Yield and optimum fungicide dose rates for winter wheat (Triticum aestivum L.) varieties with contrasting ratings for resistance to septoria tritici blotch

Field Crops Research 204 (2017) 89–100

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Yield and optimum fungicide dose rates for winter wheat (Triticum aestivum L.) varieties with contrasting ratings for resistance to septoria tritici blotch Joseph P. Lynch, Elizabeth Glynn, Steven Kildea, John Spink ∗ Teagasc Crops Research Department,Oak Park, Carlow, Co., Carlow, Ireland

a r t i c l e

i n f o

Article history: Received 10 June 2016 Received in revised form 12 January 2017 Accepted 12 January 2017 Keywords: Wheat Yield Fungicide Septoria tritici blotch Azole Economic optimum

a b s t r a c t This study evaluated the effect of varieties of winter wheat that differ in septoria tritici blotch (STB) resistance rating on yield and profit margin obtained when treated with varying application rates of an azole-only fungicide or an azole-plus-SDHI fungicide. Seven field trials were conducted over four growing seasons in the south-east of Ireland, 2012–2015. Each trial consisted of a factorial arrangement of two fungicide types (azole and azole + SDHI), five applications rates (0, 0.25, 0.50, 1.0 and 2.0 times the recommended dosage per application, applied twice during the growing season) and three varieties that differed in reported STB-resistance ratings (5, 7 and 8 on a scale of 0–9; SR5, SR7, SR8). The severity of STB and grain yield were determined and analysed by analysis of variance. The economic optimum rate of each fungicide product was also determined for each variety at each site-season. When untreated, the STB severity was lower for SR8 at GS71 than the other varieties at four of the five evaluated sites, while SR7 incurred lower STB severity than SR5 at only three of the five sites that comparisons were available. The STB severity for the SR8 variety was unaffected by the application rate of either fungicide at most sites. Increased rate of fungicide increased yield, and this effect was generally consistent among the SR5 and SR7 varieties at site-seasons with medium or high STB pressure. However, the yield benefit to increased rate of either fungicide type was minimal for SR8. The variation in the economic optimum rate of either fungicide type evaluated was greater between site-seasons than between the SR5 and SR7 varieties. However, the SR8 variety had a comparatively low economic optimum rate of an azole + SDHI in four of the five site-seasons. When averaged across all site-seasons, the margin above fungicide cost was greater for crops treated with the azole + SDHI treatment than an azole-only treatment for all varieties evaluated. These findings indicate that the range of STB resistance exhibited by the majority of varieties currently grown in Ireland (5–7 on the 0–9 scale) do not consistently confer differences in the optimum fungicide application rate. However the results also suggest that varieties exhibiting strong STB resistance may allow confident reductions to fungicide programs, should they become commonly available in the future. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Despite achieving among the highest wheat (Triticum aestivum L.) yields globally (Burke et al., 2011), the economic sustainability of winter wheat production in Ireland is under increasing pressure due to high variability in grain prices and a plateauing of yearly wheat yields, which had until recently been increasing steadily

Abbreviations: Fopt , optimum fungicide dose; GLA, green leaf area; STB, septoria tritici blotch. ∗ Corresponding author. E-mail address: [email protected] (J. Spink). http://dx.doi.org/10.1016/j.fcr.2017.01.012 0378-4290/© 2017 Elsevier B.V. All rights reserved.

since the 1960s (CSO, 2015). Significant improvements in the efficiency of winter wheat production systems in Ireland are therefore required to ensure the crops sustainability. In addition to improved varietal genetics, a reduction in cost of production may be achieved through the optimisation of agronomic practises based on variety choice. One of the primary factors leading to a high cost of production in winter wheat in north-western Europe is control of septoria tritici blotch (STB), caused by the fungal pathogen Zymoseptoria tritici, which thrives in temperate climates where mild winter temperatures and high rainfall provide the ideal conditions for rapid pathogen transfer and disease development (Thomas et al., 1989; Orton et al., 2011; O’Driscoll et al., 2014). Control of STB has become

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reliant on the use of multiple applications of a suite of fungicide products including the demethylation inhibitors (DMIs), quinone outside inhibitors (QoI), succinate dehydrogenase inhibitors (SDHI) and multi-site active ingredients. Subsequently, strains of Z. tritici have developed increased insensitivity to many commercially popular azole and QoI fungicides and thus STB control has become increasingly reliant on the SDHI fungicides (Orton et al., 2011; O’Driscoll et al., 2014; Dooley et al., 2016b). Furthermore, the recent discovery of field isolates resistant to SDHIs brings into question the long term viability of this group of chemistry (Dooley et al., 2016a). In the absence of new effective fungicide chemistry developments other methods of STB control need to be utilised to maintain or improve current winter wheat yield and production efficiency in high disease pressure regions. The use of varieties with good resistance to STB is encouraged as part of integrated pest management strategies (Clark and Hillocks, 2014). Despite this there is a lack of evidence reporting the potential of such resistant varieties to alter the requirements for fungicide inputs, especially in the context of averting the risk of economic losses in seasons with high STB pressure, which is an important consideration of fungicide dose selection (te Beest et al., 2013). The ratings of winter wheat varieties for resistance to various diseases on recommended lists are the typical source of information on varietal STB resistance for growers in Ireland and the UK (AHDB, 2016; DAFM, 2016). Therefore, the range of choice that growers have for varietal STB-resistance in crops grown in this region are reflected by the range that is present on the recommended lists (4–8 on a scale of 0–9, with 9 representing very STB-resistant). The objective of this study was to evaluate the effect of varieties of winter wheat that differ in their STB resistance rating on the yield and profit margin obtained when treated with varying application rates of fungicide.

