ORIGINAL ARTICLE

Harmful winter wheat diseases and possibilities for theirintegrated control in Latvia

Biruta Bankinaa*, Zinta Gailea, Oskars Balodisa,b, Gunita Bimšteinea, Merabs Katamadzea,Dzintra Kreitaa, Līga Pauraa and Ilze Priekulec

aLatvia University of Agriculture, Lielā iela 2, Jelgava, LV 3001, Latvia; bLatvian Rural Advisory and TrainingCentre, Rīgas iela 34, Ozolnieku region, Jelgava, LV-3018, Latvia; cUAB Makhteshim-Agan Benelux & Nordic,Ulonu 2, Vilnius, Lithuania

(Received 13 January 2014; accepted 22 July 2014)

Decision support systems based on control thresholds are an important part of integrated disease controlsystems in winter wheat. Various warning and forecast systems have been developed in different countries (inLatvia – under the framework of Danish-Baltic joint project ‘Development and Implementation of an Internet-based Decision Support System for Integrated Pest Management’), but results were contradictory. The aim ofthis investigation was to identify the possibilities for integrated disease control under intensive management ofwinter wheat. Different schemes of fungicide treatment, including a decision support system, were tested infield trials from 2008 to 2012 in two places. Leaf blotches caused by Pyrenophora tritici-repentis and Zymoseptoriatritici are recognised as the most harmful wheat diseases in Latvia. The total impact of diseases, which wascalculated as area under disease progress curves, was affected by the year and the scheme of fungicidetreatment. Significantly higher yields were obtained in treatments with fungicide application, but differencesbetween control strategies were not significant. The obtained results confirmed that two fungicide applicationsare not necessary (except certain cases). The results suggest that recommendations should be based on thedisease incidence on the third or second leaf of wheat.

Keywords: Pyrenophora tritici-repentis; Zymoseptoria tritici; decision support system

Introduction

Winter wheat is an economically important crop inLatvia and all over the world. In intensive wheatproduction diseases are an important risk factor,which depending on season and cultivars can causesignificant reduction of yields. High-yield losses arepossible in years with an epidemic of winter wheatdiseases. Fungicides for control of winter wheat leafdiseases are widely used in Latvia, and the fungicidetreatment schemes are mostly based on the crop’sgrowth stages (GS). However, these schemes havebeen developed in countries with a milder climateand a longer vegetation period than Latvia.Tan spot (caused by Pyrenophora tritici-repentis)

and Septoria leaf blotch (caused by Zymoseptoriatritici, teleomorph Mycosphaerella graminicola) havebeen the most important and harmful wheat diseases

in Latvia in the last 10 years. Powdery mildew(caused by Blumeria graminis) is common, but theincidence and severity are low. At the stage of milkripeness the incidence has been found to be 10%,which relate to less than 1% severity. Yellow rust(caused by Puccinia striiformis) and brown rust(caused by Puccinia tritici) were observed only spor-adically (Bankina & Priekule 2005; Bankina et al.2011).Leaf blotches are the most harmful wheat diseases

all over the world, including Southern Sweden,where these diseases cause approximately 74% ofthe total yield losses. The disease severity on thethird leaf at GS 55 and on the second leaf at GS 75has negatively correlated with the yield (Wiik 2009).Epidemics of Septoria leaf blotch do not occurevery year. For example, in northern Luxembourg,

*Corresponding author. Email: Biruta.Bankina@llu.lv

Acta Agriculturae Scandinavica, Section B — Soil & Plant Science, 2014Vol. 64, No. 7, 615–622, http://dx.doi.org/10.1080/09064710.2014.949296

