Reduced Fungicide Dose in Cereals: Which Parameters To Consider

Pesticide Dose: Effects on the Environment and Target and Non-Target Organisms Downloaded from pubs.acs.org by UNIV OF NEW SOUTH WALES on 09/19/17. For personal use only.

Chapter 6

Reduced Fungicide Dose in Cereals: Which Parameters To Consider? Lise Nistrup Jørgensen*,1 and Jens Erik Ørum2 1Department

of Agroecology, Aarhus University, Forsøgsvej 1, Flakkebjerg, 4200 Slagelse, Denmark 2University of Copenhagen, Food and Resource Economics Institute, Rolighedsvej, Copenhagen, Denmark *E-mail: [email protected].

Often the fungicide rates that European farmers apply are lower than the labelled rates. The use of ‘adjusted appropriate rates’ is mainly driven by results from field trials showing sufficient control and better net yield responses compared to full rates. The optimal rate depends on several factors of which the intrinsic activity of the active ingredient, the pathogen for control, disease pressure and timing of treatments are the most important. Use of economic thresholds and monitoring systems can improve the use of appropriate rates. Reduced fungicide rates using single-site inhibitors can help to delay resistance development as long as the numbers of treatments are not increased as a consequence of applying reduced rates.

Introduction Cereal diseases are major targets for fungicide use, particularly in Northern Europe, where yield levels are high and disease pressure is often significant. In this region, septoria tritici blotch caused by the fungus Zymoseptoria tritici is the most yield-reducing disease in winter wheat (1). Yield responses to fungicide applications in winter wheat are commonly in the range of 1-2 ton/ha, equal to 10-20% losses depending on level of cultivar resistance (2). Implementing IPM tools like resistant cultivars and adjustment of sowing time has been shown to diminish the need for fungicide treatments (1, 3). Farmers are constantly aiming at reducing production cost in order to optimize their net profit, © 2017 American Chemical Society


and fungicides are seen as a significant cost. The wish to cut cost and rates is particularly high when grain prices are low. Reducing fungicide input and cost can be addressed by considering the number of treatments as well as the rate. A range of dose response curves are typically observed for a specific fungicide against a specific disease as normally seen in data provided for authorization of new fungicides. Such data originate from trials carried out at different sites and in different seasons. Thus, the results reflect the inherent variability normally seen in biological systems (4). Using reduced fungicide rates in cereals has been exploited extensively in Denmark for more than 30 years, and a similar trend has also been seen in other Northern European countries. Successful use of reduced and appropriate rates requires good knowledge of the strengths and weaknesses of specific fungicides as well as knowledge of the presence of diseases and the disease pressure in the field in order to minimize the risk of control failures. The aim of this paper is to present our experience with reduced fungicide rates and to discuss the factors influencing the recommended and applied rate.

Factors Influencing Choice of Dose Rates Labelled Rates The recommended labelled rate fixed by the agrochemical companies is normally at a level providing a high and consistent level of control typically aiming at 80-90% control in 80-90% of the cases (4). In the EU, applicants must provide a biological assessment dossier that justifies the label claims. This also includes data to verify the minimum effective dose for major targets (5). In principle, the minimum effective dose could vary between targets, but often the labelled rate is either given as a range or a maximum dose per treatment followed be a maximum rate per season. With a few exceptions, farmers in the EU countries are allowed to use lower rates, but this will be at their own risk. Following new and stricter criteria for EU authorization, applicants are often asked to reduce the labelled rates due to such factors as non-acceptable risks of ground water contamination or human exposure. Such a reduction can compromise efficacy and the label text might need to be adjusted accordingly.

Optimal Rate Depends on Intrinsic Activity on Target Diseases Fungicides, in general, show highly variable dose responses against different diseases, which is important to consider when giving advice to farmers. An example with control of yellow rust (Puccinia striiformis) using four azoles is shown in Figure 1. In this case, even 25% of the labelled rate of epoxiconazole and tebuconazole provided more than 80% control. For prothiococonazole a reduction to 50% of the labelled rate leads to a significant reduction in control and can therefore not be recommended. 74


