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Evaluating fungicides for controlling Cercospora leaf spot on sugar beet
ARTICLE IN PRESS
Crop Protection 24 (2005) 79–86 www.elsevier.com/locate/cropro
Evaluating fungicides for controlling Cercospora leaf spot on sugar beet Mohamed F.R. Khana,, Larry J. Smithb a
227 Walster Hall, North Dakota State University & University of Minnesota, Fargo, ND 58105-5758, USA b Northwest Research and Outreach Center, University of Minnesota, Crookston, MN 56716, USA Received 6 April 2004; received in revised form 24 May 2004; accepted 30 June 2004
Abstract Cercospora leaf spot, caused by the fungus, Cercospora beticola, continues to be a devastating foliar disease of sugar beet (Beta vulgaris), in Minnesota and North Dakota. Commercial sugar beet varieties grown in Minnesota and North Dakota generally have only moderate resistance and require fungicide applications to obtain adequate levels of protection against C. beticola. Trials were conducted in 1999 at Foxhome and Crookston, Minnesota and in 2000 at Breckenridge and Crookston, Minnesota to determine the efficacy of labeled and experimental fungicides for controlling Cercospora leaf spot. Natural inocula were relied on for infection, and disease pressure was high at all sites in both years. Except for azoxystrobin applied alone at Foxhome, and azoxystrobin, and fentin hydroxide, applied alone, and fenbuconazole applied with an adjuvant at Breckenridge, the fungicide treatments provided better Cercospora leaf spot control, and resulted in higher recoverable sucrose yields than non-treated controls. Tetraconazole and pyraclostrobin, when applied alone, consistently provided effective Cercospora leaf spot control and resulted in high sucrose yield. r 2004 Elsevier Ltd. All rights reserved. Keywords: Cercospora beticola; Beta vulgaris; Fungicides; Strobilurin; Pyraclostrobin
1. Introduction Cercospora leaf spot, caused by the fungus Cercospora beticola Sacc., occurs in all sugar beet (Beta vulgaris L.) production areas in the United States (Ruppel, 1986; Kerr and Weiss, 1990), and is the most destructive foliar disease of sugar beet in Minnesota and North Dakota. The disease reduces root and extractable sucrose yields, and increases impurity concentrations, resulting in higher processing losses (Smith and Ruppel 1973; Lamey et al., 1987; Shane and Teng, 1992; Lamey et al., 1996). Losses in recoverable sucrose as high as 30% are common under heavy disease conditions and revenue losses as high as 43% have been reported (Lamey et al., 1987; Shane and Teng, 1992; Lamey et al., Corresponding author. Tel: +1-701-231-8596; fax: +1-701-2316186. E-mail address: [email protected] (M.F.R. Khan).
0261-2194/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2004.06.010
1996). Roots of diseased plants do not store as well as roots from healthy plants in storage piles that are processed in a 7–9 month period in North Dakota and Minnesota (Smith and Ruppel, 1973). Cercospora leaf spot is managed by fungicide applications, reducing inoculum by crop rotation and tillage, and by planting disease tolerant varieties (Miller et al., 1994). Four to five genes are responsible for Cercospora leaf spot resistance (Smith and Gaskill, 1970). Combining high levels of Cercospora leaf spot resistance with high yield in sugar beet is difficult (Smith and Campbell, 1996). As a result, commercial varieties generally have only moderate levels of resistance and require fungicide applications to obtain adequate levels of protection against Cercospora leaf spot (Miller et al., 1994). In 1998, revenue losses by American Crystal growers in Minnesota and North Dakota were over $45 M from reduced tonnage and quality despite the use of $20 M in fungicide applications (Cattanach, 2000). The major
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fungicides used in 1998 were fentin hydroxide, mancozeb and thiophanate methyl (Dexter and Luecke, 1999). Most growers experienced inconsistent leaf spot control, probably because of ineffective fungicides as a result of a high population of benzimidazole resistant and fentin hydroxide tolerant strains of C. beticola (Bugbee, 1982; Bugbee, 1995; Weiland and Smith, 1999), or untimely applications. Cercospora leaf spot was most severe in the warmer southern Minnesota sugar beet growing district, resulting in some growers applying 11 fungicide applications compared to about 3–4 applications in most years. There was an urgent need to find new chemistry fungicides that will provide effective Cercospora leaf spot control and result in high extractable sucrose. The acreage of sugar beet grown in the United States is small relative to corn (Zea mays L.), wheat (Triticum aestivum L.), cotton (Gossypium hirsutum L.), and soybean (Glycine max (L.) Merrill). As such, very few fungicides are developed primarily for controlling diseases of sugar beet. The availability of triazoles and strobilurins used on other crops presented and opportunity to test these products for controlling Cercospora leaf spot of sugar beet. Data could then be used to obtain registration for use on sugar beet. The objective of this research was to evaluate the efficacy of labeled and experimental fungicides to control Cercospora leaf spot on sugar beet.
2. Materials and methods Field trials were conducted at Foxhome and Crookston, Minnesota in 1999, and Breckenridge and Crookston, Minnesota, in 2000. The research sites were about 150 km apart. All experiments were arranged in a randomized complete block design with four replications. Treatments were considered fixed effects and replicates random effects for the analysis of variance. The least significant difference (LSD) procedure was used to compare treatments when the F-test for treatments was significant (p ¼ 0:05). The data analysis was performed with the ANOVA procedure of the Agriculture Research Manager, version 6.0 software package (Gylling Data Management Inc., Brookings, South Dakota, 1999). Field plots consisted of six 11-meter rows spaced 56 cm apart. Plots were planted with a commercial planter on 26 April in 1999 at Foxhome and Crookston, and 24, 26 April in 2000 at Crookston and Breckenridge, respectively. ‘HM Valley’, a sugar beet cultivar susceptible to Cercospora leaf spot with a Kleinwanzlebener Saatzucht (KWS) scale score of 5.16 (see below) (Steen, 1999), was planted at all sites. Terbufos (Counter 15G) was applied at 3.7 kg a.i/ha modified in-furrow at planting time to control sugar beet root maggot (Tetanops myopaeformis von Ro¨der; Diptera: Otitidae).
