herbicide mode of action an overview developed by cheryl wilen, uc ipm not for distribution without...
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Herbicide Mode of Action
An overview
Developed by Cheryl Wilen, UC IPM Not for distribution without permission
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Pesticides
Any chemical (natural or synthetic) that mitigates (kills, controls, repels) a pest (animal or plant, etc)
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HerbicideA chemical substance used to kill
undesirable plants. Will kill any plant not just weeds PRE and POE Contact and systemic
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The entire sequence of events that happen from the time the herbicide is absorbed to the eventual plant response (usually death).
The way a herbicide kills or inhibits the growth of susceptible plants!!
Mode-of-Action
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Why understand herbicide MOA?
Better understanding of how herbicides perform
Diagnosing herbicide injury Resistance Management
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Mode of Action
Three phases* Absorption* Movement to target site* Inhibit biochemical reaction at target
site
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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
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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.
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Cell (Plasma) Membranes
Contain the cellular contents and regulate the entry and exit of substances that promote or inhibit plant metabolism
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Entry and Movement of Herbicides in Plants
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Can be absorbed by underground parts as well as developing shoots
•Broadleaf species: primarily absorb through roots sometimes by hypocotyls and new shoots•Grass species: absorption occurs more through the coleoptilar node or crown node
SOIL APPLIED HERBICIDES
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Leaves are the most important site of entry
Factors that are important for leaf absorption:•retention of herbicide on leaf surface•penetration of cuticle•movement into cell cytoplasm•movement through plant
FOLIAR APPLIED HERBICIDES
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Plant Processes Affected by Herbicides
Photosynthesis Amino acid and protein synthesis Cell division Cell membranes Pigment synthesis Growth regulation Growth inhibition Fatty acid synthesis
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Common Modes of Action (MOA)
•Growth regulator•Seedling growth inhibitors•Cell membrane disruptors•Photosynthesis inhibitors•Amino acid synthesis inhibitors•Pigment synthesis inhibitors•Lipid synthesis inhibitors
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Growth Regulator Herbicides
Affect several plant process•cell division•cell enlargement•protein synthesis•respiration
Upset normal hormonal balance
Examples:2,4-DMCPADicambaBanvelClopyralid
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Growth Regulator Herbicides
•Usually applied to foliage but can have residual soil activity•Translocated (systemic)•Generally affects broadleaf plants but grasses can be injured too
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Growth Regulator Herbicides
SymptomologyBroadleaves:
stem twists and curlsleaves become cupped,
crinkled, or have a “strapped” deformity
Grasses:Brace roots fusedLeaf rolling or crinkling
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Seedling Growth Inhibitors
Root inhibitors•inhibit cell division (mitosis) of developing roots•taken up by roots
Balan, Surflan, PendimethalinShoot inhibitors
•disrupt protein synthesis, cell division, cell enlargement•weakens cell membranes•taken up by shoot as it grows through treated soil
Devrinol, Oxidiazon, Pennant (Metolachlor)
Stubby roots
Swelling at base
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Oxadiazon
Ronstar 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
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Cell Membrane Disrupters
•Destroy cell membranes•Contents leak out•Plant dessicates
DiquatOxyfluorfenFlumioxazin
Drift may injure susceptible crops by spotting leaves
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Photosynthesis Inhibitors
•Interrupt one or more steps of photosynthesis•Most are PRE but are also active POST•Translocate
Atrazine, Simazine, Linuron
X
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Amino Acid Synthesis Inhibitors•Inhibit the production of amino acids, resulting in inhibition of plant proteins•Translocated •Most have soil and foliar activity
Halosulfuron (Manage)Glyphosate (Roundup)
X
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Amino Acid Synthesis Inhibitors
Often takes days to weeks to see damage
Grasses: Stunting, purple tintBroadleaves: Purpling or reddening of veins, meristem necrosis
Glyphosate: Yellowing followed by death
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Pigment Inhibitors
•Inhibit formation of chlorophyll•Plants cannot photosynthesize
Zorial or Solicam (norflurazon), Command, Callisto(none are used in ornamentals or turf)
•Absorbed by roots and translocated via xylem (Amitrol will move in phloem)•Some have a long (12 months) soil residual
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Pigment Inhibitors
Foliage becomes white than diesEspecially apparent on new growth
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Lipid Synthesis Inhibitors
•Disrupt lipid biosynthesis in grasses•Cell membrane integrity is compromised in meristems (the growing points)•Most are foliar applied •Movement in both the phloem and xylem Usually takes 7-10 days for significant injury
Sethoxydim (Poast), fluazifop (Fusilade), clethodim (Prism)
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Leaves turn reddish
Discoloration at and above nodes
Leaves in whorl easily be separated from rest of the plant
Lipid Synthesis Inhibitors