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Herbicide Mode of Action

Tri River Pest Management WorkshopFebruary 15th, 2011

Herbicide Mode of Action

Tri River Pest Management WorkshopFebruary 15th, 2011

Dr. Scott Nissen

Colorado State University

Fort Collins, CO

Dr. Scott Nissen

Colorado State University

Fort Collins, CO

How many ways (modes of action) do herbicides kill weeds?How many ways (modes of action) do herbicides kill weeds?

A. More than 20

B. Between 10 and 20

C. Between 5 and 7

D. All herbicides work the same way

A. More than 20

B. Between 10 and 20

C. Between 5 and 7

D. All herbicides work the same way

I can tell a herbicide’s mode of action by looking at the container or the label.

I can tell a herbicide’s mode of action by looking at the container or the label.

A. You bet you’re a–- I can.

B. I have heard this talk so many times I could do it my sleep.

C. What’s a label?

D. Why should I care?

A. You bet you’re a–- I can.

B. I have heard this talk so many times I could do it my sleep.

C. What’s a label?

D. Why should I care?

How many ways (modes of action) do herbicides kill weeds?How many ways (modes of action) do herbicides kill weeds?

A. More than 20

B. Between 10 and 20

C. Between 5 and 7

D. All herbicides work the same way

A. More than 20

B. Between 10 and 20

C. Between 5 and 7

D. All herbicides work the same way

I can tell a herbicide’s mode of action by looking at the container or the label.

I can tell a herbicide’s mode of action by looking at the container or the label.

A. You bet you’re a–- I can.

B. I have heard this talk so many times I could do it my sleep.

C. What’s a label?

D. Why should I care?

A. You bet you’re a–- I can.

B. I have heard this talk so many times I could do it my sleep.

C. What’s a label?

D. Why should I care?

Increase in Unique Cases of Herbicide-Resistant Weeds Worldwide

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50

100

150

200

250

1950 1960 1970 1980 1990 2000

Year

Num

ber

of R

esist

ant

Bioty

pes

Source: Ian Heapwww.weedscience.com

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Source: Ian Heapwww.weedscience.com

0

10

20

30

40

50

60

70

80

1950 1960 1970 1980 1990 2000

Year

Num

ber

of R

esist

ant

Spe

cies Post Grass Herbicides

Amino Acid Inhibitors (ALS)Photosynthesis Inhibitors (Triazine)Photosynthesis Inhibitors (Ureas)Paraquat/DiquatRoundup (glyphosate)Growth InhibitorsPGRs (2,4-D)

Understanding Herbicide MOA: Key to Resistance ManagementUnderstanding Herbicide MOA: Key to Resistance Management

� Yellow starthistle-PGR resistant

� Ragweeds-PSII, ALS, glyphosate, PPO

� Sunflower-ALS

� Downy Brome-ALS, ACCase

� Flixweed-ALS

� Kochia-ALS, PSII, glyphosate (?)

� Russian thistle-ALS

� Yellow starthistle-PGR resistant

� Ragweeds-PSII, ALS, glyphosate, PPO

� Sunflower-ALS

� Downy Brome-ALS, ACCase

� Flixweed-ALS

� Kochia-ALS, PSII, glyphosate (?)

� Russian thistle-ALS

Photosystem II

“Atrazine”

ALS resistant kochia

Downy bromeDowny brome

ALS resistant kochia as a secondary invader following tamarisk removal.

� Raptor� Accent� Ally� Amber� Exceed� Express� Harmony� Arsenal

� Raptor� Accent� Ally� Amber� Exceed� Express� Harmony� Arsenal

� Plateau� Milestone� Permit� Telar� Escort� Beyond� Habitat� Maverick

� Plateau� Milestone� Permit� Telar� Escort� Beyond� Habitat� Maverick

How many MOAs are represented How many MOAs are represented in this list of common herbicides? in this list of common herbicides?

Organizing Herbicides by MOAsOrganizing Herbicides by MOAs

� 1. Light activated herbicides �� Photosynthesis InhibitorsPhotosynthesis Inhibitors

�� Pigment inhibitorsPigment inhibitors

�� Membrane disruptersMembrane disrupters

� 2. Plant growth regulators

� 3. Amino acid inhibitors

� 4. Fatty acid inhibitors

� 5. Growth inhibitors

� 1. Light activated herbicides �� Photosynthesis InhibitorsPhotosynthesis Inhibitors

�� Pigment inhibitorsPigment inhibitors

�� Membrane disruptersMembrane disrupters

� 2. Plant growth regulators

� 3. Amino acid inhibitors

� 4. Fatty acid inhibitors

� 5. Growth inhibitors

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Herbicide Mode of Action and Symptoms

Herbicide Mode of Action and Symptoms

Plant Growth RegulatorsPlant Growth Regulators

Herbicide Mode of Action and

Symptomology:

Herbicide Mode of Action and

Symptomology:

Light-Activated Herbicides(Photosystem II)

Light-Activated Herbicides(Photosystem II)

Light Activated Herbicides Share a Common MOALight Activated Herbicides Share a Common MOA

� Lipid peroxidation� Production of reactive oxygen, hydroxyl radicals

� Membrane leakage and cell death, usually starting with the chloroplast

� Lipid peroxidation� Production of reactive oxygen, hydroxyl radicals

� Membrane leakage and cell death, usually starting with the chloroplast

Light Activated HerbicidesLight Activated Herbicides

� PSII-Atrazine� PSI-Reglone (diquat)� Cartenoid Inhibitors

� Command (FMC)� Callisto (Syngenta)