Plots were thinned manually at the six-leaf stage to 86,450 plants ha1. Weeds were controlled with recommended herbicides (Khan, 1999), cultivation, and hand weeding. Fungicide spray treatments were applied with a fournozzle boom sprayer calibrated to deliver 690 k Pa pressure at 187 l ha1 of solution to the middle fourrows of plots. The fungicides applied, either alone, in alternation, or in mixtures were mancozeb (ethylenebisdithiocarbamate, penncozeb 75 DF, Cerexagri, Section 3 Label—see below) at 1.65 kg a.i/ha; thiophanate methyl (benzimidazole, Topsin M 70 WSB, Cerexagri, Section 3 Label) at 0.39 kg a.i/ha; fentin hydroxide (triphenyltin hydroxide, Super Tin 80 WP, Griffin LLC, Section 3 Label) at 0.28 kg a.i/ha; azoxystrobin (strobilurin, Quadris 2.08 SC, Syngenta, Section 3 Label) at 0.17 kg a.i/ha; tetraconazole (triazole, Eminent 125 SL, Sipcam Agro USA Inc., Section 18 Emergency Exemption—see below—in 1999 and 2000) at 0.11 kg a.i/ha; propiconazole+trifloxystrobin mixture (triazole+strobilurin, Stratego 2.1 EC, Bayer CropScience, Experimental Compound) at 0.18 kg a.i/ha ; pyraclostrobin (strobilurin, BAS 500 2.09 EC, BASF, Experimental Compound that received a Section 3 label in 2002) at 0.17 kg a.i/ha; fenbuconazole (triazole, RH-7592 75 WP, Dow Agro Sciences, Experimental Compound) at 0.14 kg a.i/ha+Latron CS-7 (adjuvant, Rohm and Haas) at 0.12% v/v; and non-treated controls. Fungicides with Section 3 label indicate they were approved by the Environmental Protection Agency (EPA) of the United States for use on sugar beet. Section 18 exemption by the EPA indicates that the compound was unregistered, but the EPA authorized its use under specific conditions. Foxhome, 1999 Fungicides were applied 19 July, 2, 17, 30 August, and 10 September for 14-day treatment interval, and 19 July, 9, and 30 August for 21-day treatment interval. Crookston, 1999 Fungicides were applied 16, 30 July, 19, 27 August, and 10 September for 14-day treatment interval, and 16 July, 6 and 27 August for 21-day treatment interval. Breckenridge, 2000 Fungicides were applied 25 July, 8, 22, August, and 7 September for 14-day treatment interval, and 25 July, 15 August, and 7 September for 21-day treatment interval. Crookston, 2000 Fungicides were applied 26 July, 9, 22 August, and 7 September for 14-day treatment interval, and 26 July, 16 August, and 7 September for 21-day treatment interval. Treatments were applied as close as possible to the 14or 21-day interval as field and weather conditions permitted. There were five applications of treatments at 14-day interval in 1999, and four applications of
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treatments at 14-day interval in 2000. There were three applications of treatments at 21-day interval in both 1999 and 2000. The application number for each treatment at each site is indicated in the tables for the respective sites. Cercospora leaf spot severity was rated on the KWS scale of 1–9. A rating of one indicated there was no disease, a rating of three indicated that all outer leaves displayed typical symptoms and was the early stages of economic loss level, and a rating of nine indicated that the plants had only new leaf growth, all earlier leaves being dead, and severe economic loss. Cercospora leaf spot severity was assessed throughout the season. However, the rating done one day prior to harvest was reported since recoverable sucrose is inversely proportional to Cercospora leaf spot disease severity (Smith and Ruppel, 1973). Plots were defoliated mechanically and harvested using a mechanical harvester on 21 and 30 September, 1999 at Foxhome and Crookston, respectively, and 26, and 29 September, 2000 at Breckenridge and Crookston, respectively. The middle two-rows of each plot were harvested and weighed for root yield. Ten random roots from each plot, not including roots on the ends of the plot, were analyzed for quality at the American Crystal Sugar Company Quality Tare Laboratory, East Grand Forks, Minnesota. Characters evaluated were root yield, recoverable sucrose per hectare, and sucrose concentration.
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3. Results Foxhome, 1999 Cercospora leaf spot progressed rapidly after it was first detected on 13 July. At harvest, the non-treated control had only re-growth canopy and a KWS Cercospora leaf spot severity rating of 8.9 which was significantly higher than the fungicide treatments (Table 1). Fungicide treatments resulted in higher, although not always significant, root yield per hectare than the nontreated control. All fungicide treatments, except azoxystrobin applied alone, resulted in significantly higher recoverable sucrose per hectare than the non-treated control. All fungicide treatments resulted in significantly higher sucrose concentration than the non-treated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root yield, and recoverable sucrose compared to fenbuconazole with an adjuvant. Pyraclostrobin provided better Cercospora leaf spot control, and higher root and sucrose yield than azoxystrobin. Tetraconazole applied at 14-day interval resulted in better Cercospora leaf spot control, and higher root and sucrose yield than when applied at 21day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest recoverable sucrose yield. Of the labeled compounds used in alternation, tetraconazole followed by fentin hydroxide provided the most effective leaf spot
Table 1 Cercospora leaf spot control at Foxhome in 1999 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield
Sucrose yield
Sucrose concentration
(Mg ha1)
(kg ha1)
(%)
Tetraconazole / pyraclostrobin Tetraconazole (Propiconazole+trifloxystrobin)/ fentin hydroxide *Tetraconazole Tetraconazole/ fentin hydroxide Pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide Fentin hydroxide/ tetraconazole Fenbuconazole+Latron CS-7
1,3,5 2,4 1–5 1,3,5 2,4 1–3 1,3,5 2,4 1–5 1,4 2 3,5 1,3,5 2,4 1–5
3.3 d
61 ab
8519 a
15.5 a
3.5 cd 4.3 bcd
63 a 59 ab
8268 a 7986 ab
15.2 a 15.2 a
5.1 bc 3.9 cd
61 ab 56 abc
7811 ab 7567 abc
15.2 a 14.8 a
3.8 cd 6.0 b
59 ab 63 a
7521 abc 7122 abc
14.8 a 14.5 a
5.0 bcd
52 a–d
6927 a–d
14.7 a
5.9 b
46 bcd
6644 bcd
14.6 a
Fentin hydroxide Azoxystrobin Non-treated control
1–5 1–5
6.0 b 5.0 bcd 8.9 a
50 a–d 41 cd 39 d
6170 cd 5380 de 4462 e
14.5 a 14.7 a 13.1 b
1.9
15.8
1596
1.3
LSD (P ¼ 0:05)
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) c Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 21 September.
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Table 2 Cercospora leaf spot control at Crookston in 1999 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
Pyraclostrobin Tetraconazole / pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb / fentin hydroxide Fentin hydroxide Tetraconazole *Tetraconazole Fentin hydroxide / tetraconazole (Propiconazole+trifloxystrobin) / fentin hydroxide Fenbuconazole+Latron CS-7 Tetraconazole / fentin hydroxide Azoxystrobin Non-treated control
1–5 1,3,5 2,4 1,4 2 3,5 1–5 1–5 1–3 1,3,5 2,4 1,3,5 2,4 1–5 1,3,5 2,4 1–5
LSD (P ¼ 0:05)
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
2.8 e 2.6 e
64 abc 66 a
10775 a 10744 a
17.9 a 17.4 ab
3.5 cd
65 ab
10720 a
17.6 ab
4.4 3.0 3.5 3.6
63 64 63 62
10427 10333 10306 10162
17.7 17.5 17.5 17.5
b de cd cd
a–d abc a–d bcd
ab ab ab ab
ab ab ab ab
3.1 de
62 bcd
10157 ab
17.5 ab
3.9 bc 3.5 cd
61 d 62 bcd
9925 b 9729 b
17.6 ab 17.0 b
3.6 cd 7.5 a
61 d 50 e
9720 b 7418 c
17.3 ab 16.3 c
0.7
3.3
742
0.7
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) c Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 30 September.
control and highest sucrose yield. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone but better disease control and higher root and sucrose yields than fentin hydroxide applied alone. Crookston, 1999 Cercospora leaf spot damage was high; the nontreated control had a Cercospora leaf spot severity rating of 7.5 which was significantly higher than all fungicide treatments (Table 2). All fungicide treatments resulted in significantly higher root yield per hectare, recoverable sucrose per hectare, and sucrose concentration than the non-treated control. Tetraconazole resulted in better Cercospora leaf spot control, root yield, and recoverable sucrose compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, and higher root and sucrose yield than azoxystrobin. Tetraconazole provided better Cercospora leaf spot control, and higher root and sucrose yield when applied at 14-day interval compared to 21-day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest sucrose yield. Of the labeled compounds used in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest root and sucrose yield.
Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control but lower sucrose yield and concentration than fentin hydroxide applied alone. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone, and better disease control but lower root and sucrose yields than fentin hydroxide applied alone. Breckenridge, 2000 Cercospora leaf spot was first detected on 14 July. Disease pressure, moderate in late July and August, became high in September with the non-treated control having a KWS rating of 7.9 at harvest which was significantly higher than the fungicide treatments (Table 3). Except for azoxystrobin, fungicide treatments resulted in significantly higher root yields than the nontreated control. All treatments, except azoxystrobin, and fentin hydroxide applied alone, and fenbuconazole applied with an adjuvant, resulted in significantly higher recoverable sucrose per hectare than the non-treated control. The use of fungicides did not significantly increase sugar concentration compared to the nontreated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root and sucrose yield compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration than azoxystrobin.
