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Issue No. 24, Article 4/November 9, 2012

Update on Frogeye Leaf Spot and Strobilurin Fungicide Resistance

As reported in previous articles in the Bulletin (see articles from 2010 and 2011), strains of the fungus that causes frogeye leaf spot of soybean, Cercospora sojina, that are resistant to strobilurin fungicides have been observed in the U.S. since 2010.

In 2012, soybean samples affected by frogeye leaf spot were collected in Illinois and tested for strobilurin fungicide–resistant strains of Cercospora sojina. In addition, colleagues from other states sent soybean samples to my laboratory for testing. In 2012, several new counties and parishes were found to have resistant strains (see Table 1).

Table 1. Counties and parishes in the U.S. in which strobilurin fungicide-resistant strains of Cercospora sojina have been detected (2010-2012).

State

County/parish

Alabama

Limestone

Arkansas

Chicot, Desha, Drew, Lawrence, Phillips, Poinsett, St. Francis

Illinois

Alexander, Champaign, Gallatin, Jackson, Pope, Warren

Kentucky

Ballard, Butler, Caldwell, Calloway, Carlisle, Daviess, Graves, Henderson, Hickman, Hopkins, Livingston, Marshall, McLean, Union, Webster

Louisiana

Avoyelles, Concordia, East Carroll, Ouachita, Pointe Coupee, Rapides, Tensas

Mississippi

Carroll, Coahoma

Missouri

Pemiscot, Ste. Genevieve

Tennessee

Dyer, Gibson, Lauderdale, Lawrence

Counties and parishes in bold were newly identified in 2012.

A soybean foliar fungicide trial was conducted this year at the University of Illinois Dixon Springs Agricultural Center in a field with a history of frogeye leaf spot and a strobilurin fungicide–resistant strain of Cercospora sojina. Several different foliar fungicides from different chemistry classes were applied at the R3 soybean stage and evaluated for their efficacy in managing frogeye leaf spot caused by the resistant strain. The results of this trial are shown in Table 2.

Table 2. Effect of foliar fungicides applied at the R3 growth stage on frogeye leaf spot severity and soybean yield at Dixon Springs, Illinois, in 2012.

FRAC Group Codea

Fungicide

Rate/A

Frogeye leaf spot severity (%)b

Yield (bu/A)

 

Non-treated

 

56

49

3 + 11

Quadris Top

8 fl oz

20

67

3 + 11

Quadris Xtra

6.8 fl oz

37

60

11

Quadris + COC

6 fl oz + 1% v/v

53

55

3 + 11

Quilt Xcel

10.5 fl oz

34

62

3

Domark

4 fl oz

38

61

3

Proline

3 fl oz

14

64

3

TopGuard

7 fl oz

22

63

3 + 11

Stratego YLD

4 fl oz

41

58

3

Folicur

4 fl oz

38

59

1

Topsin 4.5 L

20 fl oz

19

64

11

Headline

6 fl oz

60

49

7 + 11

Priaxor

4 fl oz

35

53

7 + 11

Priaxor

6 fl oz

36

57

11

Evito

2 fl oz

58

50

3 + 11

Evito T

4 fl oz

46

54

M

Echo 720

2.25 pts

15

61

1 + 3

Topsin + Folicur

16 fl oz + 4 fl oz

18

70

3 + 11

Quadris + Domark

5 fl oz + 4 fl oz

44

61

3 + 11

Quadris + Folicur

5 fl oz + 4 fl oz

39

60

 

 

LSD 0.05c

9

7

aFungicide Resistance Action Committee (FRAC) Group Code is a system of numbers and letters that organizes fungicide active ingredients by their modes of action. Fungi resistant to one fungicide in a FRAC Group will be resistant to all fungicides in that group. Strobilurin fungicides are in FRAC Group 11.
bFrogeye leaf spot severity ratings were collected on August 30, 2012, by evaluating the percentage of the leaf area affected in the upper third of the soybean canopy.
cLeast significant difference (LSD) with a 95% confidence level. This number can be used to determine which treatments were statistically different from one another.

Strobilurin fungicides (FRAC Group 11) were not effective in controlling frogeye leaf spot, but fungicides from different chemistry classes (FRAC groups) did reduce severity. The treatments with the statistically lowest levels of frogeye leaf spot severity were Quadris Top, Proline, TopGuard, Topsin 4.5 L, Echo 720, and Topsin 4.5 L + Folicur. Results of this trial indicate that fungicides from other chemistry classes can control frogeye leaf spot caused by strobilurin fungicide–resistant strains of Cercospora sojina.

What does all of this mean? As you plan for the 2013 growing season, it is important to consider the increasing detections of strobilurin fungicide–resistant strains of Cercospora sojina across several soybean-producing states and how this problem might affect your management decisions. Slowing the problem requires using the best fungicide resistance management practices, including these:

  • Use resistant varieties and cultural practices (crop rotation, tillage) to help manage frogeye leaf spot. If disease levels are kept low using alternative management practices, fungicides may not be needed.
  • Do not rely on a single class of fungicides to control plant diseases. Fungicides from different chemistry classes (FRAC groups) can be mixed to reduce the selection pressure being placed on the fungal population.
  • Apply foliar fungicides only when warranted to control plant diseases. Applying for other reasons may result in exerting a selection pressure on the fungal population. Every time a fungicide is applied, a selection pressure is exerted on the fungal population, and individual fungal isolates may be selected that are not as sensitive to the fungicide. Minimizing this selection pressure is key in prolonging the effectiveness and lifespan of a fungicide.

Funding for this research was provided by the United Soybean Board. Thank you to E. Sikora, T. Faske, D. Hershman, T. Price, B. Padgett, T. Allen, H. Young-Kelly, and M. Newman for help in identifying soybean samples that had strobilurin fungicide–resistant strains of Cercospora sojina in their states.--Carl A. Bradley

Author:
Carl A. Bradley

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