© 2014 Taylor & Francis

fungicide treatments based on a risk simulationmodel were not recommended between 2003 and2006 or in 2007 (Jarroudi et al. 2009). In SouthernSweden, yield increases resulting from a singlefungicide treatment were observed each year from1983 to 2007; however, only 13 out of these 25 yearshad significant yield increases (Wiik & Rosenqvist2010). Similar results were obtained also in Neb-raska. Two-year experiments showed a negativeeconomic return in most cases in 2006, when thedisease severity was low (1–5%); however, in 2007,an essential additional yield and subsequent eco-nomic effect were obtained because of the highpressure from diseases that year (Wegulo et al. 2011).A precise diagnosis of the diseases and a deter-

mination of pathogen-specific thresholds are neces-sary to develop integrated pest management (IPM)programmes. Fungicide sprays should be appliedwhen infection material transfers from the primarysource of infection to the upper leaves (Verreet et al.2000). Globally, different forecasting and warningsystems have been developed to avoid unnecessaryfungicide spraying. Forecasting of disease epidemicsdiffers among different countries. For example, the120-day period prior to GS 31 is important forevaluation of the risk of Septoria leaf blotch epidemicin the UK, where high amounts of precipitation andhigh temperatures increase the development of thedisease (Beest et al. 2009); however, this system isnot useful for colder climate zones where tilleringcontinues after overwintering. Septoria blotch has along latent period of 21–37 days, according to thefindings of Viljanen-Rollinson (Viljanen-Rollinsonet al. 2005), which means that symptom-orientedfungicide sprays are not completely effective and thatmodels based on weather conditions are required.An incidence of Septoria blotch higher than 50% onthe sixth leaf was recognised as an action thresholdin Rheinland (Germany), especially when combinedwith leaf wetness during the following two days(Heger et al. 2003). Another forecasting methodhas been developed by Danish researchers andconsists of the number of rainy days (precipitationmore than 1 mm) during the 30 days after the startof stem elongation being marked as the crucialperiod for Septoria development (Hansen et al.1994; Jørgensen et al. 1996).In Germany, the mildew threshold for treatment

with fungicides was set at 70% of infected plants orleaves (Verreet et al. 2000). The Danish PC-PDisease programme = the Danish Crop ProtectionOnline (CPO) system for diseases has recommendeddifferent mildew thresholds varying from 10% to75% incidence, depending on cultivar resistance andgrowth stage of the crop (Jørgensen et al. 1996).

Fewer thresholds have been developed for tanspot. Incubation period of tan spot is short (5–6days), and thresholds were based on incidence ofdisease. For susceptible cultivars, Danish researcherssuggest using the disease incidence as a threshold:Higher than 75% during early stages of plant devel-opment, >25% at the stages of 33–60 and >50%after flowering (Jørgensen, Nielsen, et al. 2008).Within the framework of the joint Danish–Baltic

project ‘Development and Implementation of anInternet-based Decision Support System for Inte-grated Pest Management’, field trials in all regions ofLatvia were conducted from 1998 to 2002. Recom-mendations for fungicide sprays were based on theagronomic parameters in each field, growth stage ofwheat, resistance of the variety, meteorological con-ditions (number of rainy days in the critical period ofwheat development) and assessment of diseases inthe fields (Jørgensen et al. 1996). Application ofcomputer programs is costly in terms of time andmoney, so they were not used, and reduced fungi-cide doses are not permitted in Latvia; however, themain results (disease development thresholds) seemto be useful under the conditions present in Latvia.The aim of the present investigation was to

identify the most harmful winter wheat diseases inLatvia in recent years and to identify possibilities forintegrating disease control with intensive manage-ment of winter wheat – specific control strategiesusing one and two pre-planned applications werecompared with thresholds-based spray strategies,relying on field assessments before applications.

Materials and methods

The field trials were conducted at the Research andStudy farms ‘Vecauce’ (latitude: N56°28′; longitude:E22°53′) and ‘Peterlauki’ (latitude: N56°54′; longit-ude: E23°72′) of the Latvia University of Agriculture(LLU) from 2008 until 2012.Soil conditions, seed treatments, wheat sowing

times and rates and crop management practicesrepresented typical methods of intensive winterwheat production under the conditions of CentralLatvia, which is the most important region for wheatproduction in the country.High-yielding cultivars were used during the

experimental period: ‘Zentos’ was used in all treat-ments in Peterlauki, ‘Olivin’ was used in croprotation and ‘Tarso’ was used in repeated wheatsowings in Vecauce.The total nitrogen dose in the trials varied from

160 to 180 kg ha−1. The nitrogen dose was 80 kgha−1 N in Peterlauki at the start of vegetation, plus70 kg ha−1 N at the beginning stage of stemelongation, plus 30 kg ha−1 N at the stage of

616 B. Bankina et al.

heading; in Vecauce, the dose was N 70 + N 60 + N30 kg ha−1, respectively.The meteorological conditions varied between the

seasons, but the conditions for wheat production andleaf disease development were generally suitable inall years investigated.