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Table 3 Cercospora leaf spot control at Breckenridge in 2000 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
Tetraconazole Pyraclostrobin *Tetraconazole Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide Fentin hydroxide / tetraconazole (Propiconazole+trifloxystrobin) / fentin hydroxide Tetraconazole/ fentin hydroxide Tetraconazole / pyraclostrobin Fenbuconazole+Latron CS-7 Azoxystrobin 2.08 SC
1–4 1–4 1–3 1,4 2 3 1,3 2,4 1,3 2,4 1,3 2,4 1,3 2,4 1–4 1–4
Fentin hydroxide 80 WP Non-treated control
1–4
LSD (P ¼ 0:05)
1.6 2.3 2.2 1.8
f def def ef
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
70 66 68 63
11538 11122 11049 10898
18.2 18.7 18.0 18.9
a abc ab b–e
a ab abc abc
ab ab ab ab
2.9 cde
63 b–e
10850 abc
19.0 a
2.7 c–f
62 cde
10813 abc
19.1 a
3.4 c
65 a–d
10713 abc
18.2 ab
2.4 c–f
64 bcd
10678 abc
18.3 ab
3.3 cd 4.5 b
64 bcd 65 a–d
10385 a–d 10162 bcd
18.1 ab 17.6 b
5.5 b 7.9 a
61 de 58 e
9925 cd 9283 d
18.1 ab 17.9 ab
1.1
5
1182
1.4
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 22 September. c
Tetraconazole provided better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration when applied at 14-day interval compared to 21-day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest sucrose yield. Of the labeled compounds used in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest sucrose yield. Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control, higher root and sucrose yield, and higher sucrose concentration than fentin hydroxide applied alone. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone, but better disease control and higher root and sucrose yields than fentin hydroxide applied alone. Crookston, 2000 Cercospora leaf spot damage was high with the nontreated control having a Cercospora leaf spot severity rating of 7.5 which was significantly higher than all fungicide treatments (Table 4). All fungicide treatments resulted in significantly higher root yields, recoverable sucrose per hectare, and sucrose concentration than the
non-treated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root and sucrose yield compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration than azoxystrobin. Tetraconazole applied at 14-day interval provided better Cercospora leaf spot control, but lower sucrose yield and concentration compared to when applied at 21-day interval. Comparing all fungicide treatments in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest sucrose yield. Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control and higher sucrose yield and concentration than fentin hydroxide applied alone. Tetraconazole applied alone resulted in better disease control and higher root and sucrose yield than tetraconazole in alternation with fentin hydroxide. Tetraconazole followed by fentin hydroxide in alternation resulted in better disease control, and higher root and sucrose yield than fentin hydroxide applied alone. Fentin hydroxide followed by tetraconazole in alternation resulted in better disease control but lower sucrose yield than fentin hydroxide applied alone.
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Table 4 Cercospora leaf spot control at Crookston in 2000 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
*Tetraconazole Pyraclostrobin Tetraconazole Tetraconazole / pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide (Propiconazole+trifloxystrobin) / fentin hydroxide Fenbuconazole+Latron CS-7 Azoxystrobin Tetraconazole / fentin hydroxide Fentin hydroxide Fentin hydroxide / tetraconazole Non-treated control LSD (P ¼ 0:05)
1–3 1–5 1–5 1,3,5 2,4 1,4 2 3,5 1,3,5 2,4 1–5 1–5 1,3,5 2,4 1–5 1,3,5 2,4
2.7 1.8 1.8 2.2
d–g g g efg
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
63 62 63 62
11063 10586 10437 10425
18.6 18.3 17.8 17.9
a abc a abc
a ab abc abc
a ab ab ab
2.8 def
61 a–d
10144 a–d
17.9 ab
2.0 fg
59 b–e
10053 b–e
18.2 ab
3.0 cde 3.8 c 2.8 def
59 b–e 59 b–e 57 de
9971 b–e 9673 c–f 9578 def
18.1 ab 17.8 ab 18.0 ab
4.8 b 3.2 cd
56 e 56 e
9263 ef 9055 f
17.7 ab 17.5 b
7.5 a
43 f
6690 g
16.7 c
0.9
3.9
843
0.8
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 22 September. c
4. Discussion Cercospora leaf spot severity based on leaf spot ratings at harvest was high at all research sites in both 1999 and 2000, but was generally higher at Foxhome and Breckenridge than at Crookston. Cercospora leaf spot is more severe in the southern Red River Valley (Foxhome and Breckenridge) resulting in more fungicide use compared to the northern Red River Valley (Crookston). Higher Cercospora leaf spot severity in Wilkins County—where the Foxhome and Breckenridge research sites were located—resulted in an average of 4.58 and 4.25 fungicide applications by growers, in 1999 and 2000, respectively, compared to Polk County— where the Crookston research site was located—with an average of 3.19 and 2.65 fungicide applications by growers, in 1999 and 2000, respectively (Dexter and Luecke, 2000; Dexter and Luecke, 2001). Compared to the non-treated control, all fungicide treatments, except azoxystrobin applied alone at Foxhome, and azoxystrobin, and fentin hydroxide, applied alone, and fenbuconazole applied with an adjuvant at Breckenridge consistently provided significantly better Cercospora leaf spot control, and significantly higher recoverable sucrose. Fentin hydroxide, an organic tin compound used primarily as a protectant, when used alone, provided less effective Cercospora leaf spot control than the other fungicide treatments, especially at Foxhome and Breckenridge. Growers in Minnesota
and North Dakota started to rely on fentin hydroxide for Cercospora control in 1982 (Dexter et al., 1986) after C. beticola had developed resistance to the benzimidazole class of fungicides in 1981 (Bugbee, 1982). Growers reported Cercospora leaf spot control failures in 1994, and later that year, fentin hydroxide tolerant C. beticola strains were discovered in southern Minnesota (Bugbee, 1995). In 1998, fentin hydroxide, mancozeb, thiophanate methyl, azoxystrobin, copper, and mixtures of fentin hydroxide with mancozeb or thiophanate methyl provided inadequate Cercospora leaf spot control. Despite the use of 3.74 fungicide applications, of which fentin hydroxide comprised 2.61 applications, averaged over all counties in Minnesota and North Dakota (Dexter and Luecke, 1999), growers still lost $75 M in revenue from reduced tonnage and quality (Cattanach, 2000). In 1998, 83% and 99% of Cercospora leaf spot lesions tested from the Minn-Dak factory district, which include the Foxhome and Breckenridge research sites, were tolerant to 1 ppm and 0.2 ppm of fentin hydroxide, respectively. In the Crookston factory district, that includes the Crookston research site, 50% and 68% of Cercospora leaf spot lesions tested were tolerant to 1 ppm and 0.2 ppm of fentin hydroxide, respectively (Weiland and Smith, 1999). The continued presence of a high population of C. beticola strains tolerant to fentin hydroxide in 1999 and 2000, more so in the Minn-Dak factory district (Weiland, 2000, 2001), probably resulted in the inconsistent performance of fentin hydroxide
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applied alone. Azoxystrobin and pyraclostrobin, strobilurins, inhibit fungal respiration by blocking electron transport at the quinol-oxidizing site (Sauter et al., 1995). Pyraclostrobin, when used alone, consistently provided more effective Cercospora leaf spot control that resulted in higher yields, and higher recoverable sucrose than azoxystrobin at all locations. Tetraconazole and fenbuconazole, triazoles, inhibit the oxidative sterol 14a-demethylation in the ergosterol biosynthesis pathway of many fungi (Siegel, 1981) and thus control pathogens from all major fungal groups, with the exception of Stramenopila (Scheinpflug, 1988; Sisler, 1988). Tetraconazole, when used alone, consistently provided more effective Cercospora leaf spot control that resulted in higher yields, and higher recoverable sucrose than fenbuconazole used with an adjuvant at all locations. Tetraconazole in alternation with pyraclostrobin, consistently provided good Cercospora leaf spot control that resulted in high yields and high recoverable sucrose probably because both of these fungicides, when used alone, effectively controlled Cercospora leaf spot. The older labeled fungicides, including those that belong to chemical classes with known resistance problems, in alternation with tetraconazole, or propiconazole plus trifloxystrobin, provided significantly better Cercospora leaf spot control and always resulted in significantly higher recoverable sucrose compared to the non-treated control. The consistent effective disease control provided by tetraconazole, pyraclostrobin, and the mixture of propiconazole with trifloxystrobin was probably because of their unique modes of action, coupled with limited exposure of the pathogen to these fungicides. Field isolates of C. beticola have developed resistance to the benzimidazole class (Georgopoulos and Dovas, 1973; Ruppel and Scott, 1974; Bugbee, 1982; Weiland and Halloin, 2001), increased tolerance to the organotin class (Bugbee, 1995), and reduced sensitivity to the triazole class (Karaoglanidis et al., 2000) of fungicides. There is no report of C. beticola resistance to the strobilurin class of fungicide, first used for Cercospora control in 1998 (Dexter and Luecke, 1999). The strobilurins, however, because of their site-specific activity, are prone to fungicide resistance problems, and other fungal pathogens have rapidly developed strobilurin-resistant isolates (Chin et al., 2001; Vincelli and Dixon, 2002). In this research, strobilurin, organotin, and triazole class of fungicides were used as stand alone treatments throughout the season to determine their efficacy against C. beticola. However, to reduce the risk of selecting fungicide resistant strains of C. beticola or reducing their prevalence in the population, fungicides from the same class of chemistry should not be used back-to-back or as a stand alone throughout the season. This research suggests that the newer classes of fungicides, such as the strobilurins and triazoles, which
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are susceptible to fungicide resistance, when registered or available through emergency exemptions, be used in alternation with the older registered protectant fungicides to provide effective Cercospora leaf spot control and maintain high yield of recoverable sucrose whilst reducing selection pressure for the development of fungicide resistant C. beticola strains. Sugar beet growers in Greece use a successful strategy of always mixing a triazole fungicide with a protectant fungicide to control Cercospora leaf spot and manage fungicide resistance (Karaoglandis et al., 2000). Research is needed to evaluate tank mixes of fungicides with diverse modes of action on control and resistance management of C. beticola populations in the Red River Valley sugar beet production region.