� PPO Inhibitors-Kixor (BASF)

� PSII-Atrazine� PSI-Reglone (diquat)� Cartenoid Inhibitors

� Command (FMC)� Callisto (Syngenta)

� PPO Inhibitors-Kixor (BASF)

Common PSII InhibitorsCommon PSII Inhibitors

� Atrazine� Basagran (bentazon)� Velpar (hexazinone)� Spike, Brush-Bullet (tebuthiuron)� Hyvar, Bareground (bromacil)� Karmex, Krovar (diuron)

� Atrazine� Basagran (bentazon)� Velpar (hexazinone)� Spike, Brush-Bullet (tebuthiuron)� Hyvar, Bareground (bromacil)� Karmex, Krovar (diuron)

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Photosystem II InjuryPhotosystem II Injury

burning of leaf margin

interveinal chlorosis

younger leaves show less injury

Herbicides that Mimic Natural Herbicides that Mimic Natural Plant HormonesPlant Hormones--PGRPGR

� Natural plant hormones control� growth, development� tissue differentiation� RNA and DNA synthesis

� Herbicides with this mode of action can do the same things at low concentrations

� At high concentrations these compounds are herbicides

� Natural plant hormones control� growth, development� tissue differentiation� RNA and DNA synthesis

� Herbicides with this mode of action can do the same things at low concentrations

� At high concentrations these compounds are herbicides

�Mode of action�we understand symptoms�we don’t understand why grasses are resistant and broadleaf plants are susceptible

�differences in metabolism?

�Mode of action�we understand symptoms�we don’t understand why grasses are resistant and broadleaf plants are susceptible

�differences in metabolism?

PGR Mode of ActionPGR Mode of Action PGR Mode of ActionPGR Mode of Action

� Symptoms� abnormal growth resulting in twisted stems

� stem swelling due to rapid and uncontrolled cell division

� differentiated cells changing to immature cells

� Symptoms� abnormal growth resulting in twisted stems

� stem swelling due to rapid and uncontrolled cell division

� differentiated cells changing to immature cells

Canada thistle response to Milestone 7 DAT Canada thistle response to Milestone 14 DAT

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Common Leaf Symptoms Common Leaf Symptoms from PGR Exposurefrom PGR Exposure

normal

cupped and blistered fromPGR exposure

callus formation on roots

translocation to growing point

swollen hypocotyl

lack of root development

Root Absorption of PGR Herbicides

Herbicide Mode of Action and SymptomsHerbicide Mode of

Action and Symptoms

Amino Acid InhibitorsAmino Acid Inhibitors

Amino Acid InhibitorsAmino Acid Inhibitors

� Very important group of herbicides� ALS inhibitors

� First sulfonylurea introduced in 1979 by DuPont (Telar)

� First imidazolinone introduced in 1981 by American Cyanamid (Arsenal/Habitat)

� EPSP synthase inhibitor� Roundup introduced 1971

� Very important group of herbicides� ALS inhibitors

� First sulfonylurea introduced in 1979 by DuPont (Telar)

� First imidazolinone introduced in 1981 by American Cyanamid (Arsenal/Habitat)

� EPSP synthase inhibitor� Roundup introduced 1971

General Characteristics of General Characteristics of Amino Acid InhibitorsAmino Acid Inhibitors

� Excellent foliar and root absorption

� Broad weed spectrum

� Translocation to shoot and root meristems

� Plants stop growing shortly after application

� Plant death can take 7 to 10 days

� Resistance has developed

� Excellent foliar and root absorption

� Broad weed spectrum

� Translocation to shoot and root meristems

� Plants stop growing shortly after application

� Plant death can take 7 to 10 days

� Resistance has developed

Important TerminologyImportant Terminology

� Sulfonylurea (SU) herbicide� Telar, Escort

� Imidazolinone (IMI) herbicides� Plateau, Habitat

� These chemical families inhibit branch chain amino acid production-acetolactate synthase or ALS

� Sulfonylurea (SU) herbicide� Telar, Escort

� Imidazolinone (IMI) herbicides� Plateau, Habitat

� These chemical families inhibit branch chain amino acid production-acetolactate synthase or ALS

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A) SU (Telar) binding to channel leading to ALS active site. Notice that herbicide is deeper active site then IMI molecule. Also interacts with more amino acids.

B) IMI (Plateau) binding to channel leading to ALS active site. Notice that herbicide is closer to the surface site then SU molecule. Also interacts with fewer amino acids. Plateau injury symptoms expressed by leafy spurge

Roundup (glyphosate)Roundup (glyphosate)� Inhibits the production of aromatic amino

acids

� Competitive with PEP for the enzyme 5-enolpyruvylshikimate-3-phosphate synthase or EPSP synthase.

� Non-selective, no soil activity

� Monsanto has developed Roundup Ready corn, soybeans, sugar beets, alfalfa, cotton, rice.

� Inhibits the production of aromatic amino acids

� Competitive with PEP for the enzyme 5-enolpyruvylshikimate-3-phosphate synthase or EPSP synthase.

� Non-selective, no soil activity

� Monsanto has developed Roundup Ready corn, soybeans, sugar beets, alfalfa, cotton, rice.

•Structurally similar

•Glyphosate dissociation from EPSPS is 2300 times slower than PEP!

PEP

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Questions? Questions?