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Turfgrass Herbicide Mode-of-Action. Tim R. Murphy The University of Georgia. Herbicide Mode-of Action. Mode-of-Action - The entire sequence of events that happen from the time the herbicide is absorbed to the eventual plant response (usually death). Mode of Action. Three phases Absorption

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turfgrass herbicide mode of action

Turfgrass Herbicide Mode-of-Action

Tim R. Murphy

The University of Georgia


Herbicide Mode-of Action

Mode-of-Action- The entire sequence of

events that happen from the time the herbicide is absorbed to the eventual plant response (usually death).

mode of action
Mode of Action
  • Three phases
    • Absorption
    • Movement to target site
    • Inhibit biochemical reaction
herbicide mode of action
Herbicide Mode-of-Action
  • The way a herbicide kills or inhibits the growth of susceptible plants!!
why understand herbicide moa
Why understand herbicide MOA?
  • Better understanding of how herbicides perform.
  • Diagnosing herbicide injury.
  • Professionalism.
  • Public relations.
herbicide classification
Herbicide Classification
  • Selectivity
  • Herbicide movement in plants
  • Timing of application
  • Chemical Family
  • Mode of Action
herbicide classification selectivity
Herbicide Classification - Selectivity
  • Selective
    • controls or suppresses one species of plant without seriously affecting the growth of another plant species.
  • Example
    • 2,4-D will control many broadleaf weeds without seriously affecting turfgrasses.
herbicide classification selectivity1
Herbicide Classification - Selectivity
  • Nonselective
    • Nonselective herbicides control plants regardless of species.
  • Examples
    • Roundup Pro, Finale, Reward, Scythe

Contact herbicides do not move in the plant

Phloem mobile herbicides move up and down in the plant

Xylem mobile herbicides move up in the plant

Translocated herbicides move.


Herbicide Movement in Plants

Phloem Mobile

Move to points of active growth or food storage.

Affect shoot tips, young buds and leaves.

Can cause contact burn.

Xylem Mobile

Taken up by primarily by roots.

Found in older leaves that are exporting sugars.

Do not move downward when foliar applied.


  • Rupture cell membranes.
  • Symptoms appear within hours.
  • Need complete coverage for kill.
plant processes affected by turfgrass herbicides
Plant Processes Affected by Turfgrass Herbicides
  • Photosynthesis
  • Amino acid and protein synthesis
  • Cell division
  • Cell membranes
  • Pigment synthesis (none registered in turf)
  • Growth regulation
  • Growth inhibition
  • Fatty acid synthesis

CO2 + H2O + Sunlight = C6H12O6


Carbon dioxide + water + sunlight

will yield


  • Atrazine – Aatrex, Simazine - Princep
  • Metribuzin – Sencor
  • Used only on warm-season turfgrasses
  • Root and shoot absorbed (except simazine)
  • Inhibits photosynthesis, eventual membrane disruption, requires light
  • Pre and Post control

Photosynthetic inhibitors

Atrazine, simazine, Sencor

  • Light and active photosynthesis needed for activity
  • These herbicides interrupt energy flow resulting in free radicals
  • Free radicals react with cell membranes

Atrazine on bentgrass

  • Basagran T/O
    • All warm- and cool-season turfgrasses
    • contact, foliage absorbed
    • limited translocation
    • can move in xylem if root absorbed
    • inhibits photosynthetic electron transport



Injury Symptoms-

Photosynthesis Inhibitors

Source: W. K. Vencill, UGA

  • Ronstar
    • all turfgrasses, except centipede
    • readily shoot absorbed, less so by roots
    • can be foliage absorbed, “contact burn”
    • does not significantly translocate
    • MOA is related to photosynthesis, light required
    • inhibits key enzyme in chlorophyll synthesis, toxic radicals formed, disrupts cell membranes
amino acids
Amino acids
  • Building blocks of plant proteins
  • Enzymes mediate biochemical reactions
  • Storage roles - seed reserves
  • Structural roles
  • Image, Plateau
    • warm-season grasses, except bahiagrass
    • rapid shoot and root absorption
    • translocates to meristematic areas
    • inhibits leucine, isoleucine and valine synthesis
    • growth is impaired and plants die over 1 to 3 wk period
  • Manor, Corsair, Manage,TranXit, Monument, Revolver
    • rapid shoot and root absorption
    • translocates to meristematic areas
    • inhibits leucine, isoleucine and valine synthesis
    • growth is impaired and plants die over 1 to 3 wk period
  • Manor, Blade – warm-season grasses, except bahiagrass; also on Ky. Bluegrass
  • Corsair – Ky. Bluegrass, bahiagrass, bermudagrass
  • Manage – all warm- and cool-season turfgrasses
  • Revolver – bermuda, ‘Meyer’ zoysia
  • Monument – bermuda, zoysia
  • TranXit – bermuda, zoysia, centipede, overseeded bermudagrass
  • Isopropylamine salt - Roundup Pro , GlyphoMate 41, Razor Pro, Prosecutor
  • Ammonium salt - Roundup ProDry
  • Diammonium salt - TouchDown Pro
    • foliage absorbed
    • extensively translocated in phloem
    • inhibits tryptophan, tyrosine and phenylalanine synthesis
    • growth is impaired and plants die over 1 to 3 wk period
cell division
Cell Division
  • Plant growth is due to cell division and cell elongation.
  • Benefin - Balan
  • Oryzalin - Surflan
  • Pendimethalin - Pendulum
  • Prodiamine - Barricade
  • Trifluralin - Treflan
    • Shoot and root absorbed, no translocation
    • Inhibit cell division (tubulin formation)
    • Used on most warm- and cool-season turfgrasses
  • Dimension
  • MOA is similar to dinitroanilines
    • Shoot and root absorbed, no translocation
    • Inhibit cell division (tubulin formation)
    • Has POST activity on seedling crabgrass
    • Used on most warm- and cool-season turfgrasses
dna root inhibition
DNA Root inhibition

