Herbicide Mode of Action

An overview

Developed by Cheryl Wilen, UC IPM Not for distribution without permission

Pesticides

Any chemical (natural or synthetic) that mitigates (kills, controls, repels) a pest (animal or plant, etc)

HerbicideA chemical substance used to kill

undesirable plants. Will kill any plant not just weeds PRE and POE Contact and systemic

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

Why understand herbicide MOA?

Better understanding of how herbicides perform

Diagnosing herbicide injury Resistance Management

Mode of Action

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

site

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

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.

Cell (Plasma) Membranes

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

Entry and Movement of Herbicides in Plants

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

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

Plant Processes Affected by Herbicides

Photosynthesis Amino acid and protein synthesis Cell division Cell membranes Pigment synthesis Growth regulation Growth inhibition Fatty acid synthesis

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

Growth Regulator Herbicides

Affect several plant process•cell division•cell enlargement•protein synthesis•respiration

Upset normal hormonal balance

Examples:2,4-DMCPADicambaBanvelClopyralid

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

Growth Regulator Herbicides

SymptomologyBroadleaves:

stem twists and curlsleaves become cupped,

crinkled, or have a “strapped” deformity

Grasses:Brace roots fusedLeaf rolling or crinkling

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

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

Cell Membrane Disrupters

•Destroy cell membranes•Contents leak out•Plant dessicates

DiquatOxyfluorfenFlumioxazin

Drift may injure susceptible crops by spotting leaves

Photosynthesis Inhibitors

•Interrupt one or more steps of photosynthesis•Most are PRE but are also active POST•Translocate

Atrazine, Simazine, Linuron

X

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

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

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

Pigment Inhibitors

Foliage becomes white than diesEspecially apparent on new growth

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)

Leaves turn reddish

Discoloration at and above nodes

Leaves in whorl easily be separated from rest of the plant

Lipid Synthesis Inhibitors