Herbicide Mode of Action

1. Herbicide Mode of Action An overview Developed by Cheryl Wilen, UC IPM Not for distribution without permission

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

3. Herbicide A chemical substance used to kill undesirable plants. • Will kill any plant not just weeds • PRE and POE • Contact and systemic

4. 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!!

5. Why understand herbicide MOA? • Better understanding of how herbicides perform • Diagnosing herbicide injury • Resistance Management

6. Mode of Action • Three phases • Absorption • Movement to target site • Inhibit biochemical reaction at target site

7. Herbicide Movement in Plants Phloem Mobile Move to points of active growth or food storage. Affect shoot tips, young buds and leaves. Can cause contact burn. Contact • 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.

8. 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.

9. Cell (Plasma) Membranes • Contain the cellular contents and regulate the entry and exit of substances that promote or inhibit plant metabolism

10. Entry and Movement of Herbicides in Plants

11. 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

12. 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

13. Plant Processes Affected by Herbicides • Photosynthesis • Amino acid and protein synthesis • Cell division • Cell membranes • Pigment synthesis • Growth regulation • Growth inhibition • Fatty acid synthesis

14. 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

15. Growth Regulator Herbicides • Affect several plant process • cell division • cell enlargement • protein synthesis • respiration • Upset normal hormonal balance Examples: 2,4-D MCPADicamba Banvel Clopyralid

16. 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

17. 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

18. Seedling Growth Inhibitors • 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) Swelling at base Stubby roots

19. 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

20. Cell Membrane Disrupters • Destroy cell membranes • Contents leak out • Plant dessicates • Diquat • Oxyfluorfen • Flumioxazin • Drift may injure susceptible crops by spotting leaves

21. Photosynthesis Inhibitors • Interrupt one or more steps of photosynthesis • Most are PRE but are also active POST • Translocate • Atrazine, Simazine, Linuron X

22. 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

23. 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

24. 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

25. Pigment Inhibitors Foliage becomes white than dies Especially apparent on new growth

26. 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)

27. Lipid Synthesis Inhibitors • Leaves turn reddish • Discoloration at and above nodes • Leaves in whorl easily be separated from rest of the plant