2. Materials and methods

The fungicide products were selected to represent an azole-only fungicide (Opus Max, epoxiconazole 83 g/l; BASF, Ludwigshafen, Germany) and an azole-plus-SDHI mixture fungicide (Aviator Xpro, prothioconazole 150 g/l + bixafen 75 g/l; Bayer Crop Science, Monheim am Rhein, Germany). Each fungicide was applied at either 0.25, 0.5, 1.0 or 2.0 times the maximum dosage rate per application (1.5 and 1.25 l/ha for Opus Max and Aviator Xpro, respectively) to each variety. Fungicides were applied twice during the growing season, firstly at GS33 (third last leaf fully emerged) and again at GS39 (final leaf fully emerged; Zadoks et al., 1974). Sprays were applied to each plot in a rate of 200 l/ha of water using a hand-held pressurised plot sprayer equipped with flat fan nozzles (F-110-03) selected to produce a medium spray quality at 200 kPa pressure. All other crop management, including herbicides, insecticides and nutrient management were applied at rates to minimise crop stress and facilitate a high yielding crop. 2.2. Disease assessments Disease assessments were conducted by estimating the percentage STB and green leaf area (GLA) on the flag leaf. Disease assessments were conducted at Carlow 2012 on 15 July 2012 (GS71/73), while for Kildalton 2012 an assessment of GLA was conducted on 5 July 2012 (GS71), with no assessment of STB available. During Carlow 2013 disease pressure was very low, and therefore no disease assessment data were available for this site-season. Disease assessments were conducted on 8 July 2014 (GS71) and 15 July 2015 (GS71) at Carlow and 21 July 2014 (GS71/73) and 7 July 2015 (GS71) at Killeagh. Plots were harvested using a Sampo 2010 (Sampo Rosenlew Ltd, Finland) and 1 kg subsamples of grain taken for further analysis. Grain dry matter (DM) content and hectolitre weight were determined using a Dickey John GAC2100 (DICKEYJohn Corporation, Illinois, USA). 2.3. Statistical analysis

2.1. Experimental sites and design Seven field trials were conducted during the 2012–2015 seasons (Table 1). All weather data was sourced from the closest Met Éireann weather stations (Appendix A). All crops were established, treated and harvested according to conventional practise. At each site the experiment had a complete randomised block design with 4 replicate blocks of a factorial arrangement of variety and fungicide treatments on 2.2 × 12 m plots. The winter wheat varieties used were selected for contrasting rating of resistance to STB, based on the recommended winter wheat variety list produced for the Republic of Ireland (DAFM, 2012), and differed with site-season (outlined in Table 1). They included Stigg (SR8, rating: 8/9; Limagrain, Puy-de-Dôme, France); Lion (SR7, rating: 7/9, Nickerson, Lincolnshire, UK); Einstein (SR5, rating: 5/9, Limagrain, Puy-de-Dôme, France);and JB Diego (SR5; rating: 5/9, Josef Breun, Germany).

Grain yields were adjusted to 85% DM content prior to statistical analysis. Data for each individual site-season were subjected to a three way analysis of variance for a factorial arrangement of variety x fungicide type x fungicide rate treatments plus untreated control. When effects of rate and interactions between rate and variety or fungicide product were significant a contrast analysis was conducted to evaluate whether relationship was a significantly linear or quadratic response. Individual treatment contrasts were made using Fishers least significant differences test. All procedures were conducted using the 14th edition of GenStat (VSN International Ltd., Hemel Hempstead, UK; 2011). 2.4. Economic optimum rate Margin over fungicide costs for each plot was calculated by estimating revenue from yield and subtracting the cost of fungicide

Table 1 Trial site information. Year

Site

Previous crop

Sowing date

2012 2012 2013 2014 2014 2015 2015

Carlow town, Carlow Kildalton, Kilkenny Carlow town, Carlow Carlow town, Carlow Killeagh, Cork Carlow town, Carlow Killeagh, Cork

Winter oilseed rape Spring oats Winter oilseed rape Winter oats Winter oilseed rape Winter oats Winter oilseed rape

18 October 5 October 25 October 14 October 16 November 13 October 24 October

a

SR = Septoria resistance rating (scale of 1–9).

