Turfgrass Herbicide Mode-of-Action

Tim R. Murphy

The University of Georgia

Mode-of-Action - The entire sequence ofevents that happen from the time the herbicide is absorbed to the eventual plant response (usually death).

Herbicide Mode-of Action

Mode of Action

Three phases* Absorption* Movement to target site* Inhibit biochemical reaction

Herbicide Mode-of-Action

The way a herbicide kills or inhibits the growth of susceptible plants!!

Why understand herbicide MOA?

Better understanding of how herbicides perform.

Diagnosing herbicide injury. Professionalism. Public relations.

Herbicide Classification

Selectivity Herbicide movement in plants Timing of application Chemical Family Mode of Action

Herbicide Classification - Selectivity

Selective controls or suppresses one species of plant

without seriously affecting the growth of another plant species.

Example2,4-D will control many broadleaf weeds without

seriously affecting turfgrasses.

Herbicide Classification - Selectivity

Nonselective Nonselective herbicides control plants

regardless of species. Examples

Roundup Pro, Finale, Reward, Scythe

Phloem mobile herbicides move up and down in the plant

Xylem mobile herbicides move up in the plant

Contact herbicides do not move in the plant

Translocated herbicides move.

Contact Rupture cell

membranes. Symptoms

appear within hours.

Need complete coverage for kill.

Xylem MobileTaken up by primarily by roots.Found in older leaves that are exporting sugars.

Do not move downward when foliar applied.

Phloem MobileMove to points of active growth or food storage.Affect shoot tips, young buds and leaves.Can cause contact burn.

Herbicide Movement in Plants

Plant leaf cuticle.

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

Photosynthesis

CO2 + H2O + Sunlight = C6H12O6

OR

Carbon dioxide + water + sunlight

will yield

Sugar

Triazines

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

Light and active photosynthesis needed for activity

These herbicides interrupt energy flow resulting in free radicals

Free radicals react with cell membranes

Photosynthetic inhibitors

Atrazine on bentgrass

Atrazine, simazine, Sencor

Bentazon

Basagran T/OAll warm- and cool-season turfgrassescontact, foliage absorbedlimited translocationcan move in xylem if root absorbedinhibits photosynthetic electron transport

BladexBladexCotoran

Source: W. K. Vencill, UGA

Injury Symptoms-Photosynthesis Inhibitors

Oxadiazon

Ronstarall turfgrasses, except centipedereadily shoot absorbed, less so by rootscan be foliage absorbed, “contact burn”does not significantly translocateMOA is related to photosynthesis, light requiredinhibits key enzyme in chlorophyll synthesis,

toxic radicals formed, disrupts cell membranes

Amino acids

Building blocks of plant proteinsEnzymes mediate biochemical

reactionsStorage roles - seed reservesStructural roles

Imidazolinones

Image, Plateauwarm-season grasses, except bahiagrassrapid shoot and root absorptiontranslocates to meristematic areasinhibits leucine, isoleucine and valine synthesisgrowth is impaired and plants die over 1 to 3 wk

period

Image on bermuda

Sulfonylureas

Manor, Corsair, Manage,TranXit, Monument, Revolverrapid shoot and root absorptiontranslocates to meristematic areasinhibits leucine, isoleucine and valine synthesisgrowth is impaired and plants die over 1 to 3 wk

period

Sulfonylureas

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

Glyphosate

Isopropylamine salt - Roundup Pro , GlyphoMate 41, Razor Pro, Prosecutor

Ammonium salt - Roundup ProDry Diammonium salt - TouchDown Pro

foliage absorbedextensively translocated in phloem inhibits tryptophan, tyrosine and phenylalanine synthesisgrowth is impaired and plants die over 1 to 3 wk period

Cell Division

Plant growth is due to cell division and cell elongation.

Dinitroanilines 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

Dithiopyr

Dimension MOA is similar to dinitroanilines

Shoot and root absorbed, no translocationInhibit cell division (tubulin formation)Has POST activity on seedling crabgrassUsed on most warm- and cool-season

turfgrasses

DNA Root inhibition

pendimethalin on zoysiagrass Dimension on

bermudagrass

Surflan on St. Augustinegrass

Swollen, club shaped roots

Cell Membranes

Contain the cellular contents and regulate the entry and exit of substances that promote or inhibit plant metabolism

Diquat Reward LS

contact, very rapidly absorbed,does not translocate requires light for activitydisrupts electron flow in photosynthesis leads to formation of free radicals which damages cell

membranescellular fluids leak, followed by deathused on dormant bermudagrass

Glufosinate Finale

readily absorbed by leavesprimarily 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 photosynthesisused on dormant bermudagrass

Carfentrazone

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

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

Finale on zoysia

Finale, Reward, Paraquat

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

Phenoxys 2,4-D, dichlorprop mecoprop, MCPA

Benzoics Banvel, Vanquish

Picolinic Acids Confront Turflon Lontrel SpotLight

Quinolinecarboxylic Drive

Trimec on bermuda

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 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

2,4-D Injury on Soybean

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

Bensulide - Betasan, Pre-Sanroot absorbed, essentially no translocationdisrupts cell division and enlargement

Metolachor - Pennantmainly shoot absorbed, some root uptaketranslocates in xyleminhibit root and shoot growthdisrupts protein, fatty acid and lipid 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

Cyclohexanedionessethoxydim - Vantage, Poastused only on centipedegrass and fine fescueclethodim – Envoyused only on centipedegrass

Rapidly absorbed by leaves, extensively translocated via phloem to meristems

Inhibit key enzyme in fatty acid synthesis

Fatty Acid Inhibitors

Aryl-oxy-phenoxiesfenoxaprop - Acclaim Extrafluazifop - Fusliade IIdiclofop - Illoxan

Rapidly absorbed by leaves, extensively translocated via phloem to meristems, (diclofop translocation is slow)

Inhibit key enzyme in fatty acid synthesis

Fatty Acid Inhibitors

Aryl-oxy-phenoxiesfenoxaprop - Acclaim Extra

used on cool-season grasses and zoysiagrass

fluazifop - Fusilade IIUsed only on tall fescue and bermudagrass

diclofop – IlloxanUsed only on bermudagrass

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)

Source: W. K. Vencill, UGA

“rotted” meristem

Non-Classified Herbicides

Organic Arsenicals

MSMA, DSMA, CMA, CAMAfoliage absorbedextensive movement in xylem and phloeminterfere with ATP productionalso interfere with enzyme activity and cell

divisionUsed 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

TurfWeedManagement

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