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Angela R. Post

PLNT 3113

Principles of Weed Science

Lipid biosynthesis inhibitor herbicides (ACCase)

Brief discussion of lipid biosynthesis Examples of chemical structures Characteristics Uses Selectivity Mode and mechanism of action

Acetyl-coenzyme A carboxylase (ACCase)

Rate limiting step in lipid biosynthesis

Usually sensitive in grasses, but not broadleaf

plants

Introduced for POST control of annual and perennial grasses in 1975

Some selectivity between some grass crops and weeds (wheat and wild oat – diclofop, tralkoxydim)

Non-grass species resistant Readily absorbed by foliage Ambimobile (phloem and xylem) Spray additive usually needed for

maximum activity

Wild oat control in winter wheat

Untreated control Fenoxaprop-ethyl 90 g ai/ha

1 to 3 leaf wild oat

Wild oat spray - diclofop

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Fluazifop-butyl in soybean

Untreated Treated

Fluazifop-butyl

Soybean and oat

Diclofop-methyl

Wheat and oat

Cotton and Sorghum halepense

Readily absorbed by roots and shoots (about 60-90%)

Lipophilic – readily absorbed into cuticle, may be slow to desorb into apoplast ◦ Rainfastness – depends on product

Uptake best at high humidity and warm temperature

Spray coverage important

Cl O

Cl

O CHCO2CH

3

CH3

N O

FCl

O C

H

CO2CH2

CH3

C CH

Variable – 2 to 50% ◦ Diclofop ~2% translocation, mostly acropetal in xylem, a

little basipetal in phloem

Accumulates in meristematic tissues including

intercalary meristems

Adjuvants sometimes can increase translocation

High RH and warmer temperature and increase

translocation

Water stress reduces translocation

Free acids translocate not the ester

Works best when applied to actively

growing, non-stressed grasses

Death is slow – week or more

◦ Root and shoot growth stops quickly

◦ Water soaked appearance at point of herbicide

contact

◦ Chlorosis in leaves within 2 to 6 days

◦ Necrosis begins at meristems and progresses

to older tissues

Slowly degrades in soil

Most have little soil activity under normal

conditions ◦ Diclofop only herbicide labeled for soil application –

winter wheat (PPI, Bromus tectorum)

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Lipids are essential components of membranes without which new membranes are not formed

These herbicides are active at meristematic regions in the plant where they stop production of cell membranes

Aryloxyphenoxypropionates (fops)

diclofop

Hoelon

Cyclohexanediones (dims) sethoxydim

(Poast, Poast Plus, Vantage)

Chemical properties -Are not volatile

-Will not photo-decompose

-No leaching concerns

Mode of Action – inhibit lipid production in grasses -Lipids are used for what?

Location of activity in plant -Site of action is the enzyme acetyl-CoA (ACCase)

Herbicidal properties -Fairly slow acting

-Susceptible grasses will immediately stop growing

-Death of grass will occur in 5 to 10 days after treatment

-Readily translocated

-Move to meristems

Selectivity -Broadleaves are tolerant due to lack of susceptible ACCase enzyme

-Resistant weeds are known to exist – gene alteration

-Ryegrass, fescue

-Activity reduced if mixed with some herbicides

-Very sensitive to drought -- may even fail

-Most “require” a surfactant or COC

-Rainfast in 1 to 3 hours

Symptoms include

“Stem Slipage in Grasses”

Rotten Whorl

in Corn

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Symptoms include

“White Flash in Broadleaves”

Not a major concern but common with Assure II

-Clethodim / Select

-Fluazifop / Fusilade 2000

-Quizalofop / Assure II

-Diclofop / Hoelon

-Fenoxaprop / Puma

Labeled in Cotton, soybeans, peanuts & alfalfa

-Performs well under Oklahoma’s dry conditions -Under very dry conditions activity declines

-Careful with tank mixes due to antagonism

Untreated July 29

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Select Max AI =

clethodim July 29

-Has UV light protection

-Vantage - selected turf and horticulture uses -centipedegrass and fine fescues

-Poast Plus - cotton, soybeans, strawberries, alfalfa, sunflowers, & peanuts

-Very sensitive to drought

-PRE and POST

-Does NOT control perennial grasses

-Good for ryegrass and wild oats in wheat

-Very little use in OK – Why? -Full-season grazing restriction

-Expensive

-POST grass control in rice and wheat

-Good for wild oats in wheat

-POST on cotton and soybeans

-Need an adjuvant

-Performs well in OK

Untreated July 29

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Assure II + atrazine AI = quizalofop

July 29

-POST on cotton, soybeans, ornamentals

-NOT for use on peanuts or alfalfa

-Has performed very well in OK -Affected by drought, but less so than Poast Plus

Fluazifop-P

Structure:

Method of application: Foliar.

Translocation: Phloem mobile; Symplastic

Mechanism of inhibition: Inhibition of acetyl-CoA carboxylase

(ACCase), which catalyzes the first step of fatty acid synthesis.

Plants lack lipids for new growth and repair.

Symptoms: Growth stops soon after application. Discoloration

and disintegration of meristematic tissue just above the nodes.

Leaves turn yellow and/or red and seedlings fall over at the soil

surface. Death occurs between 10 and 21 days after treatment.

LIPID BIOSYNTHESIS INHIBITORS

N

O F3C O H C

CH3

C OH

O

LIPID BIOSYNTHESIS INHIBITORS

Fluazifop-P

Environmental Persistence:

- Rapidly degraded in the soil, but may have some

temporary activity on germinating grasses

Special features/problems:

-Applied as the butyl ester of fluazifop; subsequent

metabolism within the plant changes it to the herbicidally

active form

N

O CF 3 O H C

CH 3

C O

O

(CH 2 ) 3 CH 3

Fluazifop-P butyl ester

N

O CF 3 O H C

CH 3

C OH

O

Fluazifop-P

Metabolism in plants

Others: Aryloxyphenoxy propionates: (=“Fops”)

-Diclofop-methyl HOELON

-Fenoxaprop-p-ethyl SILVERADO

-Quizalofop-p-ethyl ASSURE II

Cyclohexanediones: (=“Dims”)

-Sethoxydim POAST

-Clethodim SELECT

LIPID BIOSYNTHESIS INHIBITORS

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Broadleaf plants have an

ACCase enzyme that is

relatively insensitive to

“fops” and “dims”; Some

species metabolize these

herbicides.