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Forestry Herbicide
Symptoms & Mode of
Action
John Boyd
University of Arkansas
[email protected]
501-671-2224
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What is mode of action?
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How a herbicide disrupts or
inhibits normal plant development.
– Examples
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cell membrane destruction.
block protein synthesis
interfere with photosynthesis
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Terms to remember
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photosynthesis (food)
respiration (energy)
amino acids (proteins/growth)
lipids (cell membranes)
pigments (energy/light capture)
mitosis (cell division)
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Contact
Phloem mobile herbicides
move up and down in the plant
Contact herbicides do
not move in the plant
Xylem mobile herbicides
move up in the plant
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Growth Regulator Herbicides
Family
Common Name
Trade Name
Phenoxy
2,4-D
Several
benzoic acid
dicamba
Vanquish
carboxylic acid
picloram
Tordon
carboxylic acid
triclopyr
Garlon
carboxylic acid
clopyralid
Transline
Affect cell division, cell enlargement, protein synthesis and
respiration. Act by upsetting hormone balance.
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Growth Regulator Herbicides
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Generally applied to leaves, but have
limited soil activity
Length of soil activity is herbicidedependent
Highly systemic in susceptible plants
Activity (pound for pound) varies
among herbicides
Primarily broadleaf activity but may
affect grasses
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Growth Regulator Symptoms
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Broadleaf plant stem twisting and curling
Leaves on broadleaf plants exhibit cupping,
crinkling, strapping, or drawstring affect
Symptoms on grass plants include leaf
rolling, crinkling, brace root fusion and
malformation.
Also, flower sterility and missing grain in
crops.
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Twisting of stems and leaf cupping are typical
symptoms of the growth regulator herbicides.
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Leaf strapping is another typical symptom of these herbicides.
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Deformed pin oak leaf caused by 2,4-D. This is
referred to as the drawstring effect.
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Vanquish (dicamba)
untreated
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Upward cupping of leaves on azalea. Common symptom
with dicamba exposure.
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Upward cupping of
leaves on lilac from 2,4-D
exposure.
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Photosynthesis Inhibitors
Family
Common Name Trade Name
triazine
triazine
triazinone
uracil
atrazine
simazine
hexazinone
bromacil
AAtrex
Princep
Velpar
Hyvar
urea
urea
diuron
tebuthiuron
Karmex
Spike
These herbicides interrupt photosynthesis the process by
which green plants convert light energy into food.
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PI’s are Xylem Mobile
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Velpar, atrazine, simazine, Spike,
Hyvar
– Taken up by primarily by roots.
– Symptoms appear in older leaves that are
exporting sugars
– Do not move downward when foliar
applied.
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Photosynthesis Inhibitors
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Most PI’s are soil applied.
All PI’s have at least some
postemergence activity
Soil-applied PI’s translocate through
the xylem.
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Photosynthesis Inhibitors
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Interveinal or veinal yellowing
followed by death of plant tissue from
leaf margins inward
Postemergence applications cause
rapid burning of plant tissue
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Princep (simazine)
Symptoms often appear on the margins of the oldest
leaves (pear).
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Karmex (diuron)
on oak. Symptoms
often appear on
the margins of the
oldest leaves.
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With photosynthesis inhibitors, symptoms start on the margins of
the older leaves and move inward.
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With photosynthesis inhibitors, symptoms start on the margins of
the older leaves and move inward.
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Pigment Inhibitor Herbicides
Family
Common Name
Trade Name
isoxazolidinone
clomazone
Command
pyridazinone
norflurazon
Solicam
not classifield
fluridone
Sonar
These herbicides cause the green pigments (chlorophyll)
in plants to be destroyed. Without chlorophyll, plants
cannot photosynthesize and will die.
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Pigment inhibitors
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Pigment inhibitors cause white leaves in
susceptible plants
New growth will not contain the green
pigments needed for photosynthesis
Symptoms may be found on the cotyledon
leaves and true leaves
White growth may be observed within veins
(primarily with Zorial) and between veins
(primarily with Command)
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Pigment Inhibitor Herbicides
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Zorial (norflurazon).
Inhibit carotenoid
formation followed by
loss of chlorophyll.
New growth is white,
sometimes tinged with
pink or purple.
Zorial usually follows
the leaf veins.
Zorial
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Command (clomazone)
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New growth is
white,
sometimes
tinged with pink
or purple.
Command:
white between
the veins.
Maple
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Root Growth Inhibitors
Family
Common Name Trade Name
dinitroanilines
oryzalin
Surflan
dinitroanilines
pendimethalin
Pendulum
dinitroanilines
prodiamine
Barricade
The root inhibitors interrupt cell division (mitosis)
stopping root growth in seedling plants.