References Bugbee, W.M., 1982. Sugarbeet disease research—1981. 1981 Sugarbeet Res. Ext. Rep. 12, 155. Bugbee, W.M., 1995. Cercospora beticola tolerant to triphenyltin hydroxide. J. Sugar Beet Res. 32, 167–174. Cattanach, A., 2000. Managing Cercospora leafspot tolerance or resistance to fungicide. American Crystal Sugar Company Agnotes—Issue #364, 1 p. Chin, K.M., Wirz, M., Laird, D., 2001. Sensitivity of Mycosphaerella fijiensis from banana to trifloxystrobin. Plant Dis 85, 1264–1270. Dexter, A.G., Cattanach, A.W., Luecke, J.L., 1986. Survey of fungicide use in sugarbeet—1985. 1985 Sugarbeet Res. Ext. Rep. 16, 131–132. Dexter, A.G., Luecke, J.L., 1999. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—1998. 1998 Sugarbeet Res. Ext. Rep. 29, 243–245. Dexter, A.G., Luecke, J.L., 2000. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—1999. 1999 Sugarbeet Res. Ext. Rep. 30, 229–233. Dexter, A.G., Luecke, J.L., 2001. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—2000. 2000 Sugarbeet Res. Ext. Rep. 31, 239–242. Georgopoulos, S.G., Dovas, C., 1973. Occurrence of Cercospora beticola strains resistant to benzimidazole fungicides in northern Greece. Plant Dis 62, 321–324. Karaoglandis, G.S., Ioannidis, P.M., Thanassoulopoulos, C.C., 2000. Reduced sensitivity of Cercospora beticola isolates to steroldemythylation-inhibiting fungicides. Plant Pathol 49, 567–572. Kerr, E.D., Weiss, A., 1990. Fungicide efficacy and yield responses to fungicide treatments based on predictions of Cercospora leaf spot of sugar beet. J. Sugar Beet Res. 27, 58–71. Khan, M. 1999 (Ed.). 1999 Sugarbeet Production Guide. North Dakota State University and University of Minnesota Extension Services, pp. 22–47. Lamey, H.A., Cattanach, A.W., Bugbee, W.M., 1987. Cercospora leaf spot of sugarbeet. North Dakota State Uni. Ext. Cir. PP-764 Revised, 4 pp. Lamey, H.A., Cattanach, A.W., Bugbee, W.M., Windels, C.E., 1996. Cercospora leaf spot of sugarbeet. North Dakota State Univ. Ext. Circ. PP-764 Revised, 4 pp. Miller, S.S., Rekoske, M., Quinn, A., 1994. Genetic resistance, fungicide protection and variety approval politics for controlling yield losses from Cercospora leaf spot infection. J. Sugar Beet Res. 31, 7–12.
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Ruppel, E.G., 1986. Cercospora leaf spot. In: Whitney, E.D., Duffus, J.E. (Eds.), Compendium of Beet Diseases and Insects. APS Press, St. Paul, MN, pp. 8–9. Ruppel, E.G., Scott, P.R., 1974. Strains of Cercospora beticola resistant to benomyl in the USA. Plant Dis. Rep. 58, 434–436. Sauter, H., Ammermann, E., Benoit, R., Brand, S., Gold, R.E., Grammenos, W., Kohle, H., Lorenz, G., Muller, B., Rohl, F., Schirmer, U., Speakman, J.B., Wenderoth, B., Wingert, H., 1995. Mitochondrial respiration as a target for antifungals: lessons from research on strobilurins. In: Dixon, G.K., Copping, L.G., Hollomon, D.W. (Eds.), Antifungal Agents: Discovery and Mode of Action. BIOS scientific Publishers, Oxford, UK, pp. 173–191. Scheinpflug, H., 1988. History of DMI fungicides and monitoring for resistance. In: Delp, C.J. (Ed.), Fungicide resistance in North America. APS Press, St. Paul, Minnesota, pp. 77–78. Shane, W.W., Teng, P.S., 1992. Impact of Cercospora leaf spot on root weight, sugar yield and purity. Plant Dis 76, 812–820. Siegel, M.R., 1981. Sterol-inhibiting fungicides: effects on sterol biosynthesis and sites of action. Plant Dis 65, 986–989. Sisler, H.D., 1988. Fungicidal action and fungal resistance mechanisms. In: Delp, C.J. (Ed.), Fungicide resistance in North America. APS Press, St. Paul, Minnesota, pp. 6–8. Smith, G.A., Campbell, L.G., 1996. Association between resistance to Cercospora and yield in commercial sugarbeet. Plant Breed 115, 28–32.
Smith, G.A., Gaskill, J.O., 1970. Inheritance of resistance to Cercospora leaf spot in sugarbeet. J. Am. Soc. Sugar Beet Technol. 16, 172–180. Smith, G.A., Ruppel, E.G., 1973. Association of Cercospora leaf spot, gross sugar, percentage sucrose and root weight in sugarbeet. Can. J. Plant Sci. 53, 695–696. Steen, R.A., 1999. Results of American Crystal’s 1998 official coded variety trials. 1998 Sugarbeet Res. Ext. Rep. 29, 301–346. Vincelli, P., Dixon, E., 2002. Resistance to QoI (strobilurin-like) fungicides in isolates of Pyricularia grisea from perennial ryegrass. Plant Dis 86, 235–240. Weiland, J.J., 2000. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 1999. 1999 Sugarbeet Res. Ext. Rep. 30, 236–239. Weiland, J.J., 2001. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 2000. 2000 Sugarbeet Res. Ext. Rep. 31, 266–271. Weiland, J.J., Halloin, J.M., 2001. Benzimidazole resistance in Cercospora beticola sampled from sugarbeet fields in Michigan, USA. Can. J. Plant Path. 23, 78–82. Weiland, J.J., Smith, G.A., 1999. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 1998. 1998 Sugarbeet Res. Ext. Rep. 29, 289–291.
