Fungicide resistance occurs when fungi evolve genetic resistance to fungicides due to natural selection. When a fungicide is used repeatedly, the resistant strains of fungi survive and become dominant in the population, leading to control failure. To prevent fungicide resistance, strategies are needed to avoid the survival and proliferation of resistant fungal strains.

1. Fungicide Resistance: Can it be prevented?
Lane P. Tredway
Associate Professor and Extension Specialist
Department of Plant Pathology
North Carolina State University

2. What is fungicide
resistance?
• an evolutionary process, ‘natural
selection’
• a pathogen is a population of
individuals
• some may be naturally resistant to a
given fungicide
• if that fungicide is used repeatedly, the
resistant strains become dominant in
the population
• control failure occurs

3. Fungicide Resistance: Survival of the Fittest
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4. Fungicide Resistance: Survival of the Fittest
• a small portion of the population x x
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has natural, genetic resistance to x x x x x x x x
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a given fungicide x
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5. Fungicide Resistance: Survival of the Fittest
• a small portion of the population x x x x x
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has natural, genetic resistance to x x x x x x x
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a given fungicide x
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• when that fungicide is applied, x x x x x x x x x x x
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the resistant strains increase in x
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6. Fungicide Resistance: Survival of the Fittest
• a small portion of the population x x
x
x
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x x
x x x x x x
has natural, genetic resistance to x x x x x x x x
x
x x x x x x x x
x
a given fungicide x
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x x x x x x x x x x x
• when that fungicide is applied, x x x x x x x x x x x
x x x x x x x x x x x x
the resistant strains increase in x
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the population x x x x x x x x x x x
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• when the resistant strains x x
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become dominant, control failure x
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occurs x x x x x x x x x
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7. Fungicide Resistance: Survival of the Fittest
• a small portion of the population
has natural, genetic resistance to
a given fungicide
• when that fungicide is applied,
the resistant strains increase in
the population
• when the resistant strains
become dominant, control failure
occurs

8. Turfgrass Fungicides: 15 Chemical Classes
• Aromatic hydrocarbon • Phenylamide
• Benzimidazole • Phenylpyrolle
• Carbamate • Phosphonate
• Carboxamide • Polyoxin
• Dicarboximide • Pyridine
• Dithiocarbamate • QoI
• DMI • QiI
• Nitrile

9. FRAC codes are displayed prominently on new fungicide labels

10. Cross Resistance and Multiple Resistance
Cross Resistance
Untreated a
• resistance is specific to a
Disarm ab chemical class
Insignia ab • if a pathogen becomes
resistant to azoxystrobin, it
Heritage a will also be resistant to
fluoxastrobin, pyraclostrobin,
Signature + Daconil b and trifloxystrobin (all QoI
0 7.5 15 22.5 30 fungicides)
Anthracnose Incidence

11. Sclerotinia homoeocarpa
Cross Resistance and
Multiple Resistance
Multiple Resistance
• pathogens may develop thiophanate-methyl propiconazole iprodione
resistance to more than
one chemical class Colletotrichum graminicola
• each resistance evolves
independently of the pyraclostrobin azoxystrobin
Unamended
others
0.003 ppm 0.031 ppm
thiophanate- pyraclostrobin azoxystrobin
methyl 0.8 ppm 8 ppm
10 ppm

12. Fungicide Resistance: Current Status

13. Fungicide Resistance: Current Status
Dollar Spot
• resistance to benzimidazoles
widespread
• reduced sensitivity to DMIs is
common, but to varying degrees
• dicarboxamide resistance is rare

14. Fungicide Resistance: Current Status
Dollar Spot
• resistance to benzimidazoles
widespread
• reduced sensitivity to DMIs is
common, but to varying degrees
• dicarboxamide resistance is rare
Anthracnose
• nearly all populations are resistant
to QoIs and benzimidazoles
• some are partially sensitive

15. Resistance Case Studies: Benzimidazoles
• single mutation in pathogen
• complete control failure
• develops quickly
• resistant strains persist
indefinitely

16. Resistance Case
Studies: DMIs
• multiple mutations in pathogen
• gradual reduction in population
sensitivity over time
• appears as a gradual reduction
in control efficacy or residual
• strains may not persist in
absence of DMI applications

17. DMI resistance develops as a gradual shift

18. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

19. Banner Maxx (2 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.011

20. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

21. Banner Maxx (1 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.011

22. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

23. Banner Maxx (0.5 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.011

24. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

25. Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

26. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.011

27. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

28. Banner Maxx (2 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.020

29. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

30. Banner Maxx (1 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.020

31. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

32. Banner Maxx (0.5 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.020

33. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

34. Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

35. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.020

36. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

37. Banner Maxx (2 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.033

38. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

39. Banner Maxx (1 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.033

40. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

41. Banner Maxx (0.5 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.033

42. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

43. Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

44. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated
DMI resistance is rate and interval dependent
EC50 = 0.033

45. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

46. Banner Maxx (2 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.054

47. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

48. Banner Maxx (1 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.054

49. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

50. Banner Maxx (0.5 fl oz)
DMI resistance is rate and interval dependent
EC50 = 0.054

51. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

52. Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

53. Banner Maxx (2 fl oz) Banner Maxx (1 fl oz)
Banner Maxx (0.5 fl oz) Untreated Control
DMI resistance is rate and interval dependent
EC50 = 0.054

54. Resistance Case
Studies: QoIs Unamende pyraclostrobin azoxystrobin
0.003 ppm 0.031 ppm
• two different mutations in
pathogen
thiophanate- pyraclostrobin azoxystrobin
methyl 0.8 ppm 8 ppm
• one mutation yields 50% Isolate GBGC2, Gatlinburg GC
benzimidazole insensitive, QoI insensitive
reduction in sensitivity
• other mutation yields complete
resistance Unamende pyraclostrobin azoxystrobin
0.003 ppm 0.031 ppm
• strains persist indefinitely
thiophanate- pyraclostrobin azoxystrobin
methyl 0.8 ppm 8 ppm
Isolate BR13A, Blowing Rock CC
benzimidazole insensitive, QoI mod. sens.

55. Some anthracnose populations are partially sensitive to
the QoI fungicides
Untreated
Compass (0.25 oz)
Insignia (0.5 oz)
Heritage (0.2 oz)
Spectro (4 oz)
26GT (3 fl oz)
3336 (4 fl oz)
Daconil Ultrex (3.2 oz)
Medallion (0.5 oz)
Endorse (4 oz)
Alude (6 fl oz)
Alude + Daconil
Signature (4 oz)
Signature + Daconil
0 6 12 18 24 30
Anthracnose Incidence (%)
Treatments applied 5/23, 6/6, 6/20, 7/11, 7/25; data collected 8/15

56. What can we learn from the medical field?
How can antibiotic resistance be
prevented?
• maintain healthy lifestyle to
minimize reliance on antibiotics
• sanitation
• diagnose illnesses acurately
• avoid repetitive use of same
antibiotics
• mix antibiotics when necessary

57. Preventing Fungicide
Resistance
• maintain healthy turf
• employ IPM strategies
• diagnose diseases accurately
• avoid repetitive use of high-risk
chemical classes
• tank-mix in high-risk situations

58. Is Fungicide Resistance Preventable?
• Rotate?
• Tank-mix?
• Use it until it quits working?

59. High Risk Medium Risk Low Risk
Turfgrass pathogens vary in their ability to develop fungicide
resistance.

60. High Risk Medium Risk Low Risk
Fungicides also vary in their susceptibility to fungicide resistance.

61. What determines the resistance risk?
• pathogens vary in their
ability to evolve
resistance
• fungicides vary in their
susceptibility to
resistance
• resistance risk is
determined by the target
pathogen and selected
fungicide

62. Calculating the Resistance Risk
-----Pathogen Resistance
Risk-----
• each pathogen is
---Fungicide Resistance Risk---
Low Risk (1) Med. Risk (2) High Risk (3)
assigned a risk value
Low Risk (1) 1 2 3
• each fungicide is
assigned a risk value
Med. Risk (2) 2 4 6
• multiply them together
High Risk (3) 3 6 9 to identify the
recommended strategy

63. Resistance Risk Determines the Best Strategy
Resistance
Risk Recommended Action
Rotate to different chemical class after EVERY application; Tank-
9
mix with low or moderate risk product for EVERY application
Rotate to different chemical class after 1-2 applications; Tank-
6
mixing with low or moderate risk product recommended
Rotate to different chemical class after 1-2 applications; Tank-
4
mixing not necessary
Rotate to different chemical class after 2-3 applications; Tank-
3
mixing not necessary
Rotate to different chemical class after 3-4 applications; Tank-
2
mixing not necessary
Rotating and tank-mixing not necessary, but recommended to avoid
1
potential side affects from continuous use of same chemical class

64. Fungicide Efficacy Rankings on turffiles.ncsu.edu

65. Disease Management Utility on turffiles.ncsu.edu

66. Example: Dollar Spot Program

67. Example: Dollar Spot Program
April 1 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)

68. Example: Dollar Spot Program
April 1 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)
April 15 - Bayleton + 26GT (1 oz + 4 fl oz)

69. Example: Dollar Spot Program
April 1 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)
April 15 - Bayleton + 26GT (1 oz + 4 fl oz)
May 1 - Banner Maxx + Daconil Ultrex (2 fl oz + 3.2 oz)

70. Example: Dollar Spot Program
April 1 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)
April 15 - Bayleton + 26GT (1 oz + 4 fl oz)
May 1 - Banner Maxx + Daconil Ultrex (2 fl oz + 3.2 oz)
May 15 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)

71. Example: Dollar Spot Program
April 1 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)
April 15 - Bayleton + 26GT (1 oz + 4 fl oz)
May 1 - Banner Maxx + Daconil Ultrex (2 fl oz + 3.2 oz)
May 15 - Emerald + Daconil Ultrex (0.13 + 3.2 oz)
June 1 - 26GT + Daconil Ultrex (4 fl oz + 3.2 oz)