Varieties sowna SR5

SR7

SR8

Einstein Einstein Einstein Einstein Einstein JB Diego JB Diego

Lion Lion Lion Lion Lion Lion Lion

– – Stigg Stigg Stigg Stigg Stigg

J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

application. Crop revenue was determined using a typical grain price at harvest for each study year, which was D 180/t at 85% DM grain for 2012, and D 150/t at 85% DM grain for 2013, 2014 and 2015 (O’Donovan and O’Mahony, 2013, 2014; O’Donovan, 2015). Fungicide cost was considered as the price of the product only (i.e. excluding other application costs) and was consistent between the years of study; D 42/ha per 1.0 rate per application for azoleonly fungicide and D 72/ha per 1.0 rate per application for the azole + SDHI fungicide. A further deduction was made from the estimated revenue from each plot for a grain quality penalty, with a deduction of D 0.63/t grain for every 1 unit reduction in hectolitre weight (kg/hl) below 72 kg/hl. Response curves were fitted to this data to derive the optimum fungicide dose (Fopt ) of either fungicide treatment for the maximum margin over fungicide cost. This approach assumed that with increased fungicide use the margin per hectare will initially increase due to the increased revenue being in excess of the increased fungicide cost until an optimum rate, after which the benefit in revenue is lower than the increased fungicide cost. Two functions were fit to the data, a quadratic and a linear plus exponential relationship, with the best fitting function selected for parallel curve analysis. The linear plus exponential function is: y = a + b.r F + c.F. while the quadratic function is: y = a + bF 2 + cF where y is the margin over fungicide cost (D /ha), F is the fungicide application rate and a, b, c and r are parameters determined by statistical fitting. A “parallel curve” approach was used for curvefitting that involved a four-stage procedure: i) fit a common curve to all varieties ii) fit separate curves for each variety allowing the “a” term to vary iii) fit separate curves for each variety by allowing a, b and c to vary, but keeping the “r” term constant in the linear-plus-exponential fits. iv) fit separate curves for each variety and allow all parameters to vary. For each scenario a test was made of improvement in fit over the previous model with fewer parameters allowed to fit. If no significant improvement (P < 0.05) was observed between two stages, the less variable model was selected. Subsequently the Fopt was determined from the linear-plus-exponential curve parameters by:

 ln Fopt =

−c b. ln(r)

ln (r)

 .

while the Fopt was estimated from the quadratic parameters by: Fopt =

−b . 2a

If Fopt was estimated to be greater than 2.0 rate, then it was assumed that Fopt was 2.0. If the fit of the curve was not significant (P > 0.05) the Fopt was assumed to be 0. 3. Results 3.1. Site and season The incident radiation recorded from May to August at the Carlow and Kildalton sites in 2012 was lower than the other siteseasons (Appendix A), while these sites, in addition to the Killeagh 2015 site, also incurred higher levels of rainfall between May and

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August during the summer when compared to the other siteseasons. Significant main effects of variety on the STB severity, GLA and yield were observed at all site-seasons evaluated with the exception of the Carlow site in 2013, where STB pressure was very low and no evaluations were conducted (Figs. 1 and 2; Appendix B). 3.2. Septoria tritici blotch When untreated, the STB severity was lower (P < 0.001; range: 44–83% lower; Fig. 1) for SR8 at GS71 than the other varieties at all 4 site-seasons that analyses were conducted (2014 and 2015 sites). However, SR7 incurred lower (P < 0.001; range: 20–60% lower) STB severity than SR5 at only three (Carlow 2012, 2014 and Killeagh 2015) of the five sites that comparisons were available. The STB severity of SR5 treated with an azole + SDHI was lower (P < 0.01; range: 27–66% lower) than crops of the same variety treated with an azole-only at all five site-seasons evaluated, while a similar effect was observed for SR7 in four of these site-seasons (range: 25–80% reduction), with no difference between fungicide types observed at Carlow 2012. However, a significant difference in STB severity between fungicide types was only observed at two of the four evaluated sites for crops of SR8, with crops treated with an azole + SDHI incurring lower (P < 0.01; range: 25–50% reduction) STB than azole-only crops at Killeagh 2014 and 2015. When averaged across fungicide types, a significant quadratic relationship (P < 0.05) was observed between STB severity and increased fungicide rate for all varieties at sites where STB evaluations were conducted, with the exception of SR8 grown at Carlow in 2015, where no rate effect was observed. The STB response to increased fungicide dose was similar between SR5 and SR7 at four of the five evaluated site-seasons, with significant reductions in STB with increased dose up to 1.0 rate. There was no significant reduction in the STB severity of SR7 above 0.25 rate of fungicide at Carlow 2012 despite continued reductions up to 2.0 rate for SR5 (Fig. 1a). In addition, the fungicide rate after which no further reduction in STB severity was observed was lower (P < 0.001) for SR8 than the others at two of the four evaluated site-seasons (0 vs 2.0 and 0.25 for Carlow 2014 and 2015, respectively). 3.3. Green leaf area The GLA of SR5 treated with an azole + SDHI was greater (P < 0.001; range: 0.3–6.4 times greater) than crops of the same variety treated with an azole-alone at all six site-seasons evaluated, while SR7 had a greater (P < 0.001; range: 0.2–3.0 times greater) GLA when treated with an azole + SDHI compared to an azole-alone at five of the six site-seasons, with no difference between fungicide types observed at Carlow 2012. No significant difference in GLA between fungicide types was observed at any site-season for crops of SR8. No significant interaction was observed between fungicide type and rate on the GLA of crops for four of the six evaluated siteseasons, while significant interactions (P < 0.001) were observed at Kildalton 2012 and Carlow 2014 whereby the increase in GLA with increased rate of the azole-only treatment was much lower than observed for the azole + SDHI treatment. Despite this, the response of GLA to increased fungicide dose, on average, was similar between SR5 and SR7 at five of the six evaluated site-seasons, with no significant increase in the GLA of SR7 crops at Carlow 2012 after 0.5 rate of fungicide, despite increases (P < 0.001) up to 1.0 rate observed in SR5 crops. A significant quadratic relationship between GLA and fungicide rate was observed for SR5 and SR7 at all sites evaluated with the exception of Kildalton 2012, where a significant linear relationship was observed (P < 0.001). A significantly quadratic relationship was observed between GLA and fungicide