Crop Protection 24 (2005) 79–86 www.elsevier.com/locate/cropro
Evaluating fungicides for controlling Cercospora leaf spot on sugar beet Mohamed F.R. Khana,, Larry J. Smithb a
227 Walster Hall, North Dakota State University & University of Minnesota, Fargo, ND 58105-5758, USA b Northwest Research and Outreach Center, University of Minnesota, Crookston, MN 56716, USA Received 6 April 2004; received in revised form 24 May 2004; accepted 30 June 2004
Abstract Cercospora leaf spot, caused by the fungus, Cercospora beticola, continues to be a devastating foliar disease of sugar beet (Beta vulgaris), in Minnesota and North Dakota. Commercial sugar beet varieties grown in Minnesota and North Dakota generally have only moderate resistance and require fungicide applications to obtain adequate levels of protection against C. beticola. Trials were conducted in 1999 at Foxhome and Crookston, Minnesota and in 2000 at Breckenridge and Crookston, Minnesota to determine the efficacy of labeled and experimental fungicides for controlling Cercospora leaf spot. Natural inocula were relied on for infection, and disease pressure was high at all sites in both years. Except for azoxystrobin applied alone at Foxhome, and azoxystrobin, and fentin hydroxide, applied alone, and fenbuconazole applied with an adjuvant at Breckenridge, the fungicide treatments provided better Cercospora leaf spot control, and resulted in higher recoverable sucrose yields than non-treated controls. Tetraconazole and pyraclostrobin, when applied alone, consistently provided effective Cercospora leaf spot control and resulted in high sucrose yield. r 2004 Elsevier Ltd. All rights reserved. Keywords: Cercospora beticola; Beta vulgaris; Fungicides; Strobilurin; Pyraclostrobin
1. Introduction Cercospora leaf spot, caused by the fungus Cercospora beticola Sacc., occurs in all sugar beet (Beta vulgaris L.) production areas in the United States (Ruppel, 1986; Kerr and Weiss, 1990), and is the most destructive foliar disease of sugar beet in Minnesota and North Dakota. The disease reduces root and extractable sucrose yields, and increases impurity concentrations, resulting in higher processing losses (Smith and Ruppel 1973; Lamey et al., 1987; Shane and Teng, 1992; Lamey et al., 1996). Losses in recoverable sucrose as high as 30% are common under heavy disease conditions and revenue losses as high as 43% have been reported (Lamey et al., 1987; Shane and Teng, 1992; Lamey et al., Corresponding author. Tel: +1-701-231-8596; fax: +1-701-2316186. E-mail address: [email protected] (M.F.R. Khan).
0261-2194/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2004.06.010
1996). Roots of diseased plants do not store as well as roots from healthy plants in storage piles that are processed in a 7–9 month period in North Dakota and Minnesota (Smith and Ruppel, 1973). Cercospora leaf spot is managed by fungicide applications, reducing inoculum by crop rotation and tillage, and by planting disease tolerant varieties (Miller et al., 1994). Four to five genes are responsible for Cercospora leaf spot resistance (Smith and Gaskill, 1970). Combining high levels of Cercospora leaf spot resistance with high yield in sugar beet is difficult (Smith and Campbell, 1996). As a result, commercial varieties generally have only moderate levels of resistance and require fungicide applications to obtain adequate levels of protection against Cercospora leaf spot (Miller et al., 1994). In 1998, revenue losses by American Crystal growers in Minnesota and North Dakota were over $45 M from reduced tonnage and quality despite the use of $20 M in fungicide applications (Cattanach, 2000). The major
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fungicides used in 1998 were fentin hydroxide, mancozeb and thiophanate methyl (Dexter and Luecke, 1999). Most growers experienced inconsistent leaf spot control, probably because of ineffective fungicides as a result of a high population of benzimidazole resistant and fentin hydroxide tolerant strains of C. beticola (Bugbee, 1982; Bugbee, 1995; Weiland and Smith, 1999), or untimely applications. Cercospora leaf spot was most severe in the warmer southern Minnesota sugar beet growing district, resulting in some growers applying 11 fungicide applications compared to about 3–4 applications in most years. There was an urgent need to find new chemistry fungicides that will provide effective Cercospora leaf spot control and result in high extractable sucrose. The acreage of sugar beet grown in the United States is small relative to corn (Zea mays L.), wheat (Triticum aestivum L.), cotton (Gossypium hirsutum L.), and soybean (Glycine max (L.) Merrill). As such, very few fungicides are developed primarily for controlling diseases of sugar beet. The availability of triazoles and strobilurins used on other crops presented and opportunity to test these products for controlling Cercospora leaf spot of sugar beet. Data could then be used to obtain registration for use on sugar beet. The objective of this research was to evaluate the efficacy of labeled and experimental fungicides to control Cercospora leaf spot on sugar beet.
2. Materials and methods Field trials were conducted at Foxhome and Crookston, Minnesota in 1999, and Breckenridge and Crookston, Minnesota, in 2000. The research sites were about 150 km apart. All experiments were arranged in a randomized complete block design with four replications. Treatments were considered fixed effects and replicates random effects for the analysis of variance. The least significant difference (LSD) procedure was used to compare treatments when the F-test for treatments was significant (p ¼ 0:05). The data analysis was performed with the ANOVA procedure of the Agriculture Research Manager, version 6.0 software package (Gylling Data Management Inc., Brookings, South Dakota, 1999). Field plots consisted of six 11-meter rows spaced 56 cm apart. Plots were planted with a commercial planter on 26 April in 1999 at Foxhome and Crookston, and 24, 26 April in 2000 at Crookston and Breckenridge, respectively. ‘HM Valley’, a sugar beet cultivar susceptible to Cercospora leaf spot with a Kleinwanzlebener Saatzucht (KWS) scale score of 5.16 (see below) (Steen, 1999), was planted at all sites. Terbufos (Counter 15G) was applied at 3.7 kg a.i/ha modified in-furrow at planting time to control sugar beet root maggot (Tetanops myopaeformis von Ro¨der; Diptera: Otitidae).
Plots were thinned manually at the six-leaf stage to 86,450 plants ha1. Weeds were controlled with recommended herbicides (Khan, 1999), cultivation, and hand weeding. Fungicide spray treatments were applied with a fournozzle boom sprayer calibrated to deliver 690 k Pa pressure at 187 l ha1 of solution to the middle fourrows of plots. The fungicides applied, either alone, in alternation, or in mixtures were mancozeb (ethylenebisdithiocarbamate, penncozeb 75 DF, Cerexagri, Section 3 Label—see below) at 1.65 kg a.i/ha; thiophanate methyl (benzimidazole, Topsin M 70 WSB, Cerexagri, Section 3 Label) at 0.39 kg a.i/ha; fentin hydroxide (triphenyltin hydroxide, Super Tin 80 WP, Griffin LLC, Section 3 Label) at 0.28 kg a.i/ha; azoxystrobin (strobilurin, Quadris 2.08 SC, Syngenta, Section 3 Label) at 0.17 kg a.i/ha; tetraconazole (triazole, Eminent 125 SL, Sipcam Agro USA Inc., Section 18 Emergency Exemption—see below—in 1999 and 2000) at 0.11 kg a.i/ha; propiconazole+trifloxystrobin mixture (triazole+strobilurin, Stratego 2.1 EC, Bayer CropScience, Experimental Compound) at 0.18 kg a.i/ha ; pyraclostrobin (strobilurin, BAS 500 2.09 EC, BASF, Experimental Compound that received a Section 3 label in 2002) at 0.17 kg a.i/ha; fenbuconazole (triazole, RH-7592 75 WP, Dow Agro Sciences, Experimental Compound) at 0.14 kg a.i/ha+Latron CS-7 (adjuvant, Rohm and Haas) at 0.12% v/v; and non-treated controls. Fungicides with Section 3 label indicate they were approved by the Environmental Protection Agency (EPA) of the United States for use on sugar beet. Section 18 exemption by the EPA indicates that the compound was unregistered, but the EPA authorized its use under specific conditions. Foxhome, 1999 Fungicides were applied 19 July, 2, 17, 30 August, and 10 September for 14-day treatment interval, and 19 July, 9, and 30 August for 21-day treatment interval. Crookston, 1999 Fungicides were applied 16, 30 July, 19, 27 August, and 10 September for 14-day treatment interval, and 16 July, 6 and 27 August for 21-day treatment interval. Breckenridge, 2000 Fungicides were applied 25 July, 8, 22, August, and 7 September for 14-day treatment interval, and 25 July, 15 August, and 7 September for 21-day treatment interval. Crookston, 2000 Fungicides were applied 26 July, 9, 22 August, and 7 September for 14-day treatment interval, and 26 July, 16 August, and 7 September for 21-day treatment interval. Treatments were applied as close as possible to the 14or 21-day interval as field and weather conditions permitted. There were five applications of treatments at 14-day interval in 1999, and four applications of
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treatments at 14-day interval in 2000. There were three applications of treatments at 21-day interval in both 1999 and 2000. The application number for each treatment at each site is indicated in the tables for the respective sites. Cercospora leaf spot severity was rated on the KWS scale of 1–9. A rating of one indicated there was no disease, a rating of three indicated that all outer leaves displayed typical symptoms and was the early stages of economic loss level, and a rating of nine indicated that the plants had only new leaf growth, all earlier leaves being dead, and severe economic loss. Cercospora leaf spot severity was assessed throughout the season. However, the rating done one day prior to harvest was reported since recoverable sucrose is inversely proportional to Cercospora leaf spot disease severity (Smith and Ruppel, 1973). Plots were defoliated mechanically and harvested using a mechanical harvester on 21 and 30 September, 1999 at Foxhome and Crookston, respectively, and 26, and 29 September, 2000 at Breckenridge and Crookston, respectively. The middle two-rows of each plot were harvested and weighed for root yield. Ten random roots from each plot, not including roots on the ends of the plot, were analyzed for quality at the American Crystal Sugar Company Quality Tare Laboratory, East Grand Forks, Minnesota. Characters evaluated were root yield, recoverable sucrose per hectare, and sucrose concentration.
