Transcript Herbicide mode of action - San Diego Master Gardeners

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)
Herbicide
A chemical substance used to kill
undesirable plants.
Will kill any plant not just weeds
 PRE and POE
 Contact and systemic

Mode-of-Action
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!!
Why understand herbicide MOA?
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Better understanding of how
herbicides perform
Diagnosing herbicide injury
Resistance Management
Mode of Action

Three phases
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Absorption
Movement to target site
Inhibit biochemical reaction at target
site
Herbicide Movement in Plants
Contact
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Rupture cell
membranes.
Symptoms
appear
within
hours.
Need
complete
coverage for
kill.
Xylem
Mobile
Taken up by
primarily by
roots.
Found in older
leaves that are
exporting
sugars.
Do not move
downward
when foliar
applied.
Phloem Mobile
Move to points of
active growth or
food storage.
Affect shoot tips,
young buds and
leaves.
Can cause
contact burn.
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.
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
SOIL APPLIED HERBICIDES
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
FOLIAR 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
Plant Processes Affected by
Herbicides
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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-D
MCPA
Dicamba
Banvel
Clopyralid
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
Symptomology
Broadleaves:
stem twists and curls
leaves become cupped,
crinkled, or have a “strapped”
deformity
Grasses:
Brace roots fused
Leaf rolling or crinkling
Seedling Growth Inhibitors
Swelling at base
Stubby roots
Root inhibitors
•inhibit cell division (mitosis) of
developing roots
•taken up by roots
Balan, Surflan,
Pendimethalin
Shoot 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)
Oxadiazon
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Ronstar
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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
Diquat
Oxyfluorfen
Flumioxazin
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 tint
Broadleaves: 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 dies
Especially 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)
Lipid Synthesis Inhibitors
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Leaves turn
reddish
Discoloration at
and above nodes
Leaves in whorl
easily be
separated from
rest of the plant