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Root Growth Inhibitors
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Plant death generally occurs before
emergence
Root inhibitors translocate very little
Generally stable in upper soil profile
Persistence is herbicide-dependent
Most effective on small-seeded grass and
broadleaf weeds
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Root Growth Inhibitors
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Roots on susceptible plants will be stubby and
thick, especially lateral roots
Root limitation may cause stunting of plants and
phosphorus deficiency symptoms
Broadleaf plants may have swollen hypocotyls
Concentration of herbicide at soil surface may
cause callus tissue to form, leading to plant
lodging
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Roots on susceptible plants will be
stubby and thick, especially lateral roots
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Cell Membrane Disrupters
Family
Common Name Trade Name
bipyridiliums
diquat
Reward
bipyridiliums
paraquat
Gramoxone
diphenylethers lactofen
Cobra
Destroy cell membranes, causing cell contents to
leak out and result in desiccation of plant tissue.
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Contact herbicides
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Do not move
Herbicides: Paraquat, diquat, Cobra
(lactofen), MSMA
– Rupture cell walls.
– Symptoms appear within hours.
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Cell Membrane Disruptors & Arsenicals
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Rapid yellowing followed by desiccation of
affected plant tissue
MSMA, DSMA symptoms generally appear
first on leaf tips
Drift may result in speckled leaf burn
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Paraquat drift on pecan causes speckling of the leaves.
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Organic Arsenicals
Family
Common Name
Trade Name
organic arsenicals
MSMA
Several
organic arsenicals
DSMA
Several
Fast acting herbicides that produce rapid leaf burn.
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Lipid Synthesis Inhibitors
(Grass Specific Herbicides)
Family
Common Name
Trade Name
cyclohexanedione
sethoxydim
Poast
cyclohexanedione
clethodim
Envoy
aryloxyphenoxy- propionate
fluazifop-P
Fusilade
aryloxyphenoxy- propionate
fenoxaprop-P
Acclaim Extra
These herbicides act by disrupting lipid biosynthesis in grass plants.
Plant cells and cellular organelles all contain lipid membranes.
Therefore, these herbicides affect cell membrane integrity in the
meristems.
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Grass herbicides symptoms on johnsongrass
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Grass Specific Herbicides
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Initial injury in the meristems occurs where
new leaves are developing
These regions will turn chlorotic, which is
followed by necrosis
The affected area will become rotten and
will easily separate from rest of plant
Injury will develop slowly (7 to 14 days)
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Grass Herbicide Symptoms
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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.
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Amino Acid Synthesis Inhibitors
(ALS inhibitors)
Family
Common Name
Trade Names
imidazolinones
imazapyr
Arsenal, Chopper
Stalker
sulfonylurea
metsulfuron
Escort
sulfonylurea
sulfometuron
Oust
These herbicides inhibit amino acid synthesis which is
necessary for the formation of plant proteins.
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ALS Characteristics
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Most of these herbicides have soil and
foliar activity except glyphosate, and
glufosinate
Systemically translocated in plants
Soil activity herbicide-dependent
Generally, low use-rate herbicides
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ALS Symptoms
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Grass symptoms include: stunting, purple
coloration, and inhibited root systems with
bottle-brush appearance
Broadleaf symptoms include: red or purple
veins, yellowing of new leaves and
blackened terminal growth
Glyphosate injury includes initial yellowing
followed by death of affected tissue
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Amino Acid Synthesis Inhibitors
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Accord, Arsenal, Oust, Escort,
Move to points of active growth or
food storage.
Affect shoot tips, young buds and
leaves.
Can cause contact burn and twisting.
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Arsenal (imazapyr)
Compact growth on azalea
Untreated
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Arsenal (imazapyr)
Bunched, compact
growth on dogwood and
sassafrass
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Arsenal symptoms on maple.
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Arsenal (imazpyr)
blackberry
sweetgum
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Amino Acid Synthesis Inhibitors
(Not ALS inhibitors)
Family
Common Name
Trade Names
Not classified
glyphosate
Accord, Roundup
Not classified
sulfosate
Touchdown
Not classified
glufosinate
Finale
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Yellowing of new growth is common symptom
of glyphosate exposure.
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Split bark on apple from Roundup exposure.
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Strapped leaves on maple due to Roundup.
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Roundup drift from the previous fall may cause strapped leaves on51
peaches, apples and pears the next spring.
Stunted, compact growth due to glyphosate.
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Finale (glufosinate) on pine
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Terminal damage from a tank mix of Accord + Arsenal
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