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Fig. 1. STB severity at GS71 on the flag leaf of winter wheat varieties contrasting in STB resistance rating to grown in Carlow during 2012 (a), Carlow during 2014 (b), Killeagh during 2014 (c), Carlow during 2015 (d) and Killeagh during 2015 (e) and treated with various doses of either an azole-alone or azole + SDHI fungicide at GS33 and GS39. Markers which obstruct others due to similar values moved adjacent. Error bars represent one standard error of the mean above or below the mean. “Proportion of fungicide full rate” refers to the recommended dose for an individual application. Indices of significant effects of variety (V), fungicide type (T), fungicide rate (R) and there interactions are stated when significant (P < 0.05) by * (P < 0.05), ** (P < 0.01) or *** (P < 0.001).

rate for SR8 grown in Killeagh in 2014 and 2015, whereas no effect of rate was observed for crops of SR8 in Carlow 2014 or 2015. 3.4. Yield When untreated, the yield of SR8 was greater (P < 0.001; range: 11–29% greater) than the yield of SR5 at four of the five siteseasons available for comparison (2014 and 2015 sites), and greater than SR7 at three of these site-seasons (both 2014 sites and Killeagh 2015; range: 19–25% greater; Fig. 2). However, SR7 only achieved a greater (P < 0.001) yield than SR5 at one of the seven site-seasons available for comparison, when crops were untreated (Carlow 2015; 10% greater). When averaged across fungicide rates, crops of SR5 treated with an azole + SDHI achieved greater yields than crops of the same variety treated with an azole-only at five of the seven site-seasons (range: 7–36% greater), with no difference observed at Carlow 2012 or 2013. Furthermore, SR7 also achieved greater (range: 10–26% greater) yields when treated with an azole + SDHI compared to azole-only at five site-seasons, with no effect at Carlow 2013 or 2015. Despite this, no significant difference was observed in the yield of crops of SR8 treated with either an azole + SDHI or an azolealone at any of the five site-seasons that the variety was included. When averaged across fungicide types, the yield response of crops to increased fungicide rates was similar between SR5 and SR7 at all seven site-seasons evaluated (Fig. 2), with the exception of a significant quadratic relationship observed between yield and rate for SR7 crops in Killeagh 2014 compared to a linear relationship (P < 0.001) for SR5 at the same site. A quadratic relationship (P < 0.05) between yield and fungicide rate was also observed for SR5 and SR7 at Carlow 2012, 2014 and 2015, and Killeagh 2015; whereas a linear relationship was observed at Kildalton 2012. However, the fungicide rate after which no further yield increase was observed was significantly lower for SR8 than the other varieties at three of the five site-seasons where comparisons were available, with no difference observed at Carlow 2013 and Killeagh 2015.

Indeed, at Killeagh 2014 and Carlow 2015, no significant effect of rate was observed for crops of SR8. When averaged across varieties, an interaction between fungicide type and rate was observed on yield at Kildalton 2012, Carlow 2014 and Carlow 2015 whereby the magnitude of response for crops treated with increasing rates of the azole-only treatment was much less than the azole + SDHI treatment. 3.5. Economic optimum rates of azole-only fungicide and differentials from margin at the optimum rate Parallel curve fitting indicated that there was no significant quadratic or linear-plus-exponential relationship between the rate of azole and margin over fungicide cost at Kildalton 2012 (P = 0.062; R2 = 11.4) while the quadratic relationship at Carlow 2013 had a poor fit (R2 = 8.6), and thus, Fopt for all varieties at these sites were assumed to be 0 rate (Fig. 3; Table 2) The quadratic response to increased azole rate was similar between all varieties at Killeagh 2015 (Fig. 3g), with the differential between the margin at Fopt and margin at below- or above-optimum application rates similar among the varieties at this site. For both 2014 sites and the Carlow 2015 site, a quadratic relationship explained more of the variation between azole-only application rate and margin over fungicide cost than a linear-plusexponential fit, and parallel curves indicated that all parameters of the quadratic equation varied between varieties evaluated (Appendix C). At all three sites SR8 was estimated to achieve the highest margin at Fopt of the azole-only treatment, compared to the other varieties. In Carlow 2014, Fopt of the azole-only treatment did not differ between SR5 and SR7 (1.2 rate) while SR8 had a lower Fopt (0.6 rate). In Killeagh 2014, SR5 had a higher Fopt of azole-only (1.9 rate) than the SR7 (1.2 rate), however SR8 had a substantially lower Fopt (0 rate) at this site. At Carlow 2015, the Fopt of the azole-only did not differ substantially between SR5 and SR7 (0.6 and 0.8, respectively), however the Fopt of SR8 was minimal at this site (0 rate). The differential between margin at Fopt and the margin for untreated