Experiment design

The trials were established in two blocks: Winterwheat sown after winter wheat and winter wheatsown after oilseed rape or bare fallow. Seven treat-ments (including schemes widely used by farmers)were tested: Control (without fungicides), Standard1 (S1; one treatment), Standard 2 (S2; two treat-ments), Standard 3 (S3; one treatment with afungicide containing strobilurin) and Standard 4(S4; two treatments with a fungicide containingstrobilurin). The fungicide applications wereconducted according to the growth stage of thewheat simultaneously in both blocks (Table 1). InDecision Support System 1 (DSS 1) and DecisionSupport System 2 (DSS 2), the fungicide and thetime of application were selected depending ondisease incidence and/or number of rainy days afterGS 31 (Table 2). Decisions made under DSS 1 andDSS 2 were based on previous results and experi-ence obtained through the joint project among theBaltic States, Poland and Denmark titled ‘Devel-opment and Implementation of an Internet-basedDecision Support System for Integrated Pest Man-agement’ (Turka & Priekule 2003; Jørgensen, Noe,et al. 2008). Fungicide applications according tothe recommendations of DSS 1 advised sprayingwhen four rainy days (mm > 1) after the start ofwheat elongation or symptoms of diseases on thethird leaf were observed. DSS 2 recommended latertimes of spraying – seven rainy days or symptomson the second leaf. In some years, fungicides weresprayed simultaneously because of the meteorolo-gical conditions.

Observations and calculations

The incidence and severity of the diseases wereevaluated each week as described previously (Priekuleet al. 2002). Green leaf areas were assessed at the stageof milk ripeness (GS 75, according scale ‘BiologischeBundesanstalt, Bundessortenamt und CHemischeIndustrie’ (BBCH). Assessments were conducted incooperation with the State Plant Protection Service.The efficacy of fungicide treatment was measured byassessing the disease severity on the two top leavesat the stage of milky ripeness and by comparison ofthe total areas under the disease progress curves(AUDPC). The AUDPCs were calculated for eachdisease (approximately 10–12 assessments for eachtreatment) separately and afterwards were summedup to demonstrate the total amount of diseases. Forstatistical analyses of the total impact of the diseases(leaf blotch, tan spot and mildew) expressed as thesum of AUDPC, a three-way analysis of variance(ANOVA) was performed that included the factorA – fungicide treatment (i = 1 … 7, control withoutfungicides and six variants with fungicides), factorB – crop rotation (repeated sowing/wheat in croprotation) and factor C – site of experiments (Vecauceor Peterlauki). The differences were consideredstatistically significant when p < 0.05.The relationship between the total sum of

AUDPC and the green leaf area, the green leaf areaand the winter wheat yield was analysed by correla-tion and regression analyses.Statistical analyses were conducted using the

software program SPSS for Windows, Version 15.The general linear modes (GLM) procedure wasused for variance and regression analyses. Linearregression curves were fitted using MS Excel.

Results

Leaf blotch (Z. tritici), tan spot (P. tritici-repentis) andmildew (B. graminis) were observed during theinvestigations, but rusts were not noted.

Table 1. The schemes of standard fungicide application for control of winter wheat diseases.

