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William E. Dyer Department of Plant Sciences
Herbicide Classification and Mode of Action
Classification can be based on:
1. Chemical structure or family
2-choro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide
2. Cropping system or ecosystem
3. Mode of action
= the plant process or pathway inhibited by the herbicide
Why Know Herbicide Modes of Action?
• Better understanding of how herbicides
perform • Improve herbicide performance • Diagnose herbicide injury • Prevent and manage herbicide resistance
Contact
Absorption
Toxicity
Movement
Site of Action
Herbicide Mode of Action
Site of Absorption and Translocation
Foliar Contact (Gramoxone)
Root Contact (Treflan)
Phloem (Roundup)
Xylem (Spike)
Xylem and Phloem (Banvel, Tordon)
Rangeland herbicides
Herbicide Classification - Selectivity -
• Selective: controls or suppresses one species of plant without seriously affecting the growth of another plant species – 2,4-D
• Nonselective: control plants regardless of species – Roundup
Some Things to Remember….
• Photosynthesis (food) • Pigments (energy/light capture) • Respiration (energy) • Amino acids (proteins/growth) • Lipids (cell membranes) • Hormones (auxin and others)
Classification by Mode of Action Group
1) Lipid Biosynthesis Inhibitors 2) Amino Acid Biosynthesis Inhibitors 3) Cell Division Inhibitors 4) Synthetic Auxins 5,6,7) Photosynthesis Inhibitors 9) EPSP Synthase Inhibitor 14) Pigment Inhibitors 22) Membrane Disruptors 16,17,26) Unknown
Herbicide Mode of Action Groups
Group 1: Lipid synthesis inhibitors (“grass killers”)
Block ACCase enzyme
Trade Names: Hoelon, Fusilade, Poast, Axial, Puma, Assure
Symptoms
Mechanism of resistance: Altered target site
Altered target site
The normal
situation
A resistant enzyme
Herbicide Mode of Action Groups
Group 2: Amino acid synthesis inhibitors; blocks ALS enzyme
Trade names: Glean, Telar, Finesse, Muster, Permit, Ally, Escort, Canvas, Finesse, Accent, Basis, Beacon, Exceed, Peak, Matrix, Oust, Maverick, Harmony, Pinnacle, Cheyenne, Reliance, Amber, Rave, Upbeet, Arsenal, Assert, Lightning, Motive, Plateau, Pursuit, Raptor, Sceptor, Broadstrike, FirstRate, StrongArm, Python
Mechanism of Resistance: altered target site
• Translocates to new growth in xylem and phloem • Plants stop growing shortly after application, but plant death
may be slow (10+ days)
Plateau Injury Symptoms
Chlorosis of youngest tissue
Stunting
Chlorosis of youngest tissue
ALS Inhibitor Symptoms
Herbicide Mode of Action Groups
Group 4: Synthetic auxins
Overwhelm natural auxin pathways
Trade names: 2,4-D, MCPA, Banvel, Clarity, Milestone, Perspective, Tordon, Stinger, Curtail, Transline, Starane, Garlon, Remedy, Renovate, Paramount, Drive
Mechanism of Resistance: ???
• Translocates in both xylem and phloem • Symptoms most obvious in new growth • Selectively kill broadleaf plants
• Soil residue varies from a few days to several years
Synthetic Auxin Symptoms
Phenoxy on Red Oak University of Illinois extension
Phenoxy drift on tomato Ohio State University
Benzoic acid on soybean
Photo: HMOA and Crop Injury Symptoms Univ. of Minnesota Extension
Washington State University Extension
Pyrdine on tomato
Synthetic Auxin Symptoms
Twisting needles due to drift of triclopyr
Herbicide Mode of Action Groups
Groups 5,6,7: Photosystem II inhibitors
Block photosynthesis
Trade names: Aatrex, Princep, Simazine, Velpar, Sencor, Hyvar X, Sinbar, Basagran, Buctril, Bronate, Karmex, Direx, Lorox, Linex, Spike
Mechanism of Resistance: altered target site
• Injures old growth first, moves only upward in xylem • Injury symptoms: yellowing (chlorosis) of leaf tissue
followed by death (necrosis) of the tissue
Injury from PSII herbicide Interveinal, tip, and
margin chlorosis Interveinal chlorosis
Leaves necrotic
Consequences of photosynthetic inhibition
Herbicide Mode of Action Groups
Group 9: Glycines (glyphosate)
Inhibit amino acid biosynthesis
Trade names: Roundup, Gallup, Kleenup, Landmaster, Pondmaster, Ranger, Rodeo, Touchdown
Mechanism of Resistance: reduced translocation, gene amplification
• Translocates to new growth in xylem and phloem • Plants stop growing shortly after application, but plant
death may be slow (10+ days)
chlorosis shortened internodes
stem proliferation
Glyphosate Injury Symptoms
Herbicide Mode of Action Groups
Group 22: Bipyridiliums (paraquat)
Divert electrons from photosynthesis
Trade names: Reglone, Gramoxone, Scythe, many others
Mechanism of Resistance: enhanced detoxification
Symptoms
Other Herbicide Mode of Action Families
Groups 16,17,26: Unknown
Group 3: Microtubule assembly inhibitors
Group 14: Protox inhibitors
Group 15: Long chain fatty acid synthesis inhibitors
Why aren’t there any new herbicides?
• Half of all current herbicides inhibit one of only three target enzymes.
• All herbicides now on the market were discovered by accident.
• New ‘target-based’ technologies MAY lead to new herbicides.
R S R
Banvel-resistant kochia
http://msuextension.org/publications/AgandNaturalResources/MT200506AG
More Information?