J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

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Fig. 2. Grain yield of winter wheat varieties contrasting in resistance to STB grown in Carlow during 2012 (a), Kildalton during 2012 (b), Carlow during 2013 (c), Carlow during 2014 (d), Killeagh during 2014 (e), Carlow during 2015 (f) and Killeagh during 2015 (g) and treated with various doses of either an azole-alone or azole+SDHI fungicide at GS33 and GS39. Markers which obstruct others due to similar values moved adjacent. Error bars represent one standard error of the mean above or below the mean. “Proportion of fungicide full rate” refers to the recommended dose for an individual application. Indices of significant effects of variety (V), fungicide type (T), fungicide rate (R) and there interactions are stated when significant (P < 0.05) by * (P < 0.05), ** (P < 0.01) or *** (P < 0.001).

plots was much lower for SR8 at the 2014 sites than other varieties, however the differential was similar or higher than other varieties for rates that were above the Fopt of azole-only (Fig. 3d and e). When averaged across all site-seasons that three varieties were evaluated, margin at Fopt was higher for SR8 than the other varieties

(1457, 1322, and 1303 D /ha for SR8, SR5 and SR7, respectively), and the Fopt was substantially lower, but there was little difference between SR5 and SR7 (0.3, 1.0 and 0.9 rate for SR8, SR5 and SR7, respectively).

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Fig. 3. Fitted curves for data of the relationship between the estimated margin above fungicide cost of winter wheat varieties contrasting in resistance to STB grown in Carlow during 2012 (a), Kildalton during 2012 (b), Carlow during 2013 (c), Carlow during 2014 (d), Killeagh during 2014 (e), Carlow during 2015 (f) and Killeagh during 2015 (g) and treated with various doses, up to twice the recommended rate per application, of an azole-alone fungicide at GS33 and GS39. Points on each graph represent the mean values of 4 replicate plots per treatment. SEO = standard error of the obervation.

3.6. Economic optimum rates of azole + SDHI fungicide and differentials from margin at the optimum rate When averaged across site-seasons that had a significant fit and varieties, crops treated with an azole + SDHI had a greater estimated margin than crops treated with an azole-only (1440 vs 1358 D /ha,

respectively). Analysis of crops treated with the azole + SDHI indicated that the fitted response curve was identical for all varieties when grown in Kildalton 2012 (quadratic relationship, P < 0.001) and Carlow 2013 (linear-plus-exponential relationship, P < 0.001, Table 2), while only the magnitude of the curves differed between varieties at Killeagh 2015 (linear-plus-exponential fit, P < 0.001). At

J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

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Table 2 Estimation of the optimal proportion of full rate per application of fungicide treatments to achieve greatest margin over fungicide cost for winter wheat varieties grown in seven site-seasons. Fit type

Significance of fit

Variety

P-value

R2 b

SEa

Azole-alone Carlow 2012

Quadratic

<0.001

49.9

86.9

Kildalton 2012

Quadratic

0.062

11.4

120.0

Carlow 2013

Quadratic

0.028

8.6

178.0

Carlow 2014

Quadratic

<0.001

80.8

79.9

Killeagh 2014

Quadratic

<0.001

54.0

82.4

Carlow 2015

Quadratic

<0.001

50.1

77.2

Killeagh 2015

Quadratic

<0.001

57.1

98.3

Azole + SDHI Carlow 2012

Linear + exponential

<0.001

52.3

111.0

Kildalton 2012

Linear + exponential

<0.001

52.3

111.0

Carlow 2013

Quadratic

<0.001

42.9

160.0

Carlow 2014

Linear + exponential

<0.001

68.3

96.5

Killeagh 2014

Linear + exponential

<0.001

74.7

71.6

Carlow 2015

Quadratic

<0.001

58.8

66.5

Killeagh 2015

Linear + exponential

<0.001

41.2

118.0

Optimum fungicide rate (× full rate) Dose

Marginb (D /ha)

SR5 SR7 SR5 SR7 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8

1.0 1.3 0.0 0.0 0.0 0.0 0.0 1.2 1.2 0.6 1.9 1.2 0.0 0.6 0.8 0.0 1.1 1.1 1.1

1264 1428 ne ne 1418 1418 1418 1241 1335 1589 1175 1248 1347 1632 1532 1673 1144 982 1256

SR5 SR7 SR5 SR7 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8

0.5 0.8 2.0 2.0 0.0 0.0 0.0 2.0 2.0 0.3 0.7 0.9 0.3 0.7 1.0 0.0 0.9 0.9 0.9

1262 1462 1011 1011 1373 1373 1373 1507 1604 1609 1331 1426 1393 1672 1624 1651 1305 1189 1338

Grain prices of D 180, D 150, D 150 and D 150 were used for sites during 2012, 2013, 2014 and 2015 respectively. Data were fitted to linear-plus-exponential curves and quadratic curves, with the estimates selected from the best fitting curve. Estimated optimum margins and margins were not calculated for fits that were not significant (ne), while optimum rates were assumed to be zero when data did not have a significant relationship. a SE = standard error of the observation. b Margin over fungicide cost.