Designations of treatmentDevelopment stages of wheat when

treatment/s was/were done Active ingredients of fungicides Dose (L per ha)

Control Without fungicidesStandard 1 (S1) 51–55 Triazole, morpholinea 1.50Standard 2 (S2) 32–33 and 55–59 Triazole, morpholinea 0.75

triazole, morpholinea 1.50Standard 3 (S3) 51–55 Strobilurin, triazole, morpholineb 1.50Standard 4 (S4) 32–33 and 55–59 Strobilurin, triazole, morpholineb 0.75

strobilurin, triazolec 1.50

aEpoxiconazole 0.084 kg L−1, fenpropiomorph 0.250 kg L−1.bKresoxim-methyl 0.083 kg L−1; epoxiconazole 0.083 kg L ha−1; fenpropiomorph 0.317 kg L−1.cPyraclostrobin 0.085 kg L−1; epoxiconazole 0.0625 kg L−1.

Acta Agriculturae Scandinavica, Section B — Soil & Plant Science 617

The severity of mildew did not exceed 5% in mostcases, except in Vecauce in 2009 and 2012 in thefields with crop rotation (the severity was 5.4% and14.9%, respectively, at the time after flowering, i.e.,GS 69–71). Mildew was not observed at earliergrowth stages, except in Vecauce in 2009 when thefirst symptoms of mildew (severity of 3%) werefound at the stem elongation stage (GS 37–39).The most important diseases during the investiga-

tions were leaf blotches. Tan spot dominated in2008 and partly in 2009, but Septoria leaf blotchdominated in 2010, 2011 and 2012. The develop-ment and harmfulness of tan spot depended on theyear, trial site and crop rotation. Sowing wheat afterwheat increased the severity of tan spot, but theseverity of Septoria leaf blotch did not depend oncrop rotation (Figures 1 and 2).The wheat leaf disease progression during the

vegetation season is shown in Figure 3. The curvesfor tan spot development are slightly different whencompared to the curves for Septoria leaf blotchprogress. The progression of tan spot was morerapid, although it started later than Septoria leafblotch.

Table 2. Fungicide treatment according to the recommendations of decision support system for the control of winter wheatdiseases.

Substantiation of applicationaTime (dd. mm) and development

stages (DS) of application

Peterlauki Vecauce

Designation of treatment Year Substantiation DS Date DS Date

Decision Support System 1(DSS 1)

2008 One application – four rainy days after thestart of wheat elongation.

59 21.06 No fungicide

2009 One application – four rainy days after thestart of wheat elongation.

49 30.05 49 02.06

2010 Two applications:(1) Four rainy days after the start of wheatelongation and

37 03.06c 37 04.06

(2) Incidence of Septoria reached 30% on thethird leaves.

65 29.06b

2011 Symptoms of diseases on the third leaf. 59 13.06 59 22.062012 Symptoms of diseases on the third leaf. 59 21.06 59 20.06

Decision Support System 2(DSS 2)

2008 One application – seven rainy days after thestart of wheat elongation.

71 01.07 No fungicide

2009 One application – seven rainy days after thestart of wheat elongation.

55 04.06 55 08.06

2010 Two applications:(1) Seven rainy days after the start of wheatelongation and

37 13.06c 37 04.06

(2) Incidence of Septoria reached 30% on thesecond leaves

65 29.06

2011 Symptoms of diseases on the second leaf. 59 13.06 59 22.062012 Symptoms of diseases on the second leaf. 67 03.07 67 04.07

aEpoxiconazole 0.084 kg L−1, fenpropiomorph 0.250 kg L−1, dose 1.5 L−1 was used in all treatments.bFungicide spraying was delayed because the meteorological conditions did not allow spraying.cFungicide spraying was not done because the meteorological conditions did not allow spraying.

0

5

10

15

20

25

30

35

40

P V P V P V P V P

2008 2009 2010 2011 2012

Seve

rity

(%

)

Tan spot

Septoria blotch

Figure 1. Severity of winter wheat diseases depending onyear and trial site at the stage of milk ripeness (GS 69–71)in repeated wheat sowings: P – Pēterlauki; V – Vecauce.

0

5

10

15

20

25

30

35

40

P V P V P V P V P

2008 2009 2010 2011 2012

Seve

rity

(%

)

Tan spotSeptoria blotch

Figure 2. Severity of winter wheat diseases depending onyear and site at the stage of milk ripeness (GS 69–71) infields with crop rotation: P – Pēterlauki; V – Vecauce.

