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Forestry Herbicide Symptoms & Mode of Action. John Boyd University of Arkansas [email protected] 501-671-2224. What is mode of action?. How a herbicide disrupts or inhibits normal plant development. Examples cell membrane destruction. block protein synthesis

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Forestry Herbicide Symptoms & Mode of Action

John Boyd

University of Arkansas

[email protected]


what is mode of action
What is mode of action?
  • How a herbicide disrupts or inhibits normal plant development.
    • Examples
      • cell membrane destruction.
      • block protein synthesis
      • interfere with photosynthesis
terms to remember
Terms to remember
  • photosynthesis (food)
  • respiration (energy)
  • amino acids (proteins/growth)
  • lipids (cell membranes)
  • pigments (energy/light capture)
  • mitosis (cell division)

Phloem mobile herbicides move up and down in the plant

Contact herbicides do not move in the plant

Xylem mobileherbicides move up in the plant

growth regulator herbicides
Growth Regulator Herbicides

Affect cell division, cell enlargement, protein synthesis and respiration. Act by upsetting hormone balance.

growth regulator herbicides6
Growth Regulator Herbicides
  • Generally applied to leaves, but have limited soil activity
  • Length of soil activity is herbicide-dependent
  • Highly systemic in susceptible plants
  • Activity (pound for pound) varies among herbicides
  • Primarily broadleaf activity but may affect grasses
growth regulator symptoms
Growth Regulator Symptoms
  • 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.
photosynthesis inhibitors
Photosynthesis Inhibitors

These herbicides interrupt photosynthesis the process by which green plants convert light energy into food.

pi s are xylem mobile
PI’s are Xylem Mobile
  • 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.
photosynthesis inhibitors16
Photosynthesis Inhibitors
  • Most PI’s are soil applied.
  • All PI’s have at least some postemergence activity
  • Soil-applied PI’s translocate through the xylem.
photosynthesis inhibitors17
Photosynthesis Inhibitors
  • Interveinal or veinal yellowing followed by death of plant tissue from leaf margins inward
  • Postemergence applications cause rapid burning of plant tissue
princep simazine
Princep (simazine)

Symptoms often appear on the margins of the oldest leaves (pear).

With photosynthesis inhibitors, symptoms start on the margins of the older leaves and move inward.
With photosynthesis inhibitors, symptoms start on the margins of the older leaves and move inward.
pigment inhibitor herbicides
Pigment Inhibitor Herbicides

These herbicides cause the green pigments (chlorophyll) in plants to be destroyed. Without chlorophyll, plants cannot photosynthesize and will die.

pigment inhibitors
Pigment inhibitors
  • 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)
pigment inhibitor herbicides24
Pigment Inhibitor Herbicides
  • 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.


command clomazone
Command (clomazone)
  • New growth is white, sometimes tinged with pink or purple.
  • Command: white between the veins.


root growth inhibitors
Root Growth Inhibitors

The root inhibitors interrupt cell division (mitosis) stopping root growth in seedling plants.

root growth inhibitors27
Root Growth Inhibitors
  • 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
root growth inhibitors28
Root Growth Inhibitors
  • 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
cell membrane disrupters
Cell Membrane Disrupters

Destroy cell membranes, causing cell contents to leak out and result in desiccation of plant tissue.

contact herbicides
Contact herbicides
  • Do not move
  • Herbicides: Paraquat, diquat, Cobra (lactofen), MSMA
    • Rupture cell walls.
    • Symptoms appear within hours.
cell membrane disruptors arsenicals
Cell Membrane Disruptors & Arsenicals
  • 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
organic arsenicals
Organic Arsenicals

Fast acting herbicides that produce rapid leaf burn.

lipid synthesis inhibitors grass specific herbicides
Lipid Synthesis Inhibitors(Grass Specific Herbicides)

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.

grass specific herbicides
Grass Specific Herbicides
  • 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)
grass herbicide symptoms
Grass Herbicide Symptoms
  • 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.
amino acid synthesis inhibitors als inhibitors
Amino Acid Synthesis Inhibitors(ALS inhibitors)

These herbicides inhibit amino acid synthesis which is necessary for the formation of plant proteins.

als characteristics
ALS Characteristics
  • 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
als symptoms
ALS Symptoms
  • 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
amino acid synthesis inhibitors
Amino Acid Synthesis Inhibitors
  • 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.
arsenal imazapyr
Arsenal (imazapyr)


Compact growth on azalea

arsenal imazapyr44
Arsenal (imazapyr)

Bunched, compact growth on dogwood and sassafrass

arsenal imazpyr
Arsenal (imazpyr)



Roundup drift from the previous fall may cause strapped leaves on peaches, apples and pears the next spring.