these sites the Fopt of the azole + SDHI was 2.0 rate in Kildalton 2012, 0 rate in Carlow 2013 and 0.9 rate in Killeagh 2015. At the Killeagh 2015 site SR7 had a lower margin at Fopt of azole + SDHI than the other varieties, which didn’t differ substantially. At Carlow 2012 and both 2014 sites, a linear-plus-exponential relationship explained the relationship between azole + SDHI application rate and margin over fungicide cost better than a quadratic relationship, with parallel curves indicating that all co-ordinates varied between varieties evaluated, with the exception of the “r” term. At Carlow 2015, a quadratic relationship had a significant fit, with parallel curves indicating that all coordinates varied between varieties evaluated. At Carlow 2012, SR7 was estimated to have a greater margin at Fopt of the azole + SDHI than SR5 (1462 vs 1262 D /ha, respectively), with an Fopt of 0.8 and 0.5, respectively. However, the differential between the margin at Fopt and the margin when untreated was much more substantial for SR7 than SR5 (Fig. 4a). For Carlow 2014 and Killeagh 2014 the Fopt of the azole + SDHI treatment was similar between SR7 and SR5, however the Fopt

differed significantly with SR8 achieving a lower Fopt of azole + SDHI (0.3 rate for both site seasons), than SR5 (2.0 and 0.7 rate for Carlow and Killeagh, respectively) and SR7 (2.0 and 0.9 rate for Carlow and Killeagh, respectively). The SR5 crop achieved a lower margin at Fopt at these sites than the other varieties. At Carlow 2015, SR8 had a substantially lower Fopt (0 rate) than the other varieties (0.7 and 1.0 rate for SR5 and SR7, respectively). At these 3 site-seasons, the differential between the margin at Fopt and the margin when untreated was substantially lower for SR8 than the other varieties, with this effect reversed for rates of 1.0 and 2.0 (Fig. 4d–f). When averaged across all site-seasons that three varieties were evaluated (i.e. 2014 and 2015 sites), the margin at Fopt did not differ substantially between the varieties, while the Fopt of SR8 (0.3 rate) was much lower than the other two varieties, which were greater than 0.8 rate. The differential between the margin at Fopt and the margin at a selected rate was greater when SR8 was treated with 2.0 rate than when untreated, whereas this effect was reversed for the other varieties, on average across the 2013–2015 sites.

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Fig. 4. Fitted linear plus exponential curves for data of the relationship between the estimated margin above fungicide cost of winter wheat varieties contrasting in resistance to STB grown in Carlow during 2012 (a), Kildalton during 2012 (b), Carlow during 2013 (c), Carlow during 2014 (d), Killeagh during 2014 (e), Carlow during 2015 (f) and Killeagh during 2015 (g) and treated with various doses, up to twice the recommended rate per application, of an azole + SDHI fungicide at GS33 and GS39. Points on each graph represent the mean values of 4 replicate plots per treatment. SEO = standard error of the obervation.

4. Discussion 4.1. STB pressure and weather conditions During the 2011–2012 growing season, at both the Carlow and Kildalton sites, warm temperatures during the winter and early spring resulted in a medium-high STB pressure during the growing

season as exhibited by the mean of 44% STB severity on flag leaf of untreated crops at GS71 for Carlow. A low incidence of solar radiation and a high amount of rainfall in the second half of the growing season contributed to the generally lower than average yields achieved in Carlow and the substantially lower yields achieved in Kildalton. These conditions resulted in greater, and more linear, responses observed for crops treated with the azole + SDHI at

J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

Kildalton 2012. The 2012–2013 growing season facilitated a high yielding crop, with cooler temperatures in the early season, and low rainfall and high levels of sunlight in the late season. Consequently, the low early season temperatures and lack of rainfall resulted in low STB pressure, and no significant disease severity data was obtained during this growing season. Furthermore the lack of impact of any treatments on yield in Carlow 2013 indicates that disease pressure was minimal. During 2014, the incident radiation levels and temperature profile during the season facilitated a high yield crop, while the relatively warm temperatures and high rainfall in the early season allowing for high STB pressure at both the Carlow and Killeagh sites, with means of 50% and 74% STB severity on flag leaf of untreated crops at GS71 for Carlow and Killeagh, respectively. The weather conditions differed somewhat between the Carlow and Killeagh sites during 2015, with cooler winter temperatures and lower rainfall during the summer facilitating lower STB-pressure at the Carlow site than Killeagh (means of 17% and 43% STB severity on flag leaf of untreated crops at GS71 for Carlow and Killeagh, respectively). Previous studies have reported a “damaging epidemic” of STB to relate to a minimum of 5% of leaf area exhibiting symptoms of the disease when averaged across the top three leaves at GS75 (Pietravalle et al., 2003; te Beest et al., 2009). Thus the results of the present study indicate that six of the seven site-seasons evaluated incurred at least medium-high STB pressure during the growing season. The relationship between the weather data and the STB disease levels at the site-seasons in the present study support the wellestablished theory that warm early season temperatures and high rainfall during the mid-season promote STB infections (Fones and Gurr, 2015). Therefore, site-seasons used during the course of this study provided a good opportunity for evaluating the objectives in different environmental contexts. 4.2. Varietal STB resistance rating and STB severity The observed trends in STB severity across the evaluated siteseasons in the present study were largely reflected by concurrent trends in GLA, across all evaluated varieties. These results further support the consensus that STB is currently the most destructive foliar wheat disease in Ireland (Jess et al., 2014). All seven of the experiments described in the present study included the SR5 and SR7 treatments, which represented varieties that were at opposite extremities, in terms of STB resistance rating (5 vs 7/9 resistance score, respectively), of the range of conventionally sown winter wheat varieties currently grown in Ireland (DAFM, 2012). Despite the differential in STB-resistance rating, no difference was observed in terms of either STB severity or GLA between SR5 and SR7 at three of the six sites evaluated. At only one of these sites, Carlow 2015, can relatively moderate STB pressure be suggested as a contributing factor. Previous studies have reported inconsistencies in varietal effects on STB severity at different trial environments, particularly for varieties considered susceptible to STB (Simón et al., 2005; Zalewski et al., 2009; Schilly et al., 2011), with increased selection pressure and virulent strains of the pathogen potentially contributing to the reduction in hostplant resistance. However, a definitive explanation for this effect is beyond the scope of the present study. Despite this, SR8 incurred lower STB severity than SR5 in every experiment where it was included and the STB pressure was medium-high (i.e. the 2014 and 2015 seasons). This variety, Stigg, had previously been reported to have exceptional STB resistance by Angus et al. (2010) and Hubbard et al. (2014). Therefore, the results indicate that variety selection does have potential to facilitate a reduction in severity of STB in medium-high disease pressure