618 B. Bankina et al.

The development of tan spot was influenced bycrop rotation. In repeated wheat sowings, the diseaseinfection started earlier and achieved a higher sever-ity level, whereas the progress of Septoria leaf blotch

was not affected by crop rotation (the difference inseverity at the stage of milk ripeness was only 1.7%).The total impact of diseases during the whole

vegetation season and the efficacy of disease sup-pression by fungicides are shown by AUDPC(Table 3).The scheme for fungicide treatment influenced

the total amount of disease in all years except 2010.Values of AUDPC were higher in the variant withoutfungicides in all trials, but differences among theschemes were inconsistent. In general, two fungicideapplications did not produce better results whencompared with a single treatment.Crop rotation had an effect on the AUDPC, but in

certain cases, the level of disease was higher in plotswith crop rotation. The site of the experiment did

Table 3. Total value of areas under the disease progress curves (AUDPC) depending on fungicide treatment, crop rotationand site.

Pēterlauki Factor C Vecauce Factor C

Factor B

Year Fungicide treatment Factor A Repeated sowings Crop rotation Repeated sowings Crop rotation

2008a Control 410 119 120 –A Standard 1 165 35 46 –B Standard 2 235 78 49 –

Standard 3 52 20 39 –Standard 4 20 20 38 –

Decision Support System 1 44 42 87 –Decision Support System 2 200 36 97 –

2009a Control 249 68 133 376A Standard 1 38 22 71 220

Standard 2 76 76 70 136Standard 3 47 37 136 211Standard 4 35 28 76 204

Decision Support System 1 101 31 131 168Decision Support System 2 91 42 103 165

2010a Control 237 368 165 258B Standard 1 181 202 188 205C Standard 2 77 182 134 151

Standard 3 138 144 175 168Standard 4 31 114 136 131

Decision Support System 1 166 194 129 134Decision Support System 2 117 164 118 166

2011a Control 47 194 184 243A Standard 1 63 148 129 136B Standard 2 50 191 106 109

Standard 3 37 74 109 128Standard 4 49 128 84 92

Decision Support System 1 38 87 122 172Decision Support System 2 53 114 136 198

2012a Control 57 185 169 364A Standard 1 65 109 82 220B Standard 2 52 157 59 120

Standard 3 51 104 92 216Standard 4 65 113 49 124

Decision Support System 1 54 103 100 275Decision Support System 2 57 98 112 258

aThe AUDPC was influenced by a factor within a year: (A) fungicide treatment in all years, except year 2010; (B) repeated sowings/croprotation in all years, except year 2009; and (C) place (Pēterlauki/Vecauce) only in year 2010 (p < 0.05).

0

2

4

6

8

10

12

14

31–33 37–39 45–49 55–59 60–63 69–71 >71

Seve

rity

(%

)

Growth stages

Tan spot: Crop rotation

Tan spot: Wheat after wheat

Septoria blotch: Crop rotation

Septoria blotch: Wheat after wheat

Figure 3. Development of winter wheat diseases (tan spotand Septoria leaf blotch) depending on crop rotation(average data, 2008–2012).

Acta Agriculturae Scandinavica, Section B — Soil & Plant Science 619

not affect the results except in 2010, when the levelof disease in Peterlauki was higher than that inVecauce.Fungicide application decreased the development

of disease and increased the leaf green area. A weaknegative correlation (r = −0.335, p < 0.05) betweenthe total sum of AUDPC and the green leaf area wasobserved. Compared to a single application, theefficacy of two fungicide treatments was not increased.The highest leaf green area was observed in variants(S3 and S4) where fungicides with active ingredientscontaining strobilurins were used (Figure 4).The green leaf area had a significant and moderate

positive correlation with the yield of winter wheat(r = 0.450, p < 0.05; Figure 5).Winter yield variations were significantly influ-

enced by year (p < 0.01), site and crop rotation (p <0.05). Yield differences between the variants rangedbetween 4.6 and 10.0 t ha−1. Substantially loweryields were obtained in variants not treated withfungicides, and differences between other treatmentswere not statistically significant (Figure 6).On average, DSS produced the same result as

standard variants. Compared to the control, theadditional yield reached 3–77%, depending on theyear and crop rotation.

