions with prothioconazole and pyraclostrobin gave the lowest levels of control ranging from 48 to 98% (avg. 72) and 25–98% (avg. 74%), respectively. Solutions with mefentrifluconazole + pyraclostrobin and bixafen + fluopyram + prothioconazole and prothioconazole + trifloxystrobin gave similar ranges of effects with averages around 80%. The remaining treatments gave similar, excellent effects with average effects above 90% and little variation.

Fig. 2
figure 2

Brown rust control (%) on flag leaf − 1 in five field trials from 2021 and 2022 in Denmark, Belgium, and the UK. Assessments were carried out at 34–43 days after application, growth stage 75–83. Fungicide products were applied at full rates in most cases. Active compound dose rates are presented in g/L. FLX Fluxapyroxad, MEF mefentrifluconazole, PYSTR pyraclostrobin, PTH prothioconazole, BNZ Benzovindiflupyr, BIX bixafen, FLU fluopyram, FLSTR fluoxastrobin, TRSTR trifloxystrobin, MTY metyltetraprole

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Field effects and yields—Ramularia

Six trials from the Ramularia trial series had sufficient levels of attack to rate the effects of treatments. The disease pressure varied widely across the trials from 7.9 to 85% (avg. 34%) (Table 4). The variation of treatment effects across the trials was small, and clear patterns could be seen in the ratings of treatment effects across the trials (Table 5). The highest effects of 90% on average were delivered by the mixture fluxapyroxad + metyltetraprole. Good effects were also seen from treatments with solo mefentrifluconazole and mixtures of fluxapyroxad and mefentrifluconazole and pyraclostrobin ranging from 74 to 76% on average. The solo solutions with prothioconazole and folpet gave the lowest effects ranging from 20 to 70% (avg. 52%) and 30–82% (avg. 55%), respectively.

Table 4 Disease severity in untreated plots on flag leaves − 1 in the Ramularia trial series
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Table 5 Overview of Ramularia control and disease severity (%) in untreated (untr.) plots on flag leaves minus one (F-1) in six field trials located in Denmark, Ireland, Sotland, and Germany in 2021–2022. Assessments were carried out at growth stage (GS) 65–81, 19–28 days after application (DAA). Dose rates are presented in g active compound/ha, and details on active compounds are presented in Table 2
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The yield responses in the six Ramularia trials roughly reflected the disease data (Fig. 3). However, very similar and small yield increases were seen from the treatments in general, and in three of the trials from 2021 statistically significant yield increases were not found (P = 0.05). The highest yields were provided by the mixture fluxapyroxad + metyltetraprole of 11.1 dt/ha on average and the lowest increases by mefentrifluconazole + sulfur, reduced dosage prothioconazole and folpet of 5.8 dt/ha, 5.2 and 3.7 dt/ha on average, respectively. The remaining treatments gave increases of 6.3–8.1 dt/ha.

Fig. 3
figure 3

Yield increases in six field trials attacked by Ramularia from 2021 and 2022 in Denmark, Ireland, Scotland, and Germany. Active compound dose rates are presented in g/L. FLX fluxapyroxad, MEF mefentrifluconazole, PYSTR pyraclostrobin, PTH prothioconazole, BNZ Benzovindiflupyr, BIX bixafen, FLU fluopyram, FOL folpet, SLF sulfur, MTY metyltetraprole

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Discussion

The results from the two trial series repeated in two seasons showed distinct patterns of field effects from most fungicides across Europe. Against net blotch, most treatments had consistently high effects in all trials. Namely, the treatments with three different active compounds all gave consistently high and similar effects. Only prothioconazole solo stood out with inferior effects of 66% on average, which was 15–32% lower than those of the remaining treatments, while solo pyraclostrobin gave good effects of 81% on average. These findings corresponding to those of previous studies were mixtures between SDHIs, especially fluxapyroxad, and prothioconazole/pyraclostrobin gave good control against net blotch (Tini et al. 2022), but where the use of solo DMIs was discouraged. Semar et al. (2007) also found good effects of pyraclostrobin against net blotch and no substantial effect of F129L mutation on the effects.

The effects were also mostly high against brown rust, but the results were more variable. Unlike other cereal diseases, the rusts have remained sensitive to all major fungicide classes, and although QoI and SDHI resistance conferring mutations have been found, the impact is limited.

The field effects against Ramularia were most variable across treatments, but clear patterns were seen across the trials. Excellent effects were delivered by fluxapyroxad + metyltetraprole, which were 15-39% higher than that of all other treatments on average, which corresponds to findings of Matsuzaki et al. (2021). However, the effects were generally weaker than those seen in the net blotch trial series also for mixtures including SDHIs, QoIs, and DMIs. This corresponds the findings of Rehfus et al. (2019) who found increasing frequencies in Central Europe of haplotypes harboring mutations in the SDHI and DMI target sites leading to decreased sensitivity. In the present study, solo prothioconazole gave inferior control in most trials with an average of 51%, while mefentrifluconazole solo gave good effects of 76% on average. Previous findings on Zymoseptoria tritici have also shown that mefentrifluconazole and prothioconazole exhibit distinct resistance profiles, while cross-resistance was found between mefentrifluconazole, difenoconazole, and tebuconazole (Heick et al. 2020).

The yield increases from the six barley trials were small, which might reflect the fact that Ramularia attacks usually happen late in the season and application against the disease is also carried out later. Thus, for most treatments very similar yield increases were seen. However, metyltetraprole + fluxapyroxad gave clearly higher yield increases, which matched the pattern seen in the field effects.