97

seasons; however, it should be noted that varieties that exhibit similar levels of STB-resistance to SR8 are not widely available currently to Irish growers (DAFM, 2016). The results of the present study highlight that the current ratings system does relate linearly to STB resistance, with the differences in STB severity between varieties with ratings of 5 and 7 much less than between the varieties rated 7 and 8. 4.3. Fungicide and variety interactions – disease The results indicated that STB severity was lower for crops treated with an azole + SDHI than crops treated with an azole-only at the majority of site-seasons that incurred a medium-high disease pressure. This likely reflects the reduction in efficacy of azolebased fungicide products in recent times, which has been widely reported previously (Cools and Fraaije, 2013; O’Driscoll et al., 2014; Dooley et al., 2016b) and has highlighted the importance of preserving the effectiveness of SDHI-based products to reduce the likelihood of severe STB epidemics. Furthermore, the lack of significant interactions between SR5 and SR7 and the response of these crops to increased rate of azole-only products highlights the limitations that varieties in the typical range of STB-resistance ratings on the national recommended lists have to support reduced use of these products. Despite this, interactions were observed at some site-seasons whereby effects of fungicide product or rate were negligible for SR8, in contrast to the other varieties. As the rate of fungicide resistance selection is in most instances related to the dose of the fungicide applied, with increasing dose increasing selection (van den Bosch et al., 2014), the ability to reduce dose whilst maintaining disease control as achieved with SR8 in the present study will undoubtedly prolong the efficacy of fungicide actives. 4.4. Variety and fungicide – yield Differences in the grain yield of varieties reflected differences observed in the STB severity for most site-seasons evaluated. Therefore, these results further highlight STB control as one of the major factors limiting winter wheat production in the temperate climates of north-western Europe, as previously reported by Hardwick et al. (2001), Mercer and Ruddock (2004) and Ishikawa et al. (2012). However, interactions between variety and fungicide treatments were observed in all but one of the trials where SR8 was grown during a site-season with medium-high disease pressure. The yield response observed for this variety to increased fungicide dose, or the use of an azole + SDHI instead of an azole-only fungicide, was much reduced or negated compared to the other varieties. These results further support the theory that a variety with strong STBresistance has the potential to maintain commercially acceptable yields under lower rate fungicide programs. 4.5. Economic optimum The focus of much previous work on variety STB-resistance and fungicide application rates has been on minimising STB severity or maximising yield benefits. However, information on the potential economic benefits of these results are generally rare, despite their high importance (Wiik and Rosenqvist, 2010). In the present study an analysis was conducted of the economic optimum application rate (Fopt ) of the azole-only and azole + SDHI fungicide treatments that considered crop yield, the cost of crop protection and grain quality in the calculation. When SR5 and SR7 are considered, the variation of Fopt with site season was greater than variation between these varieties, for both fungicide treatments. Over the course of the seven sites, the climatic conditions that encouraged the medium-high level disease epidemics occurred both prior to (2014 sites) and after (2012

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sites and Killeagh 2015) the typical fungicide spray timings during May, which highlights the difficulty in predicting the optimum fungicide dose prior to application. Furthermore, the Fopt of the azole-only for these varieties was at, or greater than, the maximum permitted dose rate of an azole for five of the seven site-seasons evaluated. As the impact of environment on disease severity can vary significantly (Thomas et al., 1989) as observed over the course of the present study, a commercial variety would need a consistently low Fopt to enable a grower to confidently reduce fungicide inputs. The results from the present study indicate that the differential in variety STB-resistance ratings between the SR7 and SR5 is not substantial enough to allow for a reduced recommended rate of either fungicide tested. Furthermore, the differential between the estimated margin at Fopt and rates below-optimum were substantial in some site-seasons for SR5 and SR7 and indicate that reduced rates of both products would be a high risk option. Although the Fopt of SR8 also varied with site-season, the differential between the estimated margin at Fopt and rates belowoptimum was minimal for crops of SR8 at four of the five evaluated site-seasons, and substantially lower than other varieties in most site-seasons with medium-high STB-pressure. These results support the findings of previous studies by Jorgensen et al. (2008), Viljanen-Rollinson et al. (2010) and Poole