Surflan on St. Augustinegrass

pendimethalin on zoysiagrass

Dimension on bermudagrass

Swollen, club shaped roots

cell membranes
Cell Membranes
  • Contain the cellular contents and regulate the entry and exit of substances that promote or inhibit plant metabolism
  • Reward LS
    • contact, very rapidly absorbed,does not translocate
    • requires light for activity
    • disrupts electron flow in photosynthesis
    • leads to formation of free radicals which damages cell membranes
    • cellular fluids leak, followed by death
    • used on dormant bermudagrass
  • Finale
    • readily absorbed by leaves
    • primarily contact activity, limited translocation
    • inhibits key enzyme involved in conversion of ammonia into amino acids
    • toxic levels of ammonia build up, causes cell membrane disruption, interferes with photosynthesis
    • used on dormant bermudagrass
  • Sold with phenoxy herbicides and dicamba as PowerZone and SpeedZone
  • Limited translocation
  • Inhibits key enzyme in chlorophyll synthesis
  • Causes buildup of phytotoxic compounds which damages cell membranes
  • Cellular fluids leak, followed by cell death
  • All turfgrasses
cell membrane disrupters
Cell Membrane Disrupters

Finale, Reward, Paraquat

  • Damage occurs within hours.
  • Membrane destruction.
  • No redistribution
  • Complete coverage for kill.

Finale on zoysia


Growth Regulator Herbicides

These herbicides affect several plant processes such as cell division, cell enlargement, protein synthesis and respiration. They act by upsetting the normal hormonal balance in plants.

growth regulator herbicides

2,4-D, dichlorprop

mecoprop, MCPA


Banvel, Vanquish

Picolinic Acids







Growth Regulator Herbicides

Trimec on bermuda

phenoxy benzoic acid picolinic acid
Phenoxy, Benzoic Acid, Picolinic Acid
  • readily absorbed by foliage, less so by roots
  • extensively translocated
  • interfere with DNA, RNA and protein synthesis
  • results in uncontrolled cell division and elongation
  • vascular tissues are plugged, 1 to 3 wks
phenoxy benzoic acid picolinic acid1
Phenoxy, Benzoic Acid, Picolinic Acid
  • Phenoxies – used on cool- and warm season turfgrasses, except St. Augustine
  • Benzoic acids - used on cool- and warm season turfgrasses
  • Lontrel – all cool- and warm-season grasses
  • Confront – cool- and warm-season grasses, except St. Augustine
  • Turflon – Ky. Bluegrass and tall fescue
growth inhibition
Growth Inhibition
  • Exact MOA not known
  • Inhibit root and shoot growth
  • Disrupt cell division, cell enlargement
  • Also may disrupt lipid synthesis, nucleic acid and protein synthesis
growth inhibitors
Growth Inhibitors
  • Bensulide - Betasan, Pre-San
    • root absorbed, essentially no translocation
    • disrupts cell division and enlargement
  • Metolachor - Pennant
    • mainly shoot absorbed, some root uptake
    • translocates in xylem
    • inhibit root and shoot growth
    • disrupts protein, fatty acid and lipid synthesis
fatty acid synthesis
Fatty Acid Synthesis
  • Fatty acids are important components of cell membranes
  • Inhibition fatty acid synthesis blocks production of lipids needed for new cell growth.
fatty acid inhibitors
Fatty Acid Inhibitors
  • Cyclohexanediones
    • sethoxydim - Vantage, Poast
    • used only on centipedegrass and fine fescue
    • clethodim – Envoy
    • used only on centipedegrass
  • Rapidly absorbed by leaves, extensively translocated via phloem to meristems
  • Inhibit key enzyme in fatty acid synthesis
fatty acid inhibitors1
Fatty Acid Inhibitors
  • Aryl-oxy-phenoxies
    • fenoxaprop - Acclaim Extra
    • fluazifop - Fusliade II
    • diclofop - Illoxan
  • Rapidly absorbed by leaves, extensively translocated via phloem to meristems, (diclofop translocation is slow)
  • Inhibit key enzyme in fatty acid synthesis
fatty acid inhibitors2
Fatty Acid Inhibitors
  • Aryl-oxy-phenoxies
    • fenoxaprop - Acclaim Extra
      • used on cool-season grasses and zoysiagrass
    • fluazifop - Fusilade II
      • Used only on tall fescue and bermudagrass
    • diclofop – Illoxan
      • Used only on bermudagrass
fatty acid synthesis inhibitors grass herbicides
Fatty Acid Synthesis Inhibitors - Grass Herbicides
  • Reddening of leaf tissue.
  • Discoloration of tissue at and above the nodes.
  • Tissue and leaves in the leaf whorl can easily be separated from the rest of the plant.
  • No broadleaf activity.

Fusilade on dallisgrass


Fatty Acid Synthesis Inhibitors

  • Initial injury in meristematic regions occurs where newest leaves are developing
  • These regions will turn chlorotic, which is followed by necrosis
  • The affected area will become “rotted” and will easily separate from rest of plant
  • Injury will develop slowly (7 to 14 days)

“rotted” meristem

Source: W. K. Vencill, UGA

organic arsenicals
Organic Arsenicals
    • foliage absorbed
    • extensive movement in xylem and phloem
    • interfere with ATP production
    • also interfere with enzyme activity and cell division
    • Used on cool-season grasses, bermuda and zoysiagrass
Leaf burn on broadleaves and grasses with yellow leaf tips on grasses
  • Terminal yellowing of grasses

MSMA on zoysia