81
3. Results Foxhome, 1999 Cercospora leaf spot progressed rapidly after it was first detected on 13 July. At harvest, the non-treated control had only re-growth canopy and a KWS Cercospora leaf spot severity rating of 8.9 which was significantly higher than the fungicide treatments (Table 1). Fungicide treatments resulted in higher, although not always significant, root yield per hectare than the nontreated control. All fungicide treatments, except azoxystrobin applied alone, resulted in significantly higher recoverable sucrose per hectare than the non-treated control. All fungicide treatments resulted in significantly higher sucrose concentration than the non-treated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root yield, and recoverable sucrose compared to fenbuconazole with an adjuvant. Pyraclostrobin provided better Cercospora leaf spot control, and higher root and sucrose yield than azoxystrobin. Tetraconazole applied at 14-day interval resulted in better Cercospora leaf spot control, and higher root and sucrose yield than when applied at 21day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest recoverable sucrose yield. Of the labeled compounds used in alternation, tetraconazole followed by fentin hydroxide provided the most effective leaf spot
Table 1 Cercospora leaf spot control at Foxhome in 1999 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield
Sucrose yield
Sucrose concentration
(Mg ha1)
(kg ha1)
(%)
Tetraconazole / pyraclostrobin Tetraconazole (Propiconazole+trifloxystrobin)/ fentin hydroxide *Tetraconazole Tetraconazole/ fentin hydroxide Pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide Fentin hydroxide/ tetraconazole Fenbuconazole+Latron CS-7
1,3,5 2,4 1–5 1,3,5 2,4 1–3 1,3,5 2,4 1–5 1,4 2 3,5 1,3,5 2,4 1–5
3.3 d
61 ab
8519 a
15.5 a
3.5 cd 4.3 bcd
63 a 59 ab
8268 a 7986 ab
15.2 a 15.2 a
5.1 bc 3.9 cd
61 ab 56 abc
7811 ab 7567 abc
15.2 a 14.8 a
3.8 cd 6.0 b
59 ab 63 a
7521 abc 7122 abc
14.8 a 14.5 a
5.0 bcd
52 a–d
6927 a–d
14.7 a
5.9 b
46 bcd
6644 bcd
14.6 a
Fentin hydroxide Azoxystrobin Non-treated control
1–5 1–5
6.0 b 5.0 bcd 8.9 a
50 a–d 41 cd 39 d
6170 cd 5380 de 4462 e
14.5 a 14.7 a 13.1 b
1.9
15.8
1596
1.3
LSD (P ¼ 0:05)
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) c Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 21 September.
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Table 2 Cercospora leaf spot control at Crookston in 1999 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
Pyraclostrobin Tetraconazole / pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb / fentin hydroxide Fentin hydroxide Tetraconazole *Tetraconazole Fentin hydroxide / tetraconazole (Propiconazole+trifloxystrobin) / fentin hydroxide Fenbuconazole+Latron CS-7 Tetraconazole / fentin hydroxide Azoxystrobin Non-treated control
1–5 1,3,5 2,4 1,4 2 3,5 1–5 1–5 1–3 1,3,5 2,4 1,3,5 2,4 1–5 1,3,5 2,4 1–5
LSD (P ¼ 0:05)
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
2.8 e 2.6 e
64 abc 66 a
10775 a 10744 a
17.9 a 17.4 ab
3.5 cd
65 ab
10720 a
17.6 ab
4.4 3.0 3.5 3.6
63 64 63 62
10427 10333 10306 10162
17.7 17.5 17.5 17.5
b de cd cd
a–d abc a–d bcd
ab ab ab ab
ab ab ab ab
3.1 de
62 bcd
10157 ab
17.5 ab
3.9 bc 3.5 cd
61 d 62 bcd
9925 b 9729 b
17.6 ab 17.0 b
3.6 cd 7.5 a
61 d 50 e
9720 b 7418 c
17.3 ab 16.3 c
0.7
3.3
742
0.7
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) c Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 30 September.
control and highest sucrose yield. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone but better disease control and higher root and sucrose yields than fentin hydroxide applied alone. Crookston, 1999 Cercospora leaf spot damage was high; the nontreated control had a Cercospora leaf spot severity rating of 7.5 which was significantly higher than all fungicide treatments (Table 2). All fungicide treatments resulted in significantly higher root yield per hectare, recoverable sucrose per hectare, and sucrose concentration than the non-treated control. Tetraconazole resulted in better Cercospora leaf spot control, root yield, and recoverable sucrose compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, and higher root and sucrose yield than azoxystrobin. Tetraconazole provided better Cercospora leaf spot control, and higher root and sucrose yield when applied at 14-day interval compared to 21-day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest sucrose yield. Of the labeled compounds used in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest root and sucrose yield.
Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control but lower sucrose yield and concentration than fentin hydroxide applied alone. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone, and better disease control but lower root and sucrose yields than fentin hydroxide applied alone. Breckenridge, 2000 Cercospora leaf spot was first detected on 14 July. Disease pressure, moderate in late July and August, became high in September with the non-treated control having a KWS rating of 7.9 at harvest which was significantly higher than the fungicide treatments (Table 3). Except for azoxystrobin, fungicide treatments resulted in significantly higher root yields than the nontreated control. All treatments, except azoxystrobin, and fentin hydroxide applied alone, and fenbuconazole applied with an adjuvant, resulted in significantly higher recoverable sucrose per hectare than the non-treated control. The use of fungicides did not significantly increase sugar concentration compared to the nontreated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root and sucrose yield compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration than azoxystrobin.