Figure 1. Control of yellow rust (Puccinia striiformis) in a winter wheat field trial using 4 different azoles applied at flag leaf emergence (BBCH GS 37-39). Plants are assessed 50 days after application on flag leaf. Level of severity on untreated plots was 50% severity on flag leaves. (see color insert) Certain diseases are known to require higher rates (50 to 100% of the labelled rate) for achieving even moderate levels of control. For example this is the case for Fusarium head blight (Fusarium spp.) and eyespot (Oculimacula spp.), which can be due to low intrinsic activity and/or difficulties in achieving optimal timing or good coverage of the target area. Other diseases including powdery mildew in barley (Blumeria graminis) and rust diseases in cereals (Puccinia striiformis, P. triticina and P. hordei) have generally been found to be controlled successfully using efficient fungicides at rates of 25-50% of the labelled rate, as also illustrated in Figure 1 for yellow rust. Optimal Timing The knowledge accumulated over the years concerning fungicide rates has led to great focus on the importance of optimizing the timing of application. Preventive treatments generally require lower input compared to treatments during the latent period (curative) or even later on well-established attacks (eradicative). In case of septoria tritici blotch timing has been found to be very important as shown in Figure 2 (6). In these field trials treatments applied at flag leaf emergence (GS 37-39) were typically providing preventative control on flag leaves and curative control on second leaves. Using full labelled rates, control levels dropped from 80-90% to 50% when moving from preventive to curative control. Similarly, the control levels of the 25% doses dropped from 70% to 20%. In a more detailed study carried out under semi-field conditions using artificial inoculation with Z. tritici, a ten-day delay in application reduced the level of control significantly and consequently also the option of using reduced 75


rates successfully, as shown for epoxiconazole (Figure 3). One quarter of the labelled rate applied preventatively two days before inoculation or curatively six days after inoculation gave almost the same control as the full labelled rate applied ten days after inoculation. Not surprisingly, the lower rates proved to be more vulnerable to the timing than full rates.

Figure 2. Different activity of three fungicides on Septoria tritici blotch applied preventatively on flag leaf (top) or curatively on 2nd leaf (bottom). Average data from 2013-15 provided kindly by AHDB, U.K. (see color insert) 76


In recent years, when the performance of the azoles has been declining due to evolution of resistance in the Z. tritici populations in Europe, particularly the efficacy of curative treatments has been found to drop significantly (6, 7). The two azoles epoxiconazole and prothioconazole have in English trials dropped from providing approximately 90% to 40% control applied curatively (7). Fungicide rates should also be adjusted according to canopy structures. Early timing on a small canopy requires less fungicide than a full canopy around heading stage. Studies examining the relationship between canopy size and fungicide rate clearly indicated the need for higher input in dense crops compared to more open and thin crops (8).

Figure 3. Control of septoria tritici blotch in semi-field pot trial artificially inoculated with Z. tritici using 3 dose rates of epoxiconazole and 4 timings; one preventive treatment - 2 days before inoculation (DBI) and 3 curative treatments 6, 10 and 18 days after inoculation (DAI). (see color insert)

Monitoring and Economic Thresholds Several forecasting systems and economic thresholds have been developed for disease control in cereals. Information on these systems for winter wheat diseases were summarized by Jørgensen et al. (1). In particular, conditions for infection following formation of the two upper leaves were found to be critical, as these leaves are very important to retain yield (9). Disease monitoring and forecasting systems can help optimize spray timing and possibly also assist in predicting the degree of disease severity expected in untreated crops, which again relates to the appropriate rate needed (10). 77


Many farmers in Northern Europe are capable of monitoring the risk of diseases ensuring that treatments are carried out preventively and at low disease pressure, allowing the use of reduced rates. National monitoring systems can support farmers decision making and provide information to farmers on how alert they have to be. A major challenge is still to determine the final level of disease and thus the potential yield loss, as this is to a major extent influenced by the weather conditions following fungicide application. Unfortunately, long-term weather forecasts are still very unreliable and cannot be included in the risk scenarios. Hence using historical data becomes important in order to assess the overall risk of economic losses using appropriate rates.

Economical Gain from Fungicide Treatments Farmers and advisors aim to apply appropriate fungicide rates, which is determined as the dose giving the best economic gain (measured as yield increase per ha) after deducting the cost of fungicide and application. The optimal rate will vary depending on the expected disease level, the timing, and the fungicide efficacy. In Denmark, fungicide inputs are expressed as Treatment Frequency Index (TFI), which equals the number of labelled rates applied per season. An analysis of Danish trial data from 5 years has shown that, on average, the highest net yield gain was obtained with fungicide inputs between 0.5 and 1.0 TFI (11). Treatments using a high TFI were associated with an increased probability of negative net yield gains (Figure 4). The data, however, also showed that although the overall responses were positive, a negative response can occur, regardless of the treatment frequency and strategy applied, reflecting that in some seasons, it is not right to spray at all. The same data set was used for predicting the optimal fungicide input. The economical optimum clearly depended on grain price and disease level. Dividing the data into the trials with susceptible and resistant cultivars and five different timings and various combinations of timings using the BBCH scale (A: GS 2531, B: GS 32-36, C: GS 37-50, D: GS 51-64 and E: GS 65-70) allowed for a more detailed economical analysis. The analysis revealed very flat response curves indicating a high degree of robustness of the rates and timings. A split strategy with 2-3 fungicide applications corresponding to a total fungicide input of 0.4-0.75 TFI was economically optimal in susceptible varieties, whereas 1-2 applications corresponding to a total fungicide input of 0.30-0.65 TFI was optimal in resistant varieties. The analysis clearly revealed that higher TFI levels were more relevant as grain prices increased. When minimizing net cost, on average there will be situations across seasons where lower rates will cause significant net costs caused by inadequate control in high disease seasons as discussed by Beest et al. 2013 (12), who similarly state that the use of disease-resistant cultivars reduces both the optimal dose, all levels of risk and the disease-related costs at all rates. Maximizing profit normally also implies acceptance of some diseases in the crop despite treatments. Use of the Danish Decision Support System ‘Crop 78