and Arnaudin (2014) who reported that disease resistance of varieties can allow for a reduced dependence on fungicide, a trait that may become more valuable in the near future in the event of further restrictions on fungicide use (Hillocks, 2012). Indeed, results from the present study also indicated that the SR8 variety treated with a 0.5 rate of either azole-only or azole + SDHI achieved a greater margin than both the SR5 and SR7 varieties treated with a full rate of the same fungicides, when averaged across the five site-seasons that three varieties were evaluated, indicating that the use of varieties with strong STB-resistance may already allow for economic benefits if they were available to growers. Despite these findings, the range of STB-resistance ratings for the majority of varieties grown in Ireland (4–7 on a scale of 1–9) during the period 2012–2016 are unlikely to confer adequate resistance to allow for confident reductions in fungicide application rates. Acknowledgements The authors would like to acknowledge the technical assistance of Jim Grace, Deirdre Doyle, John Hogan and the Teagasc farm staff, and the assistance of Dr Jim Grant with statistical analysis. Funding for this study was provided by Teagasc and the Department of Agriculture, Food and the Marine, Ireland. Appendix A. Meteorological data for sites used in the study

Average daily temperature (◦ C)

Site-season

Carlow 2012 Kildalton 2012a Carlow 2013 Carlow 2014 Killeagh 2014b Carlow 2015 Killeagh 2015b a Weather b Weather

Total solar radiation (MJ/m2 )

Total rainfall (mm)

1 Oct–28 Feb

1 Mar–31 May

1 Jun–31 Aug

1 Oct–30 Apr

1 May–31 Aug

1 Oct–30 Apr

1 May–31 Aug

8.2 8.3 6.8 7.3 8.4 6.4 8.5

8.8 8.8 7.3 9.7 9.8 8.4 9.2

14.6 14.1 15.3 15.3 15.6 14.2 14.0

408 465 426 776 837 533 551

417 562 176 287 181 281 421

11.2 11.3 11.6 12.0 13.4 13.8 14.6

17.3 17.3 24.0 19.5 22.2 20.1 21.0

data for Kildalton site was sourced from Johnstown Castle, Co. Wexford, approx. 62 km south-east of site data for Killeagh was sourced from Roches Point, Co. Cork, approx. 15 km south-west of site.

J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

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Appendix B. Average green leaf area at GS71 on the two highest leaves of winter wheat varieties contrasting in resistance to STB grown in Carlow during 2012 (a), Kildalton during 2012 (b), Carlow during 2014 (c), Killeagh during 2014 (d), Carlow during 2015 (e) and Killeagh during 2015 (f) and treated with various doses of either an azole alone or azole+ SDHI fungicide at GS33 and GS39. Markers which obstruct others due to similar values moved adjacent. Error bars represent one standard error of the mean either above or below the mean. “Proportion of fungicide full rate” refers to the recommended dose for an individual application. Indices of significant effects of variety (V), fungicide type (T), fungicide rate (R) and there interactions are stated when significant (P<0.05) by * (P<0.05), ** (P<0.01) or *** (P<0.001).

Appendix C. Estimation of parameters for regression curves fitting the relationship between the proportion of fungicide rate and the margin over fungicide cost for winter wheat varieties grown in seven site-seasons and treated with either an azole alone or an azole + SDHI. Fit type

Azole-alone Carlow 2012 Kildalton 2012

Quadratic

Carlow 2013

Quadratic

Carlow 2014

Quadratic

Killeagh 2014

Quadratic

Carlow 2015

Quadratic

Killeagh 2015

Quadratic

Azole + SDHI Carlow 2012

Linear + exponential

Kildalton 2012

Linear + exponential

Variety

Parameter estimates a

b

c

r

SR5 SR7 SR5 SR7 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8

1135 1158 582 663 1418 1418 1418 1142 1162 1567 1146 1010 1347 1618 1469 1673 1064 901 1175

−134 −171 −88 −88 −27 −27 −27 −64 −116 −73 −6 −160 −34 −45 −89 −8 −68 −68 −68

263 430 174 174 −33 −33 −33 159 284 81 21 390 −7 50 151 129 148 148 148

– – – – – – – – – – – – – – – – – – –

SR5 SR7 SR5 SR7

1315 1502 710 710

−168 −364 −97 −97

−77 −41 151 151

7.0 × 10−3 7.0 × 10−3 2.8 × 10−14 2.8 × 10−14

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J.P. Lynch et al. / Field Crops Research 204 (2017) 89–100

Carlow 2013

Quadratic

Carlow 2014

Linear + exponential

Killeagh 2014

Linear + exponential

Carlow 2015

Quadratic

Killeagh 2015

Linear + exponential

SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8 SR5 SR7 SR8

1373 1373 1373 1459 1572 1635 1455 1557 1547 1632 1444 1651 1411 1295 1444

−81 −81 −81 −343 −403 −67 −323 −561 −203 −91 −168 2 −338 −338 −338

−28 −28 −28 56 16 −67 −122 −105 −241 121 348 −121 −84 −84 −84

– – – 3.3 × 10-4 3.3 × 10-4 3.3 × 10-4 4.8 × 10-2 4.8 × 10-2 4.8 × 10-2 – – – 6.7 × 10-2 6.7 × 10-2 6.7 × 10-2

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