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Table 3 Cercospora leaf spot control at Breckenridge in 2000 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
Tetraconazole Pyraclostrobin *Tetraconazole Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide Fentin hydroxide / tetraconazole (Propiconazole+trifloxystrobin) / fentin hydroxide Tetraconazole/ fentin hydroxide Tetraconazole / pyraclostrobin Fenbuconazole+Latron CS-7 Azoxystrobin 2.08 SC
1–4 1–4 1–3 1,4 2 3 1,3 2,4 1,3 2,4 1,3 2,4 1,3 2,4 1–4 1–4
Fentin hydroxide 80 WP Non-treated control
1–4
LSD (P ¼ 0:05)
1.6 2.3 2.2 1.8
f def def ef
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
70 66 68 63
11538 11122 11049 10898
18.2 18.7 18.0 18.9
a abc ab b–e
a ab abc abc
ab ab ab ab
2.9 cde
63 b–e
10850 abc
19.0 a
2.7 c–f
62 cde
10813 abc
19.1 a
3.4 c
65 a–d
10713 abc
18.2 ab
2.4 c–f
64 bcd
10678 abc
18.3 ab
3.3 cd 4.5 b
64 bcd 65 a–d
10385 a–d 10162 bcd
18.1 ab 17.6 b
5.5 b 7.9 a
61 de 58 e
9925 cd 9283 d
18.1 ab 17.9 ab
1.1
5
1182
1.4
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 22 September. c
Tetraconazole provided better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration when applied at 14-day interval compared to 21-day interval. Comparing all fungicide treatments in alternation, tetraconazole with pyraclostrobin provided the most effective disease control and resulted in the highest sucrose yield. Of the labeled compounds used in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest sucrose yield. Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control, higher root and sucrose yield, and higher sucrose concentration than fentin hydroxide applied alone. Tetraconazole in alternation with fentin hydroxide resulted in less effective disease control and lower root and sucrose yield than tetraconazole applied alone, but better disease control and higher root and sucrose yields than fentin hydroxide applied alone. Crookston, 2000 Cercospora leaf spot damage was high with the nontreated control having a Cercospora leaf spot severity rating of 7.5 which was significantly higher than all fungicide treatments (Table 4). All fungicide treatments resulted in significantly higher root yields, recoverable sucrose per hectare, and sucrose concentration than the
non-treated control. Tetraconazole resulted in significantly better Cercospora leaf spot control, root and sucrose yield compared to fenbuconazole with an adjuvant. Pyraclostrobin provided significantly better Cercospora leaf spot control, higher root and sucrose yield, and higher sucrose concentration than azoxystrobin. Tetraconazole applied at 14-day interval provided better Cercospora leaf spot control, but lower sucrose yield and concentration compared to when applied at 21-day interval. Comparing all fungicide treatments in alternation, the four chemistry treatment of tetraconazole, thiophanate methyl plus mancozeb, and fentin hydroxide resulted in the most effective leaf spot control, and the highest sucrose yield. Tetraconazole, or the mixture of propiconazole and trifloxystrobin followed by fentin hydroxide in alternation resulted in better leaf spot control and higher sucrose yield and concentration than fentin hydroxide applied alone. Tetraconazole applied alone resulted in better disease control and higher root and sucrose yield than tetraconazole in alternation with fentin hydroxide. Tetraconazole followed by fentin hydroxide in alternation resulted in better disease control, and higher root and sucrose yield than fentin hydroxide applied alone. Fentin hydroxide followed by tetraconazole in alternation resulted in better disease control but lower sucrose yield than fentin hydroxide applied alone.
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Table 4 Cercospora leaf spot control at Crookston in 2000 with labeled and experimental fungicides Treatmentsc
Application number
CLSd
Root yield 1
*Tetraconazole Pyraclostrobin Tetraconazole Tetraconazole / pyraclostrobin Tetraconazole / thiophanate methyl+mancozeb/ fentin hydroxide (Propiconazole+trifloxystrobin) / fentin hydroxide Fenbuconazole+Latron CS-7 Azoxystrobin Tetraconazole / fentin hydroxide Fentin hydroxide Fentin hydroxide / tetraconazole Non-treated control LSD (P ¼ 0:05)
1–3 1–5 1–5 1,3,5 2,4 1,4 2 3,5 1,3,5 2,4 1–5 1–5 1,3,5 2,4 1–5 1,3,5 2,4
2.7 1.8 1.8 2.2
d–g g g efg
Sucrose yield 1
Sucrose concentration
(Mg ha )
(kg ha )
(%)
63 62 63 62
11063 10586 10437 10425
18.6 18.3 17.8 17.9
a abc a abc
a ab abc abc
a ab ab ab
2.8 def
61 a–d
10144 a–d
17.9 ab
2.0 fg
59 b–e
10053 b–e
18.2 ab
3.0 cde 3.8 c 2.8 def
59 b–e 59 b–e 57 de
9971 b–e 9673 c–f 9578 def
18.1 ab 17.8 ab 18.0 ab
4.8 b 3.2 cd
56 e 56 e
9263 ef 9055 f
17.7 ab 17.5 b
7.5 a
43 f
6690 g
16.7 c
0.9
3.9
843
0.8
Means followed by same letter do not significantly differ (P=0.05, Student–Newman–Keuls) Treatments were applied at approximately 14-day interval except for treatment marked by an * where the interval was 21 days. d Cercospora leaf spot measured on KWS scale 1–9 (no leaf spot to most leaves dead with regrowth of new leaves) rated 22 September. c
4. Discussion Cercospora leaf spot severity based on leaf spot ratings at harvest was high at all research sites in both 1999 and 2000, but was generally higher at Foxhome and Breckenridge than at Crookston. Cercospora leaf spot is more severe in the southern Red River Valley (Foxhome and Breckenridge) resulting in more fungicide use compared to the northern Red River Valley (Crookston). Higher Cercospora leaf spot severity in Wilkins County—where the Foxhome and Breckenridge research sites were located—resulted in an average of 4.58 and 4.25 fungicide applications by growers, in 1999 and 2000, respectively, compared to Polk County— where the Crookston research site was located—with an average of 3.19 and 2.65 fungicide applications by growers, in 1999 and 2000, respectively (Dexter and Luecke, 2000; Dexter and Luecke, 2001). Compared to the non-treated control, all fungicide treatments, except azoxystrobin applied alone at Foxhome, and azoxystrobin, and fentin hydroxide, applied alone, and fenbuconazole applied with an adjuvant at Breckenridge consistently provided significantly better Cercospora leaf spot control, and significantly higher recoverable sucrose. Fentin hydroxide, an organic tin compound used primarily as a protectant, when used alone, provided less effective Cercospora leaf spot control than the other fungicide treatments, especially at Foxhome and Breckenridge. Growers in Minnesota
and North Dakota started to rely on fentin hydroxide for Cercospora control in 1982 (Dexter et al., 1986) after C. beticola had developed resistance to the benzimidazole class of fungicides in 1981 (Bugbee, 1982). Growers reported Cercospora leaf spot control failures in 1994, and later that year, fentin hydroxide tolerant C. beticola strains were discovered in southern Minnesota (Bugbee, 1995). In 1998, fentin hydroxide, mancozeb, thiophanate methyl, azoxystrobin, copper, and mixtures of fentin hydroxide with mancozeb or thiophanate methyl provided inadequate Cercospora leaf spot control. Despite the use of 3.74 fungicide applications, of which fentin hydroxide comprised 2.61 applications, averaged over all counties in Minnesota and North Dakota (Dexter and Luecke, 1999), growers still lost $75 M in revenue from reduced tonnage and quality (Cattanach, 2000). In 1998, 83% and 99% of Cercospora leaf spot lesions tested from the Minn-Dak factory district, which include the Foxhome and Breckenridge research sites, were tolerant to 1 ppm and 0.2 ppm of fentin hydroxide, respectively. In the Crookston factory district, that includes the Crookston research site, 50% and 68% of Cercospora leaf spot lesions tested were tolerant to 1 ppm and 0.2 ppm of fentin hydroxide, respectively (Weiland and Smith, 1999). The continued presence of a high population of C. beticola strains tolerant to fentin hydroxide in 1999 and 2000, more so in the Minn-Dak factory district (Weiland, 2000, 2001), probably resulted in the inconsistent performance of fentin hydroxide
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applied alone. Azoxystrobin and pyraclostrobin, strobilurins, inhibit fungal respiration by blocking electron transport at the quinol-oxidizing site (Sauter et al., 1995). Pyraclostrobin, when used alone, consistently provided more effective Cercospora leaf spot control that resulted in higher yields, and higher recoverable sucrose than azoxystrobin at all locations. Tetraconazole and fenbuconazole, triazoles, inhibit the oxidative sterol 14a-demethylation in the ergosterol biosynthesis pathway of many fungi (Siegel, 1981) and thus control pathogens from all major fungal groups, with the exception of Stramenopila (Scheinpflug, 1988; Sisler, 1988). Tetraconazole, when used alone, consistently provided more effective Cercospora leaf spot control that resulted in higher yields, and higher recoverable sucrose than fenbuconazole used with an adjuvant at all locations. Tetraconazole in alternation with pyraclostrobin, consistently provided good Cercospora leaf spot control that resulted in high yields and high recoverable sucrose probably because both of these fungicides, when used alone, effectively controlled Cercospora leaf spot. The older labeled fungicides, including those that belong to chemical classes with known resistance problems, in alternation with tetraconazole, or propiconazole plus trifloxystrobin, provided significantly better Cercospora leaf spot control and always resulted in significantly higher recoverable sucrose compared to the non-treated control. The consistent effective disease control provided by tetraconazole, pyraclostrobin, and the mixture of propiconazole with trifloxystrobin was probably because of their unique modes of action, coupled with limited exposure of the pathogen to these fungicides. Field isolates of C. beticola have developed resistance to the benzimidazole class (Georgopoulos and Dovas, 1973; Ruppel and Scott, 1974; Bugbee, 1982; Weiland and Halloin, 2001), increased tolerance to the organotin class (Bugbee, 1995), and reduced sensitivity to the triazole class (Karaoglanidis et al., 2000) of fungicides. There is no report of C. beticola resistance to the strobilurin class of fungicide, first used for Cercospora control in 1998 (Dexter and Luecke, 1999). The strobilurins, however, because of their site-specific activity, are prone to fungicide resistance problems, and other fungal pathogens have rapidly developed strobilurin-resistant isolates (Chin et al., 2001; Vincelli and Dixon, 2002). In this research, strobilurin, organotin, and triazole class of fungicides were used as stand alone treatments throughout the season to determine their efficacy against C. beticola. However, to reduce the risk of selecting fungicide resistant strains of C. beticola or reducing their prevalence in the population, fungicides from the same class of chemistry should not be used back-to-back or as a stand alone throughout the season. This research suggests that the newer classes of fungicides, such as the strobilurins and triazoles, which
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are susceptible to fungicide resistance, when registered or available through emergency exemptions, be used in alternation with the older registered protectant fungicides to provide effective Cercospora leaf spot control and maintain high yield of recoverable sucrose whilst reducing selection pressure for the development of fungicide resistant C. beticola strains. Sugar beet growers in Greece use a successful strategy of always mixing a triazole fungicide with a protectant fungicide to control Cercospora leaf spot and manage fungicide resistance (Karaoglandis et al., 2000). Research is needed to evaluate tank mixes of fungicides with diverse modes of action on control and resistance management of C. beticola populations in the Red River Valley sugar beet production region.