Discussion

The key point for integrated plant protection isan accurate identification of wheat diseases. Leafblotches caused by P. tritici-repentis and Z. tritici werethe most harmful diseases in our investigations,whereas the severity of mildew was comparativelylow. Leaf blotches have been recognised as the mostharmful wheat diseases in different countries, andsimilar results were obtained in our study. Tan spothas been described as the most important wheatdisease in Lithuania, Canada, and other countries,but Septoria blotch is more harmful in Denmark, theUK, Germany and Estonia (Lõiveke & Sepp 2009).The occurrence and severity of mildew is known

to fluctuate among localities and years, and particu-larly among cultivars. In a research study in Poland,similar to our investigation, mildew was not found tobe an important disease, and its severity fluctuatedonly within a certain percentage from 1999 to 2011(Wyczling et al. 2010). Similar results were obtainedin Germany, and a significant disease level wasdetermined only for certain years and certain loca-tions (Verreet et al. 2000). Different agronomicalparameters, such as high sowing rates and high dosesof nitrogen, promoted the development of thisdisease in Denmark (Jørgensen et al. 1997).A high occurrence of yellow and brown rust has

been observed sporadically all over Europe (Beestet al. 2008), but these diseases were not observed inour investigations.Crop rotation affected the development of leaf

diseases in our trials, but this influence varieddepending on the biological traits of the pathogens.The severity of tan spot was significantly higher inrepeated wheat sowings, and crop rotation did notaffect the development of Septoria blotch, but incertain cases, the severity of mildew was higher infields with crop rotation. A similar conclusion wasreached in the Polish study described above, whichobserved higher intensity of mildew on plants grown

0

5

10

15

20

25

30

35

40

Control S 1 S 2 S 3 S 4 DSS 1 DSS 2

Lea

f gr

een

area

(%

)

Figure 4. Wheat leaf green area depending on fungicideapplication (average data) at the stage of milk ripeness.S1, Standard 1; S2, Standard 2; S3, Standard 3; S4,Standard 4; DSS 1, Decision Support System 1; DSS 2,Decision Support System 2.

4

5

6

7

8

9

10

11

0 10 20 30 40 50 60 70

Yie

ld (t

ha

)–1

Wheat leaf green area (%)

Figure 5. Correlation between the wheat yield and leafgreen area.

5.0

5.5

6.0

6.5

7.0

7.5

8.0

C S 1 S 2 S 3 S 4 DSS 1 DSS 2

Yie

ld (t

ha

)–1

Figure 6. Average yield of winter wheat depending onfungicide treatment. S1, Standard 1; S2, Standard 2;S3, Standard 3; S4, Standard 4; DSS 1, Decision SupportSystem 1; DSS 2, Decision Support System 2.

620 B. Bankina et al.

after sowing with oilseed rape than in repeated wheatsowings (Wyczling et al. 2010).Developing an optimal system of disease control is

impossible without understanding the peculiarities ofdisease life cycles, especially the crucial periods ofdisease development. Ascospores are described as aprimary infection source of Septoria leaf blotch inresearch papers from Western Europe (Cook et al.1999). In Latvia, conidia are recognised as a primaryinoculum in which the spores are dispersed by rainsplash from lower leaves infected in autumn. Theharmfulness of this disease depends on the ability ofconidia to vertically spread to upper leaves. Tele-omorph of this pathogen has not yet been found inLatvia (Bankina et al. 2011).Rapid distribution of tan spot started only after

