lecture 18 Flashcards
herbicide resistance definition
Inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type
herbicide tolerance definition
the inherent ability of a species to survive herbicide treatment
evolutionary resistance definition
the process where rare resistant individuals become the majority
needs for herbicide resistance
- a population of weeds (originally suspectable)
-genetic change - heritable mutation - time
-dose dependance - dose must kill wild type normally
two reasons resistance occurs
1) heritable variation (mutation)
2)selection
unsatisfactory pesticide performance - not due to resistance
-misapplication
-hard/dirt water
-rainfall following application
-too hot/cold/wet/dry
are resistant plants always easy to identify?
no, they may still look like they’re dying but the new growth point is not dying.
3 genetic factors influencing evolution of resistance in weeds
1) initial mutation frequency
2)number of genes / mode of inheritance
3)relative fitness
2 biological/ecological factors influencing evolution of resistance
1) weed mating system
2) soil seed reserve
(gene flow)
2 operational factors influencing evolutionary of resistance
1) selection pressure
2) herbicide use patterns (rotations and mixtures)
mutation rates influenced by:
numerous factors and vary considerably among gene loci
cytoplasmic vs nuclear inheritance
- triazine resistance - cytoplasmic (chloroplast)
all others - nuclear
single gene vs polygenic inheritance
alopecurus myosuroides - 2 additive genes
avena fatua - 2 recessive genes
most others (to date) single gene per MOA
recessive vs dominant
most dominant or partially dominant
outcrossing effect on resistance
increases spread of resistance through pollen flow
low seedbank longevity increases or decreases rate of resistance
increases
selection pressure
the more effective the herbicide the more intense the selection pressure
4 herbicide attributes causing resistance
high efficacy
frequent, repeated application
soil residual activity
single target site
3 main risk factors for evolution of herbicide resistant weeds
1) repeated application of highly efficacious herbicides with the same site of action
2) annual weed species that occur at high population densities, are widely distributed, are genetically variable, are prolific seed producers, have efficient gene dissemination.
3) simple cropping systems that favor a few dominant weed species
mechanisms of resistance
altered target site
enhanced detoxification/metabolism
reduced translocation
altered target enzyme specific activity
sequestration
reduced entry
rapid necrosis
3 classifications of resistance
1) Resistance -
2) cross resistant -
3) multiple resistant -
Cross resistance
evolved mechanism to survive application of more than one class of herbicide
multiple resistance
more than one evolved mechanism that provides a broad range of resistance or survival of more than one group of herbicide
two ways to slow rate of resistance
1) reduce number of herbicide applications
2)alter MOA’s