References Bugbee, W.M., 1982. Sugarbeet disease research—1981. 1981 Sugarbeet Res. Ext. Rep. 12, 155. Bugbee, W.M., 1995. Cercospora beticola tolerant to triphenyltin hydroxide. J. Sugar Beet Res. 32, 167–174. Cattanach, A., 2000. Managing Cercospora leafspot tolerance or resistance to fungicide. American Crystal Sugar Company Agnotes—Issue #364, 1 p. Chin, K.M., Wirz, M., Laird, D., 2001. Sensitivity of Mycosphaerella fijiensis from banana to trifloxystrobin. Plant Dis 85, 1264–1270. Dexter, A.G., Cattanach, A.W., Luecke, J.L., 1986. Survey of fungicide use in sugarbeet—1985. 1985 Sugarbeet Res. Ext. Rep. 16, 131–132. Dexter, A.G., Luecke, J.L., 1999. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—1998. 1998 Sugarbeet Res. Ext. Rep. 29, 243–245. Dexter, A.G., Luecke, J.L., 2000. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—1999. 1999 Sugarbeet Res. Ext. Rep. 30, 229–233. Dexter, A.G., Luecke, J.L., 2001. Survey of fungicide use in sugarbeet in eastern North Dakota and Minnesota—2000. 2000 Sugarbeet Res. Ext. Rep. 31, 239–242. Georgopoulos, S.G., Dovas, C., 1973. Occurrence of Cercospora beticola strains resistant to benzimidazole fungicides in northern Greece. Plant Dis 62, 321–324. Karaoglandis, G.S., Ioannidis, P.M., Thanassoulopoulos, C.C., 2000. Reduced sensitivity of Cercospora beticola isolates to steroldemythylation-inhibiting fungicides. Plant Pathol 49, 567–572. Kerr, E.D., Weiss, A., 1990. Fungicide efficacy and yield responses to fungicide treatments based on predictions of Cercospora leaf spot of sugar beet. J. Sugar Beet Res. 27, 58–71. Khan, M. 1999 (Ed.). 1999 Sugarbeet Production Guide. North Dakota State University and University of Minnesota Extension Services, pp. 22–47. Lamey, H.A., Cattanach, A.W., Bugbee, W.M., 1987. Cercospora leaf spot of sugarbeet. North Dakota State Uni. Ext. Cir. PP-764 Revised, 4 pp. Lamey, H.A., Cattanach, A.W., Bugbee, W.M., Windels, C.E., 1996. Cercospora leaf spot of sugarbeet. North Dakota State Univ. Ext. Circ. PP-764 Revised, 4 pp. Miller, S.S., Rekoske, M., Quinn, A., 1994. Genetic resistance, fungicide protection and variety approval politics for controlling yield losses from Cercospora leaf spot infection. J. Sugar Beet Res. 31, 7–12.
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Ruppel, E.G., 1986. Cercospora leaf spot. In: Whitney, E.D., Duffus, J.E. (Eds.), Compendium of Beet Diseases and Insects. APS Press, St. Paul, MN, pp. 8–9. Ruppel, E.G., Scott, P.R., 1974. Strains of Cercospora beticola resistant to benomyl in the USA. Plant Dis. Rep. 58, 434–436. Sauter, H., Ammermann, E., Benoit, R., Brand, S., Gold, R.E., Grammenos, W., Kohle, H., Lorenz, G., Muller, B., Rohl, F., Schirmer, U., Speakman, J.B., Wenderoth, B., Wingert, H., 1995. Mitochondrial respiration as a target for antifungals: lessons from research on strobilurins. In: Dixon, G.K., Copping, L.G., Hollomon, D.W. (Eds.), Antifungal Agents: Discovery and Mode of Action. BIOS scientific Publishers, Oxford, UK, pp. 173–191. Scheinpflug, H., 1988. History of DMI fungicides and monitoring for resistance. In: Delp, C.J. (Ed.), Fungicide resistance in North America. APS Press, St. Paul, Minnesota, pp. 77–78. Shane, W.W., Teng, P.S., 1992. Impact of Cercospora leaf spot on root weight, sugar yield and purity. Plant Dis 76, 812–820. Siegel, M.R., 1981. Sterol-inhibiting fungicides: effects on sterol biosynthesis and sites of action. Plant Dis 65, 986–989. Sisler, H.D., 1988. Fungicidal action and fungal resistance mechanisms. In: Delp, C.J. (Ed.), Fungicide resistance in North America. APS Press, St. Paul, Minnesota, pp. 6–8. Smith, G.A., Campbell, L.G., 1996. Association between resistance to Cercospora and yield in commercial sugarbeet. Plant Breed 115, 28–32.
Smith, G.A., Gaskill, J.O., 1970. Inheritance of resistance to Cercospora leaf spot in sugarbeet. J. Am. Soc. Sugar Beet Technol. 16, 172–180. Smith, G.A., Ruppel, E.G., 1973. Association of Cercospora leaf spot, gross sugar, percentage sucrose and root weight in sugarbeet. Can. J. Plant Sci. 53, 695–696. Steen, R.A., 1999. Results of American Crystal’s 1998 official coded variety trials. 1998 Sugarbeet Res. Ext. Rep. 29, 301–346. Vincelli, P., Dixon, E., 2002. Resistance to QoI (strobilurin-like) fungicides in isolates of Pyricularia grisea from perennial ryegrass. Plant Dis 86, 235–240. Weiland, J.J., 2000. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 1999. 1999 Sugarbeet Res. Ext. Rep. 30, 236–239. Weiland, J.J., 2001. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 2000. 2000 Sugarbeet Res. Ext. Rep. 31, 266–271. Weiland, J.J., Halloin, J.M., 2001. Benzimidazole resistance in Cercospora beticola sampled from sugarbeet fields in Michigan, USA. Can. J. Plant Path. 23, 78–82. Weiland, J.J., Smith, G.A., 1999. A survey for the prevalence and distribution of Cercospora beticola tolerant to triphenyltin hydroxide and mancozeb and resistant to thiophanate methyl in 1998. 1998 Sugarbeet Res. Ext. Rep. 29, 289–291.