flowering, possibly because of the conditionsrequired for liberation, dispersal and germination ofascospores in addition to the development of conidiaon lower senescent leaves and short incubationperiod of tan spot (Bankina & Priekule 2011). Thesefactors are very important for selecting an optimaltime of fungicide usage because early sprayings arenot effective in these conditions. Similar tendencieshave been observed by Polish researchers: Rapiddevelopment of leaf necrotic spots occurred aftergrowth stage BBCH 59, and a single fungicidetreatment showed good efficacy (Wyczling et al.2010). The same result was reported by Wegulo et al.(2009), who observed that leaf blotches (caused bydifferent pathogens together) progressed rapidly onlyafter flowering, with the disease intensity increasingexponentially up to milk ripeness. The severity ofdisease had the strongest correlation with yield lossesat the flowering stage (Wegulo et al. 2009).In our investigations, the total average value of

AUDPC in the untreated variant fluctuated between167 and 257 units depending on the year. Thedifference in these values was not significant, andthe meteorological conditions were appropriate fordisease development each year. A comparison ofAUDPC values between treated and untreated plotsis recognised as a useful tool to estimate the efficacyof different fungicide application schemes (Cooket al. 1999), but a clear difference was not observedin our investigations. The total value of AUDPC wasinfluenced by different factors and did not reflect theeffect of different fungicide application schemes;however, the AUDPC value was higher in the controltreatment (without fungicides) in almost all cases.According to the findings of other studies, fungi-

cide application increases the green leaf area (Cooket al. 1999; Wyczling et al. 2010). Our investigationsalso demonstrated that differences in leaf green areawere significant and correlated with yield.

Numerous investigations to find optimal schemesfor fungicide treatment have been performed all overthe world, particularly in Europe. Reduced doses offungicides are not allowed in Latvia, which is whyinvestigations must focus on the best time of spray-ing and on finding the damage thresholds of dis-eases. In our trials, the average additional yield intreated fields reached 9.8–13.5% when comparedwith untreated fields, but two applications were nomore effective than a single treatment. Fungicidescontaining strobilurins provided a slightly higheryield, and similar results have been obtained byWegulo et al. (2012), who found that the mean yielddifference between treated and untreated fieldsfluctuated between 12.6% and 29.4% in trials inNebraska, USA (Wegulo et al. 2012).Different strategies for fungicide spraying are

applied in Latvia, but generally two (sometimes eventhree) applications are used in intensive managementof winter wheat. A fixed control strategy with two oreven three applications per season will in most seasonsuse more fungicide than needed based on an econom-ical evaluations. The presented study indicates thatthere is a clear difference in the need for treatmentsdepending on site, year and crop rotation. This hassimilarly been seen in other European countries,where focus often has been to adjust treatments duringstem elongation, where new leaves developed (Mercer& Ruddock 2005). In Denmark, the climate is milderand diseases start to develop earlier than in Latvia;nevertheless, in these conditions, early treatment isalso less effective. Large field trials in Denmark haveconfirmed the necessity of protecting ears and upperleaves. According to data from many investigations inDenmark, it has been found that the second applica-tion around heading provide 85% of the total yieldresponse from two fungicide treatments, the firsttreatment (BBCH 31–32) has often not given profit-able output, even when only reduced rates have beenapplied (Jørgensen et al. 2010).In our study, recommendations of the DSS were

contradictory, and further investigations are requiredto improve the model; however, certain distincttendencies were observed. Tan spot frequently domi-nated during the trial period, so the number of rainydays cannot be a suitable indicator for conditions inLatvia. As ascospores spread by wind, the distributionof tan spot does not particularly depend on rainsplash. The incidence of disease on the upper leavesseems to be a more suitable indicator as also proposedin the Danish model (Jørgensen & Olsen 2007).The severity of diseases and spectrum of patho-

gens fluctuated depending on weather conditionsand localities. Consequently, optimal control ofwheat diseases is achievable only through monitoring

Acta Agriculturae Scandinavica, Section B — Soil & Plant Science 621

of site-specific diseases in combinations with weatherrisk and data analyses.

Funding

The research was supported by a grant from the Ministryof Agriculture of Latvia [grant number ZM 070410/S 35];European Agricultural Fund for Rural Development [grantnumber 020311/C – 31].

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