Protection Online - Disease and Pests’ often results in optimal profit but less optimal disease control (13).

Figure 4. Box plots characterizing the distribution of net yield gain in winter wheat grouped according to input of fungicides (TFI). Thick line inside box = median, lower box boundary = 1st quartile, upper box boundary = 3rd quartile (i.e. the box includes 50% of the data). (see color insert)

Risk of Resistance Development The use of reduced rates of fungicides has often been postulated to increase the risk of developing fungicide resistance, but clear evidence to verify that this concern is justified has not been presented. The issue has been discussed in many fora including a discussion on how plant pathogens differ from other pesticide targets like insects (14). A recent review for fungicides concluded that high fungicide rates reduce the density of sensitive fungi more than the density of resistant fungi and therefore a high dose increases the selection for resistance. A hypothesis that was confirmed by data from 16 out of 19 peer reviewed studies showing reduced sensitivity from high rates compared with low rates (15), suggesting that lower and appropriate rates might even help to delay development of resistance. It was however also shown, that if fungicide rates become too low, this could lead to more treatments per season, which again could lead to longer periods of exposure and a higher risk of resistance. 79


Figure 5. Calculated net yield gain in winter wheat in resistant (top) and susceptible cultivars (bottom) for selected strategies with 1 to 3 applications and using two prices for grain. Based on data from Denmark 1999-2003. A: GS 25-31, B: GS 32-36, C: GS 37-50, D: GS 51-64 and E: GS 65-70. The legends are ranked according to the most beneficial solutions. (see color insert)

Discussion The availability of effective, systemic fungicides has since the beginning of the 1980’s enabled farmers to control diseases and increase productivity in cereal production (16). The use of fungicides is considered a very important tool for 80


the control of cereal diseases, but today the use of pesticides is often questioned and the common European agricultural policy supports initiatives reducing the reliance on pesticides and supports the adoption of the concept of integrated pest management (IPM) (17). One of the eight IPM principles promoted by the EU particularly focuses on keeping the use of pesticides at the minimal levels that are necessary, e.g. by applying reduced rates or fewer treatments. The optimal input of fungicides in winter wheat depends on the disease pressure and the climatic conditions in the growing season, but the susceptibility of the cultivars also has a major impact on the optimal fungicide input. A survey indicated major differences between fungicide inputs in winter wheat in the UK, Germany, France and Denmark, reflecting the differences in the above-mentioned parameters (1). In Denmark, the difference in required fungicide rates between diseaseresistant and disease-susceptible cultivars is significant and is greatest in seasons with severe attacks of septoria tritici blotch and yellow rust (11). One single treatment at heading (BBCH 39-55) is often sufficient in varieties with good resistance to these diseases, whereas the more susceptible varieties needed 2-3 fungicide applications, but even in these cultivars the optimal fungicide input is generally found to be below 1 TFI per season. More focus on growing resistant cultivars with high yield potential is important as the risk of yield losses is potentially much lower in resistant cultivars compared to susceptible cultivars (13, 18). Moreover, the optimum fungicide input is highly dependent on the grain price and should be adjusted according to the expected price level. A doubling of the grain price from 100 to 200€ per ton increased the TFI optimum by 50%. Recommending and applying reduced rates should never result in significant inferior control and significant economical yield losses, and therefore a certain safety margin is often practiced by advisors and farmers. Due to increasing problems with azole resistance, particularly to septoria tritici blotch, the dose required to give sufficient control and optimum net yields has been on the rise in recent years. This applies particularly to countries where new modes of action providing new control options are limited, as it is the case in Denmark.

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Reduced Fungicide Dose in Cereals: Which Parameters To Consider

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