Download Master Gardener`s Weed Science and Managment

Herbicide Resistance: Evolution,
Prevention and Control
Jamshid Ashigh
Extension Weed Specialist/ Assistant Professor
Chemical weed control
• Herbicides are the most efficient technology for
large-scale weed control.
• The introduction of new herbicides to replace
those herbicides failing due to resistance is
essential for weed management.
Herbicide resistance world wide
Herbicides development and use
• The rate of introduction of new herbicides has
slowed dramatically.
• Due to the high cost
• Ten multi-national corporations
• In 2004 there were four remaining
• There is a strong imperative to use the
currently available herbicide resources in
more sustainable ways.
• Sustainable agriculture
• IWM
Herbicide Resistance
• Herbicide resistance is the inherited ability of a
plant to survive and reproduce following
exposure to a dose of herbicide normally lethal
to the wild type (WSSA).
• Similar to resistance to:
• Insecticides
• Fungicides
• Antibiotics
Evolution of herbicide resistance
• Weed populations are diverse
• Most individuals are susceptible to
herbicides
• A few individuals are naturally resistant
(1X10-8 to 1X10-10)
• After the herbicide is applied they are the
only ones to survive
Evolution of Herbicide resistance
• Survivors put seeds in the seed bank
• The following year
• A few more survivors
• Not noticeable
• Several years (3-10 years) of selection before
it becomes visible
• 30% of the total population
Resistance selection
Source: J.L. Gunsolus. Herbicide Resistant Weeds. 1998. North Central Region Extension Publication 468.
Major factors influencing the evolution of resistance
• Selection pressure
• Initial frequency of herbicide-resistant
individuals
– 1X10-8 to 1X10-10
• Gene flow
– Pollen and seed movement
– Resistance controlled by single gene (e.g., ALSinhibitors resistance)
Selection pressure
• Efficiency of the herbicide
• Sensitive weeds
• Frequency of use
• Applied alone
• Duration of effect
Weed characteristics that influence
the evolution of resistance
• Annual life cycle
• Produce large numbers of seed
• Genetic variability
– Outcross
What makes the weeds resistant?
• Altered site of action
• Change in target enzyme
• Enhanced metabolism
• Ability to degrade herbicide
• Decreased absorption and translocation
• Herbicide does not get to its site of action
• Sequestration
• Herbicide not available to the plant (stored in
vacuoles)
Altered site of action
• Alterations in the site of action that
prevent the herbicide from binding are the
most common mechanism of resistance
• Caused by mutation(s) in the gene(s)
• Change in the amino acids
Triazine mechanism of action
Chloroplast
Thylacoid
membrane
Source: Dr. J. Christopher Hall
Plastoquinone Binding
Source: Dr. J. Christopher Hall
Triazine Binding
Source: Dr. J. Christopher Hall
Triazine Resistance
Source: Dr. J. Christopher Hall
Confirmation of resistance in the field
• Other causes of herbicide failure have been
ruled out (e.g., plant size, time of application etc).
• The same herbicide or herbicides from the
same family have been used year after year.
• One weed that is normally controlled is not
controlled while other weeds are.
• Single weed species in patches and they are
spreading.
• Mix of dead and healthy weeds from the
same species are seen in the field.
Source: Dr. Earl Creech
Confirmation of resistance in the lab
• Seedlings.
– Pre emergence
– Post emergence
• DNA sequencing.
• Absorption, translocation and metabolism
R
Pre- emergence
S
S
R
Post- emergence
Herbicide resistant weeds
• Worldwide:
– 323 Resistant Biotypes, 187 Species (112
dicots and 75 monocots)
• United States:
– Resistant weeds have been reported from
46 states.
– In New Mexico:
• One confirmed case (Kochia)
• Suspected glyphosate-resistant Palmer
amaranth.
Source: Dr. I. Heap. www.weedscience.org
Glyphosate-resistance
• Glyphosate-resistant (Roundup ready) crops are
the most widely used transgenic crops.
– 98% of cotton in US.
• Glyphosate is also used extensively in other
crops.
• Nine weed species including Palmer amaranth
have developed resistance to Glyphosate.
• Resistance is threatening the ongoing
sustainability of Glyphosate.
Glyphosate-resistant palmer amaranth
• Glyphosate-resistant Palmer amaranth in
Georgia, Tennessee, North Carolina, South
Carolina and Arkansas.
• In late 2007, a Palmer amaranth population was
reported to have survived Glyphosate
applications in New Mexico.
• Palmer amaranth is an out-crossing plant:
– Spread faster than self-pollinated species.
Preliminary Results
1X
1X
2X
2X
Resistant
Susceptible
It is important to adopt proactive
weed management strategies to
prevent/control resistance.
Prevention and management
•
•
•
•
•
•
Prevent seed production introduction
Monitor the field
Follow label direction
Rotate herbicide
Rotate crop
Use less herbicide
•
•
Integrate other methods of weed control
with herbicides
Use herbicide mixtures
Problem with rotating herbicides
• Herbicide rotation has not precluded the
development of some types of resistance
(e.g., multiple- or cross-resistant wild oat in
Canada).
• Especially if the label rates are not followed
• Due to stacking of resistance genes
• Triazine resistance + ALS resistance
Criteria for good herbicide mixtures
• Same residual (persistence)
• Same efficacy (control the same
spectrum of weeds)
• Different mode of action
Example of a bad mixture
Herbicide
Mode of action
Residual
Pursuit
ALS
Long
Basagran
Photosystem II
None
• Follow label direction for mixture
options
Label
• Legally binding
• Contains critical info:
– How to mix & apply
– Restrictions
– Use rates
– Spill info/contact
– Soil activity
– Species
– Time of application
– Mixture options